Respiratory syncytial virus and metapneumovirus vaccines

Abstract

Provided herein are vaccine composition comprising a chemically-modified messenger ribonucleic acid (mRNA) encoding a hMPV fusion (F) glycoprotein and / or a chemically-modified mRNA encoding a hRSV F glycoprotein formulated in a lipid nanoparticle, and related methods for inducing an antigen- specific immune response.

Description

[0001]RESPIRATORY SYNCYTIAL VIRUS AND METAPNEUMOVIRUS VACCINES RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No.63 / 654,809, filed on May 31, 2024, U.S. Provisional Patent Application No. 63 / 654,879, filed on May 31, 2024, U.S. Provisional Patent Application No.63 / 655,476, filed on June 3, 2024, and U.S. Provisional Patent Application No.63 / 764,558, filed on February 27, 2025, each of which is hereby incorporated by reference in its entirety. REFERENCE TO AN ELECTRONIC SEQUENCE LISTING The content of the electronic sequence listing (M137870296WO00-SEQ-JXV.xml; Size: 218,086 bytes; and Date of Creation: May 30, 2025) is herein incorporated by reference in its entirety. BACKGROUND Respiratory diseases, encompassing a range of conditions affecting the gas exchange organs, pose significant health challenges globally. Respiratory diseases can be caused by various viruses. Two viruses of the Pneumoviridae family, human metapneumovirus (hMPV) and human respiratory syncytial virus (hRSV), present significant concern, particularly in immunosuppressed patients, older adults, and children. Though infection symptoms of infection by either virus in healthy adults typically resemble those of the “common cold” (e.g., coughing, wheezing, and fever), hMPV and hRSV are both leading causes of death in vulnerable populations. In addition, nearly 90% of the global population of infants and children under 5 years old are affected by either virus, immunity is incomplete, and reinfection occurs frequently throughout life. The continuing health problems associated with hMPV and hRSV and other infectious disease viruses are of international concern, reinforcing the importance of developing effective and safe vaccine candidates against infectious disease viruses both alone and in combination with one another. SUMMARY Provided are compositions and methods useful for, in some aspects, vaccination against human metapneumovirus (hMPV) infection and, optionally, human respiratory syncytial virus (hRSV) infection, and which offer the advantages of high efficacy, speed of development, and production scalability and reliability. The compositions and methods are based, at least in part, on the finding that certain substitutions in hMPV F glycoproteins allow for stabilization of the F glycoprotein in a prefusion conformation, and consequently, improve immunogenicity. Such stabilized prefusion hMPV F glycoproteins allow for improved hMPV vaccines, such as those containing ribonucleic acids (RNAs) (e.g., mRNAs) encoding the stabilized prefusion hMPV F glycoproteins, or other compositions for vaccination (e.g., viral vector or protein-based vaccines). Accordingly, some aspects relate to compositions comprising one or more mRNAs encoding one or more hMPV fusion glycoproteins (“F glycoproteins”) and, optionally, one or more hRSV proteins (e.g., one or more hRSV F glycoproteins). Some aspects of the present disclosure provide a human metapneumovirus type A (hMPV-A) fusion (F) protomer, wherein the hMPV F protomer comprises at least one mutation selected from the group consisting of: (i) truncation of a cytoplasmic tail; (ii) one or more substitutions of a non-cysteine residue to a cysteine (C) residue; (iii) one or more substitutions of a surface exposed residue to a serine (S) residue or lysine (K) residue; (iv) one or more substitutions of an internal residue to a valine (V) residue or threonine (T) residue; and (v) modification of a F1 / F2 cleavage site with a flexible linker. In some embodiments, the hMPV-A F protomer does not comprise a cytoplasmic tail sequence of SEQ ID NO: 49 (KKTKKPTGAPPELSGVTNNGFIPHN). In some embodiments, the one or more substitutions of a non-cysteine residue to a C residue are present at one or more of positions of 84, 140, 147, 249, 454, and 458; wherein the positions are numbered by alignment to SEQ ID NO: 14. In some embodiments, the one or more substitutions of a non-cysteine to a C residue are present at positions 84 and 249; optionally wherein the substitutions comprise V84C and A249C. In some embodiments, the one or more substitutions of a non-cysteine to a C residue are present at positions 140 and 147; optionally wherein the substitutions comprise A140C and A147C. In some embodiments, the one or more substitutions of a non-cysteine to a C residue are present at positions 454 and 458; optionally wherein the substitutions comprise D454C and V458C. In some embodiments, the one or more substitutions of a surface exposed residue to a S residue or K residue are present at positions 61, 138, or 232; wherein the positions are numbered by alignment to SEQ ID NO: 14. In some embodiments, the one or more substitutions of a surface exposed residue are selected from the group consisting of: A61S, N138K, and P232S. In some embodiments, the one or more substitutions of an internal residue to a T residue or V residue are present at positions 114 and / or 449; wherein the positions are numbered by alignment to SEQ ID NO: 14. In some embodiments, the one or more substitutions of an internal residue are selected from the group consisting of: A114T, A114V, I449T, and I449V. In some embodiments, the flexible linker replaces amino acid residues 89-112; wherein the positions are numbered by alignment to SEQ ID NO: 14. In some embodiments, the flexible linker is GSGGSG (SEQ ID NO: 141) or GS. In some embodiments, the hMPV F protomer comprises an amino acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a sequence selected from the group consisting of SEQ ID NOs: 1-3. In some embodiments, the hMPV F protomer comprises an amino acid sequence selected from the group consisting of SEQ ID NOs: 1-3. In some embodiments, the hMPV F protomer consists of, or consists essentially of, an amino acid sequence selected from the group consisting of SEQ ID NOs: 1-3. In some embodiments, the present disclosure provides a messenger ribonucleic acid (mRNA) encoding a hMPV-A F protomer. In some embodiments, the mRNA comprises an open reading frame (ORF) encoding the hMPV-A F protomer, and wherein the ORF comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a sequence selected from the group consisting of SEQ ID NOs: 18-20. In some embodiments, the ORF comprises a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 18-20. In some embodiments, the ORF consists of, or consists essentially of, a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 18-20. In some embodiments, the mRNA comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 31-33. In some embodiments, the mRNA comprises a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 31-33. In some embodiments, the mRNA consists of, or consists essentially of, a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 31-33. Some aspects of the present disclosure provide a human metapneumovirus type B (hMPV-B) fusion (F) protomer, wherein the hMPV F protomer comprises at least one mutation selected from the group consisting of: (i) truncation of a cytoplasmic tail; (ii) one or more substitutions of a non-cysteine residue to a cysteine (C) residue; and (iii) modification of a F1 / F2 cleavage site with a flexible linker. In some embodiments, the hMPV-B F protomer does not comprise a cytoplasmic tail sequence of SEQ ID NO: 49 (KKTKKPTGAPPELSGVTNNGFIPHN). In some embodiments, the one or more substitutions of a non-cysteine residue to a C residue are present at one or more of positions of 84, 140, 147, and 249; wherein the positions are numbered by alignment to SEQ ID NO: 15. In some embodiments, the one or more substitutions of a non-cysteine to a C residue are present at positions 84 and 249; optionally wherein the substitutions comprise V84C and A249C. In some embodiments, the one or more substitutions of a non-cysteine to a C residue are present at positions 140 and 147; optionally wherein the substitutions comprise A140C and A147C. In some embodiments, the flexible linker replaces amino acid residues 89-112; wherein the positions are numbered by alignment to SEQ ID NO: 15. In some embodiments, the flexible linker is GSGGSG (SEQ ID NO: 141) or GS. In some embodiments, the hMPV F protomer comprises an amino acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to SEQ ID NO: 4. In some embodiments, the hMPV F protomer comprises an amino acid sequence of SEQ ID NO: 4. In some embodiments, the hMPV F protomer consists of, or consists essentially of, an amino acid sequence of SEQ ID NO: 4. In some embodiments, the present disclosure provides a messenger ribonucleic acid (mRNA) encoding a hMPV-B F protomer. In some embodiments, the mRNA comprises an open reading frame (ORF) encoding the hMPV-B F protomer, and wherein the ORF comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a sequence of SEQ ID NO: 21. In some embodiments, the ORF comprises a nucleic acid sequence of SEQ ID NO: 21. In some embodiments, the ORF consists of, or consists essentially of, a nucleic acid sequence of SEQ ID NO: 21. In some embodiments, the mRNA comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a nucleic acid sequence of SEQ ID NO: 34. In some embodiments, mRNA comprises a nucleic acid sequence of SEQ ID NO: 34. In some embodiments, mRNA consists of, or consists essentially of, a nucleic acid sequence of SEQ ID NO: 34. In some embodiments, the present disclosure provides a composition comprising: (a) a mRNA comprising an ORF encoding an hMPV (e.g., hMPV-A or hMPV-B) protomer; and (b) an mRNA comprising an ORF encoding a human respiratory syncytial virus (hRSV) fusion (F) protein. In some embodiments, the hRSV F protein comprises: (a) an hRSV-A F protein; (b) an hRSV-B F protein; or (c) an hRSV-A F protein and an hRSV-B F protein. Some embodiments comprise (a) mRNA encoding a hMPV-A F protein, (b) mRNA encoding a hMPV-B F protein, and (c) mRNA encoding a hRSV-A F protein. In some embodiments, (a) the hRSV-A protein comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to SEQ ID NO: 13; and / or (b) the hRSV-B protein comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to any one of SEQ ID NOs: 5-12. In some embodiments, the composition comprises (a) the hRSV-A protein comprises SEQ ID NO: 13; and / or (b) the hRSV-B protein comprises any one of SEQ ID NOs: 5-12. In some embodiments, the ORF encoding the hRSV-A protein comprises a nucleic acid sequence having at 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to SEQ ID NO: 30 or SEQ ID NO: 113; and / or the ORF encoding the hRSV-B protein comprises a nucleic acid sequence having at 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to any one of SEQ ID NOs: 22-29. In some embodiments, the ORF encoding the hRSV-A protein comprises SEQ ID NO: 30 or SEQ ID NO: 113; and / or the ORF encoding the hRSV-B protein comprises any one of SEQ ID NOs: 22-29. In some embodiments, the mRNA encoding the hRSV-B protein comprises a nucleic acid sequence having at 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to any one of SEQ ID NOs: 35-42. In some embodiments, the mRNA encoding the hRSV-B protein comprises any one of SEQ ID NOs: 35-42. In some embodiments, the composition comprises: (a) a messenger ribonucleic acid (mRNA) comprising an open reading frame (ORF) encoding a human metapneumovirus-A (hMPV-A) fusion (F) protomer, wherein the hMPV-A F protomer comprises at least one stabilizing mutation relative to SEQ ID NO: 14; and (b) a mRNA comprising an ORF encoding an hMPV-B F protomer, wherein the hMPV-B F protomer comprises at least one stabilizing mutation relative to SEQ ID NO: 15. In some embodiments, the composition comprises: (a) mRNA comprising an ORF encoding an hMPV-A F protomer; and (b) mRNA comprising an ORF encoding an hMPV-B F protomer. In some embodiments, the composition further comprises an mRNA comprising an ORF encoding a hRSV F glycoprotein. In some embodiments, the hRSV F glycoprotein is an hRSV-A F glycoprotein the hRSV F glycoprotein is an hRSV-B F glycoprotein; or the hRSV F glycoprotein is an hRSV-A F glycoprotein and the composition further comprises an hRSV-B F glycoprotein. In some embodiments, the ratio of the hMPV-A mRNA to the hMPV-B mRNA is 1:1, 1:2, or 2:1. In some embodiments, the ratio of hMPV mRNA (hMPV-A, hMPV-B, or hMPV- A and hMPV-B) to hRSV mRNA (hRSV-A, hRSV-B, or hRSV-A and hRSV-B) is 1:1, 1:2, or 2:1. Some embodiments comprise combination vaccine compositions comprising messenger ribonucleic acid (mRNA) formulated in a lipid nanoparticle, wherein the combination vaccine composition comprises: (a) a first mRNA comprising a first open reading frame (ORF) encoding a human metapneumovirus-A (hMPV-A) fusion (F) glycoprotein, wherein the first ORF comprises a nucleotide sequence at least 90%, at least 95%, at least 98%, or at least 99% identical to SEQ ID NO: 19 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine; (b) a second mRNA comprising a second ORF encoding a human metapneumovirus-B (hMPV-B) F glycoprotein, wherein the second ORF comprises a nucleotide sequence at least 90%, at least 95%, at least 98%, or at least 99% identical to SEQ ID NO: 21 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine; and (c) a third mRNA comprising a third ORF encoding a human respiratory syncytial virus (hRSV) F protein, wherein the third ORF comprises a nucleotide sequence at least 90%, at least 95%, at least 98%, or at least 99% identical to SEQ ID NO: 113 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine. In some embodiments the mass ratio of (a):(b):(c) is 1:1:2 Some embodiments comprise combination vaccine compositions comprising messenger ribonucleic acid (mRNA) formulated in a lipid nanoparticle, wherein the combination vaccine composition comprises: (a) a first mRNA comprising a first open reading frame (ORF) encoding a human metapneumovirus-A (hMPV-A) fusion (F) glycoprotein, wherein the first ORF comprises a nucleotide sequence of SEQ ID NO: 19 having nucleosides consisting of N1- methylpseudouridine, adenosine, guanosine, and cytidine; (b) a second mRNA comprising a second ORF encoding a human metapneumovirus-B (hMPV-B) F glycoprotein, wherein the second ORF comprises a nucleotide sequence of SEQ ID NO: 21 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine; and (c) a third mRNA comprising a third ORF encoding a human respiratory syncytial virus (hRSV) F protein, wherein the third ORF comprises a nucleotide sequence of SEQ ID NO: 113 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine. In some embodiments the mass ratio of (a):(b):(c) is 1:1:2 In some embodiments, each mRNA comprises one or more chemically modified nucleotides. In some embodiments, each mRNA comprises N1-methylpseudouridine. In some embodiments, the ORF of each mRNA comprises nucleosides consisting of N1- methylpseudouridine, adenosine, guanosine, and cytidine. In some embodiments, each mRNA is formulated in a lipid nanoparticle. In some embodiments the lipid nanoparticle comprises an ionizable amino lipid, a non-cationic lipid, a sterol, and a PEG-modified lipid. In some embodiments the lipid nanoparticle comprises 20-60 mol% of the ionizable amino lipid, 5-25 mol% DSPC, 25-55 mol% cholesterol, and 0.5-15 mol% PEG-DMG. In some embodiments, a composition comprises an mRNA encoding an hMPV protomer formulated in a lipid nanoparticle. In some embodiments, the present disclosure provides a method comprising administering to a subject the composition. Some embodiments comprise methods of preventing hMPV infection comprising administering to a human subject a composition to thereby prevent hMPV infection. Some embodiments comprise methods of preventing hMPV and / or hRSV infection comprising administering to a human subject a composition to thereby prevent hMPV and / or hRSV infection. Some embodiments comprise use of a composition in the manufacture of a medicament for prevention of hMPV infection. Some embodiments comprise use of a composition in the manufacture of a medicament for prevention of hMPV and / or hRSV infection. Some aspects comprise a method of preventing lower respiratory tract disease (LRTD) caused by hMPV and / or hRSV comprising administering to a human subject a composition to thereby prevent LRTD caused by hMPV and / or hRSV. BRIEF DESCRIPTION OF THE DRAWINGS FIGs.1A-1B show disulfide-based prefusion stabilization of hMPV F protein. FIG.1A shows structure-based design of inter-protomer and intra-promoter disulfide bonds based on the prefusion structure of an hMPV F trimer. FIG.1B shows the location of inter-promoter and intra-promoter disulfide bonds in a stabilized prefusion hMPV F monomer, as well as the linker (residues 89-112). FIGs.2A-2B show characterization of hMPV A and hMPV B F protein variants. FIG.2A shows relative expression of hMPV A variants to a wild-type hMPV A (group 1). FIG.2B shows relative expression of hMPV B variants to a wild-type hMPV B (group 1). FIGs.3A-3C show antibody responses in mice vaccinated with hMPV compositions, with and without the addition of mRNA encoding a stabilized prefusion RSV-A F protein (mRNA-1345). FIG.3A shows hMPV-A neutralizing titers. FIG.3B shows hMPV-B neutralizing titers. FIG.3C shows RSV-A2 neutralizing titers. The horizontal dotted lines in each figure indicate the upper and lower limits of detection. FIGs.4A-4D show antibody titers in mice vaccinated with compositions including mRNA encoding hMPV F antigens alone and in combination with mRNA encoding an hRSV-A antigen (mRNA-1345). FIG.4A show antibody titers of anti-stabilized prefusion hMPV-A IgG in sera on days 21 and 36. FIG.4B shows titers of anti-stabilized prefusion hMPV-B IgG in sera on days 21 and 36. FIG.4C shows titers of anti-post-fusion stabilized hMPV-A IgG in sera on days 21 and 36. FIG.4D shows titers of anti-stabilized prefusion RSV-A2 IgG titers in sera on days 21 and 36. FIGs.5A-5D show T cell responses in mice vaccinated with compositions including mRNA encoding hMPV F antigens alone and in combination with mRNA encoding an hRSV-A antigen (mRNA-1345). FIG.5A shows CD4+ T cell responses to hRSV peptides. FIG.5B shows CD8+ T cell responses to hRSV peptides. FIG.5C shows CD4+ T cell responses to hMPV peptides. FIG.5D shows CD8+ T cell responses to hMPV peptides. FIGs.6A-6B show a schematic of the experimental design for producing hMPV-A and hMPV-B homotrimers and heterotrimers. FIG.6A shows six possible trimers that could be formed with combinations of hMPV-A F protein and hMPV-B F protein. Plasmid-encoded antigens were tagged, expressed, and purified in tandem to produce trimers, which were analyzed with electron microscopy. FIG.6B shows a schematic of an experimental method used to produce purified trimer proteins. FIGs.7A-7B show antibody responses in mice vaccinated with compositions including combinations of mRNA encoding hMPV F and hRSV antigens. FIG.7A shows titers of anti- stabilized prefusion hMPV-A IgG in sera collected on day 21. FIG.7B shows titers of anti- stabilized prefusion hMPV-A IgG in sera collected on day 32. In FIGs.7A-7B, the dotted line corresponds to the mean anti-stabilized prefusion hMPV A IgG titer in the sera of mice administered a composition comprising mRNA encoding an hRSV / A antigen and wild-type hMPV / A (hMPV-A WT); 1345(A) = SEQ ID NO: 13; 1345(B) = SEQ ID NO:10; hMPV Α WT = SEQ ID NO: 14; Α / preF DS2 = SEQ ID NO: 3; A / preF DS3 = SEQ ID NO: 2; and, B / preF DS2 = SEQ ID NO 4. FIG.8 shows titers of anti-stabilized prefusion hMPV-B IgG in sera collected on day 32 from mice vaccinated with compositions including combinations of mRNA encoding hMPV F and hRSV antigens. In FIG.8, 1345(A) = SEQ ID NO: 13; 1345(B) = SEQ ID NO:10; hMPV Α WT = SEQ ID NO: 14; Α / preF DS2 = SEQ ID NO: 3; A / preF DS3 = SEQ ID NO: 2; and, B / preF DS2 = SEQ ID NO 4. FIG.9 shows neutralizing antibody titers against hMPV A (dark gray) or hMPV B (light gray) evaluated in the sera of mice collected 36 days post-vaccination with compositions including combinations of mRNA encoding hMPV F and hRSV antigens. In FIG.9, 1345(A) = SEQ ID NO: 13; 1345(B) = SEQ ID NO:10; hMPV Α WT = SEQ ID NO: 14; Α / preF DS2 = SEQ ID NO: 3; A / preF DS3 = SEQ ID NO: 2; and, B / preF DS2 = SEQ ID NO 4. FIGs.10A-10C show antibody responses in mice vaccinated with mRNA encoding an hRSV A antigen with or without an mRNA encoding an hRSV B antigen. FIG.10A shows titers of anti-stabilized prefusion RSV A IgG in sera collected on day 21. FIG.10B shows titers of anti-stabilized prefusion RSV A IgG in sera collected on day 32. FIG.10C shows titers of anti- stabilized prefusion RSV B IgG in sera collected on day 32. FIGs.11A-11B show antibody responses in mice vaccinated with mRNA encoding an hRSV A antigen with or without an mRNA encoding an hRSV B antigen. FIG.11A shows the 50% inhibitor doses (IC50) against RSV A2 in sera collected on day 36. FIG.11B shows the RSV A2 neutralizing antibody titers following administration of an mRNA encoding RSV / A with or without an mRNA encoding a prefusion stabilized hMPV B / 2023 F protein. In FIGs. 11A-11B, the dashed line corresponds to the lower limit of quantification; ns = not significant. FIGs.12A-12B show antibody responses in mice vaccinated with mRNA encoding an hRSV A antigen with or without an mRNA encoding an hRSV B antigen. FIG.12A shows the 50% inhibitor doses (IC50) against RSV B / 2023 in sera collected on day 36. FIG.12B shows the RSV B / 2023 neutralizing antibody titers following administration of an mRNA encoding RSV / A with or without an mRNA encoding a prefusion stabilized hMPV B / 2023 F protein. In FIGs.12A-12B, the dashed line corresponds to the lower limit of quantification; ns = not significant. FIGs.13A-13B show antibody responses in mice vaccinated with mRNA encoding an hRSV A antigen with or without an mRNA encoding an hRSV B antigen. FIG.13A shows the 50% inhibitor doses (IC50) against RSV B18537 (1962 strain) in sera collected on day 36. FIG. 13B shows the RSV B18537 (1962 strain) neutralizing antibody titers following administration of an mRNA encoding RSV / A with or without an mRNA encoding a prefusion stabilized hMPV B / 2023 F protein. In FIGs.13A-13B, the dashed line corresponds to the lower limit of quantification; ns = not significant. DETAILED DESCRIPTION Respiratory viruses of the family Pneuomoviridae are leading causes of lower respiratory tract infections in humans, having a significant impact on morbidity and mortality worldwide. Human metapneumovirus (hMPV) and human respiratory syncytial virus (hRSV) are closely related members of the Pneumoviridae family. Though the two viruses cause similar symptoms in mild cases of infection, severe infections by hMPV tend to produce pneumonia while severe infections by hRSV are more likely to cause bronchiolitis (Wolf, D. G., et al. (2006). Pediatr. Infect. Dis. J, 25(4), 320-324). Further, while hRSV and hMPV share similarities in clinical presentation and life cycles, immune responses against either virus differ in course, and antisera against hRSV and hMPV are not cross-neutralizing (Wyde, P. R, et al. (2003). Antiviral Res.60, 51–59). Vulnerable populations are thus at increased risk of co-infections, which may be associated with increased disease severity and negative outcomes (Li, Y., et al. (2020). J. Glob. Health, 10(1)). Vaccines targeting one or both of hMPV and hRSV can significantly reduce the contribution of Pneumoviridae to global disease burden. In some embodiments vaccines targeting one or both of hMPV and hRSV are combined with a vaccine targeting one or both of influenza and betacoronavirus. Vaccines against Pneumoviridae viruses are designed to stimulate an immune response protective against the viruses. Various types of vaccines exist, including nucleic acid vaccines (e.g., DNA and RNA, such as self-amplifying RNA or mRNA vaccines) that use the genetic instructions for antigenic polypeptide production to stimulate the immune response. Protein- based vaccines use an antigenic polypeptide or fragment thereof, either from inactivated viruses or purified subunits. Live attenuated vaccines use weakened live viruses comprising or encoding the antigenic polypeptide(s), while viral vector vaccines employ a virus to deliver the antigenic polypeptide(s) to cells. Mutations to stabilize the antigenic polypeptide(s) can enhance the effectiveness of these diverse types of vaccines, leading to improved immune responses and protection against the viruses. Preferred compositions comprise mRNA vaccines. Some aspects relate to compositions and methods with improved vaccine efficacy, e.g., due to increased stability of the viral antigens. In some embodiments the vaccine efficacy is at least 60%, at least 70%, at least 80%, at least 90%, or at least 95%. Vaccine efficacy may be expressed as a proportionate reduction in disease attack rate (AR) between the unvaccinated (ARU) and vaccinated (ARV) study cohorts and can be calculated from the relative risk (RR) of disease among the vaccinated group with use of the following formulas: Efficacy=(ARU−ARV) / ARU×100; and Efficacy=(1−RR)×100. Human Metapneumovirus (hMPV) Human metapneumovirus (hMPV) is an enveloped negative-sense, single-stranded ribonucleic acid (RNA) virus of the Pneumoviridae family. HMPV is present in at least two subtypes, known as Subtype A and Subtype B (e.g., hMPV-A, hMPV-B), a categorization primarily resulting from differences in the surface glycoproteins. Each subtype of hMPV also contains within it at least two more sublineages (e.g., A1, A2a, A2b, B1, B2), which are not clearly associated with a particular clinical course. The envelope of hMPV contains three surface glycoproteins: fusion (F), attachment (G), and small hydrophobic (SH). The hMPV fusion (F) glycoprotein is required for host cell infection and is thought to be capable of performing both attachment and fusion steps in vivo. HMPV F glycoproteins contain invariant arginine-glycine-aspartate (RGD) motifs which bind to integrins, allowing for both binding and fusion, even in the absence of an hMPV G protein. Though both the hMPV G glycoprotein and hMPV F glycoprotein vary between subtypes and clinical isolates, the majority of variation occurs in the hMPV G glycoprotein. hMPV F glycoproteins are initially synthesized as inactive precursor monomers (F0), which, after further processing into heterodimers, are assembled into metastable prefusion trimers. Host cell protease activity at an F1 / F2 cleavage site allows the formation of biological fusion-active F1+ F2heterodimers, which are covalently linked via disulfide bonds (Schowalter et al. Journal of Virology 2006; 80:10931-10941). The fusion-active hMPV F glycoprotein presents in two conformations: prefusion and post-fusion. The prefusion hMPV F glycoprotein is typically observed on the viral surface, where it interacts with receptors and attachment factors on the surface of a host cell. Once bound to a host cell surface, the hMPV F glycoprotein enters the post-fusion conformation, such that the viral and host cell membrane fuse and the viral nucleocapsid is released into the cytoplasm of the host cell. Notably, transition of the hMPV F glycoprotein from prefusion to post-fusion conformation requires activity of a heptad repeat A (HRA) sequence of the F1 subunit, which refolds into an α-helix and mediates insertion of the protein into the target membrane. This refolding also allows for the stable post-fusion assembly of the HRA and HRB sequences of the F1 subunit to form a six-helix bundle. Targets for neutralizing antibodies exist on both the prefusion and post-fusion conformation of the hMPV F glycoprotein—indeed, the hMPV F glycoprotein has been found to be highly immunogenic in both prefusion and post-fusion form (Battles, et al. Nat Commun, 2017; 8:1528). However, some evidence suggests that a stabilized prefusion hMPV F glycoprotein may elicit increased neutralizing antibody titer over stabilized post-fusion hMPV F glycoproteins (Hsieh, et al. Nat. Commun., 2022; 13:1299). Accordingly, an hMPV F glycoprotein stabilized (e.g., mutated to exist in a labile, high-energy state) in a prefusion conformation may produce a greater neutralizing immune response. hMPV infection can result in symptoms ranging in severity from mild, cold-like symptoms to bronchiolitis, pneumonia, febrile seizures, and death. Importantly, hMPV is also among the top three causes of severe acute upper and lower respiratory tract disease in children. Several strains of hMPV are in global circulation at a given time (e.g., A, B1, B2). As cross- immunity to one antigen of an hMPV subtype may be observed after immunization with an antigen of another hMPV subtype, multivalent compositions may more comprehensively address the disease burden caused by hMPV subtypes, and variants thereof. Thus, some aspects relate to multivalent stabilized prefusion hMPV F glycoproteins that comprise mutations to prevent the transition of the protein into its post-fusion conformation. Stabilized prefusion hMPV F glycoproteins are discussed below in the section entitled “hMPV F Glycoproteins.” Those skilled in the art will appreciate that hMPV proteins discussed therein are useful in multiple types of compositions (e.g., vaccine compositions). In some embodiments, a composition comprises one or more hMPV proteins. In some embodiments, the composition comprises one or more nucleic acids (e.g., mRNAs) encoding one or more hMPV proteins. These and other compositions are discussed below in the section entitled “Vaccine Compositions.” Thus, discussion of hMPV proteins can also be applied to nucleic acids encoding said hMPV proteins and vice versa unless otherwise clear from context. Therefore, disclosure related to particular polypeptide mutations is also relevant to nucleic acids encoding those polypeptides with those mutations, unless otherwise clear from context. Likewise, disclosure related to mRNA encoding mutated hMPV proteins may also be relevant to the mutated hMPV proteins. Thus, when a composition comprising an mRNA encoding an hMPV protein having a particular mutation is disclosed, the skilled artisan can infer that the hMPV protein per se, and compositions comprising the hMPV protein, are also disclosed. In some embodiments, an hMPV protein is a recombinant protein. A "recombinant protein” refers to a protein that is produced in a heterologous organism that does not naturally produce the protein or a variant thereof. Non- limiting examples of organisms in which recombinant proteins may be produced include bacteria (e.g., Escherichia coli), yeast (e.g., Saccharomyces cerevisiae), and mammalian cells. hMPV F Glycoproteins It was surprisingly discovered that hMPV F glycoproteins having one or more mutations (e.g., substitutions, deletions, insertions) relative to a wild-type hMPV F glycoprotein sequence may exhibit increased stability, surface expression, and / or immunogenicity as compared to the wild-type hMPV F glycoprotein. Without being bound by theory, it is believed that such mutations in hMPV F glycoproteins can stabilize hMPV F glycoproteins in their respective prefusion conformations. For example, stabilizing mutations, in some embodiments, prevent the transition of the prefusion conformation hMPV F glycoprotein into its post-fusion conformation; such hMPV antigens are hereinafter referred to as “stabilized prefusion” antigens (e.g., stabilized prefusion hMPV F glycoproteins). Mutations that may be applied to hMPV F glycoproteins are described below. For clarity, mutations are described using amino acid numbering corresponding to specific hMPV F glycoprotein amino acid sequences (e.g., of SEQ ID NO: 14 or 15). A person of ordinary skill in the art will appreciate that mutations disclosed in reference to a given hMPV F glycoprotein amino acid sequence (e.g., SEQ ID NO: 14 or 15) may be applied to hMPV F glycoproteins of another hMPV subtype (e.g., A, B), hMPV F glycoproteins of a different subclass of the same hMPV subtype (e.g., A1, A2a, A2b), and / or other hMPV F glycoproteins of different isolates (e.g., clinical isolates). An hMPV F glycoprotein may comprise one or more substitutions relative to an F glycoprotein of a particular hMPV subtype or hMPV subclass. An “hMPV subtype” refers to an hMPV that is recognized as belonging to one of the hMPV genetic groups: hMPV-A and hMPV- B. hMPV subtypes are determined based on the genetic lineage of the virus. An hMPV subtype is further divided into additional sublineages. An “hMPV sublineage”, as used herein, is a subgroup of an hMPV subtype which is based on variations in the G gene and / or clade. Classic subtypes include A1, A2a, A2b, B1, and B2; however, novel subtypes have been proposed, including A2b1 and A2b2 (Nao, et al. Microorganisms, 2020, 8(9):1280). Amino acid sequences of the hMPV F glycoproteins of a virus isolate may be determined by sequencing the isolate’s genome segments and / or viral mRNA from cells infected with a particular subclass or sublineage. An hMPV F glycoprotein may comprise one or more substitutions relative to an F glycoprotein of an hMPV isolate. An “hMPV isolate” or “isolate of hMPV” refers to an hMPV that has been obtained from an infected host and grown in cell culture. Amino acid sequences of the F glycoproteins of a virus isolate may be determined by sequencing the isolate’s genome segments and / or viral mRNA from cells infected with the isolate. An hMPV F glycoprotein may comprise one or more substitutions relative to a reference hMPV F glycoprotein sequence. In the context of proteins having one or more mutations (e.g., substitutions), a “reference protein” (e.g., reference hMPV F glycoprotein) refers to a protein into which a mutation is introduced. For example, an hMPV-B F glycoprotein having the amino acid sequence of SEQ ID NO: 4 comprises V84C, A140C, A147C, and A249C substitutions relative to the reference hMPV-B F glycoprotein sequence of SEQ ID NO: 15. Mutations in the instant specification are numbered according to full-length amino acid sequences of hMPV F glycoproteins (e.g.¸SEQ ID NO: 1-4 illustrate mutations relative to SEQ ID NOs: 14 or 15), each of which includes the signal peptide of the hMPV F glycoprotein (i.e., residue 1 of each sequence is the methionine (M) encoded by the start codon of an ORF encoding the hMPV F glycoprotein). The skilled artisan will appreciate that mutations may be applied to any hMPV F glycoprotein amino acid sequence that is extant at the time this specification is filed. The skilled artisan will also appreciate that the mutations may be applied hMPV F glycoprotein amino acid sequences that do not yet exist at the time of filing this specification (e.g., an hMPV F glycoprotein amino acid sequence arising from the continued evolution of hMPV). When a given hMPV isolate is identified as circulating or as a variant of interest, the skilled artisan could apply a mutation described below to the F glycoprotein protein of that hMPV isolate, to produce a stabilized form of that isolate’s F glycoprotein. For example, the skilled artisan will appreciate that mutations described in the context of hMPV-B F glycoprotein may be applied to extant or later-arising hMPV-B F glycoprotein, as F glycoproteins within a given antigenic subgroup are typically more similar to each other than to F glycoprotein proteins of other antigenic subgroups (e.g., hMPV-A). Some embodiments relate to stabilized prefusion hMPV F glycoproteins comprising one or more mutations selected from: (i) truncation of a cytoplasmic tail; (ii) substitution of one or more of a non-cysteine residue with a cysteine (C) residue (e.g., such that a disulfide bond forms and links a neighboring protomer); (iii) replacement of one or more cavity-lining residues with a cavity-filling amino acid substitution (e.g., larger hydrophobic residues); (iv) substitution of one or more exterior surface exposed residue with a hydrophilic residue; and / or (v) modification of a cleavage site. In some embodiments, a stabilized prefusion hMPV F glycoprotein comprises a truncated cytoplasmic tail. In some embodiments, a stabilized prefusion hMPV F glycoprotein lacks a cytoplasmic tail. In some embodiments, the cytoplasmic tail comprises the C-terminal 20-30, 20- 25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids of the of the hMPV F glycoprotein (that is, the C-terminal 20-30, 20-25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids of a wild-type hMPV F glycoprotein). In some embodiments, a stabilized prefusion hMPV F glycoprotein comprises one or more cysteine substitutions, such that a disulfide bond is formed between the introduced cysteines. In some embodiments, the disulfide bond is an interprotomer disulfide bond, which covalently links two protomers of a stabilized prefusion hMPV F glycoprotein containing multiple protomers (e.g., three protomers). In some embodiments, the disulfide bond is an intraprotomer disulfide bond, which covalently links two residues of a single protomer of the stabilized prefusion hMPV F glycoprotein. In some embodiments, the disulfide bond is in the head region of the stabilized prefusion hMPV F. In some embodiments, the disulfide bond is in the stalk region of the stabilized prefusion hMPV F. In some embodiments, a stabilized prefusion hMPV F glycoprotein comprises one or more substitutions of a polar or charged cavity-lining residue with a hydrophobic residue. In some embodiments, a stabilized prefusion hMPV F glycoprotein comprises one or more substitutions of a polar or charged cavity-lining residue with a glycine. Protein interiors may comprise tightly packed side chains which influence the stability of the protein, whereas larger cavities are less stable than narrow cavities (Bueno et al. J Mol Bio.2006.358(3):701–712). In addition to a cavity in each protomer of the hMPV F glycoprotein, the assembled trimer includes a large central cavity. By replacing small, hydrophilic cavity-lining residues ((e.g., arginine (R), asparagine (N), aspartic acid (D), cysteine (C), glutamine (Q), glutamic acid (E), glycine (G), histidine (H), lysine (K), serine (S), threonine(T), tyrosine (Y)) with larger, hydrophobic residues (e.g., alanine (A), isoleucine (I), leucine (L), methionine (M), phenylalanine (F), proline (P), tryptophan (W), valine (V)), the stability of proteins can be increased by such cavity-filling mutations. Any suitable hydrophobic residue may be used in such a substitution, such as alanine (A), isoleucine (I), leucine (L), methionine (M), phenylalanine (F), proline (P), tryptophan (W), valine (V). In some embodiments, a stabilized prefusion hMPV F glycoprotein comprises one or more substitutions of exterior surface exposed residue with a hydrophilic residue. Introduction of a hydrophilic residue to the exterior surfaces of some proteins may be destabilizing while, in others, they may increase thermal stability, by, for example, increasing local packing and hydration at a particular residue. By substituting a residue with a hydrophilic residue, the stability of a protein can be increased, such that a particular conformation (e.g., prefusion) of the protein is favored in the cellular environment. Any suitable hydrophilic residue may be used in such a substitution, such as lysine (K) or serine (S). In some embodiments, a stabilized prefusion hMPV F glycoprotein comprises a modification of an F1 / F2 cleavage site with a linker (e.g., a flexible linker). The F1 / F2 cleavage site is a protease cleavage site in the F0 precursor of the hMPV glycoprotein; in the hMPV F glycoprotein of SEQ ID NO: 14, the F1 / F2 cleavage site corresponds to residues 97-102 (NPRQSR (SEQ ID NO: 70)). Proteolytic cleavage at the F1 / F2 cleavage site enables the maturation of the F0 precursor into the F1 / F2 heterodimer, and allows for the conformational changes required to fuse the viral envelope and cell membrane. An F1 / F2 cleavage site may be modified (e.g., replaced) with a flexible linker, for example, a glycine (G) and serine (S) linker. In this way, without wishing to be bound by theory, it is thought that modification of the F1 / F2 cleavage site prevents release of the fusion peptide, resulting in a stabilized prefusion hMPV F glycoprotein. Exemplary linkers are described in the section entitled “Linkers and Cleavable Peptides” below. Stabilized Prefusion hMPV-A F Glycoproteins and Nucleic Acids Some embodiments relate to stabilized hMPV-A F glycoproteins. Stabilized prefusion forms of hMPV-A F glycoproteins may comprise one or more modifications (e.g., substitutions) relative to an amino acid sequence of an F glycoprotein of an hMPV-A isolate (e.g., relative to SEQ ID NO: 14). For example, a stabilized prefusion hMPV-A F glycoprotein may comprise one or more of the following modifications: (i) truncation of a cytoplasmic tail; (ii) substitution of one or more of a non-cysteine residue with a cysteine (C) residue (e.g., such that a disulfide bond forms and links neighboring protomer); (iii) replacement of one or more cavity-lining residues with a cavity-filling amino acid substitution (e.g., larger hydrophobic residues); (iv) substitution of one or more exterior surface exposed residue with a hydrophilic residue; and / or (v) modification of a cleavage site. Such modifications may stabilize the hMPV-A F glycoprotein in a prefusion conformation (e.g., may result in a stabilized prefusion hMPV-A F glycoprotein). Those of ordinary skill in the art will appreciate that mutations disclosed in relation to a reference sequence of an hMPV-A F glycoprotein (e.g., SEQ ID NO: 14), may be applied to F glycoproteins of other hMPV-A isolates. For example, in applying an A114T substitution to an F glycoprotein of a given isolate of hMPV-A, the skilled artisan would align the amino acid sequence of that isolate’s F glycoprotein to the hMPV-A F glycoprotein sequence of SEQ ID NO: 14 and introduce a T at the residue of the isolate’s F glycoprotein amino acid sequence that aligns to A114 of SEQ ID NO: 14. In some embodiments, the stabilized prefusion form of a hMPV-A F glycoprotein comprises a truncated cytoplasmic tail. In some embodiments, the stabilized prefusion form of a hMPV-A F glycoprotein lacks a cytoplasmic tail. In some embodiments, the cytoplasmic tail comprises the C-terminal 20-30, 20-25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids of a full length reference hMPV-A F glycoprotein (e.g., SEQ ID NO: 14). In some embodiments, the cytoplasmic tail comprises the C-terminal 26 amino acids (e.g., IKKTKKPTGAPPELSGVTNNGFIPHN (SEQ ID NO: 48)) of a full length reference hMPV-A F glycoprotein (e.g., SEQ ID NO: 14). In some embodiments, the cytoplasmic tail comprises the C-terminal 25 amino acids (e.g., KKTKKPTGAPPELSGVTNNGFIPHN (SEQ ID NO: 49)) of a full length reference hMPV-A F glycoprotein (e.g., SEQ ID NO: 14). In some embodiments, the cytoplasmic tail comprises the C-terminal 20 amino acids (e.g., PTGAPPELSGVTNNGFIPHN (SEQ ID NO: 50)) of a full length reference hMPV-A F glycoprotein (e.g., SEQ ID NO: 14). In some embodiments, the cytoplasmic tail comprises the C-terminal 15 amino acids (e.g., PELSGVTNNGFIPHN (SEQ ID NO: 51)) of a full length reference hMPV-A F glycoprotein (e.g., SEQ ID NO: 14). In some embodiments, the cytoplasmic tail comprises the C-terminal 10 amino acids (e.g., VTNNGFIPHN (SEQ ID NO: 52)) of a full length reference hMPV-A F glycoprotein (e.g., SEQ ID NO: 14). In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a substitution of a non-cysteine residue with a cysteine (C) residue. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 84, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 140, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 147, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 249, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 454, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 458, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 84, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine substitution at position 84, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an V84C substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 140, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine substitution at position 140, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an A140C substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 147, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine substitution at position 147, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an A147C substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 249, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine substitution at position 249, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an A249C substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 454, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine substitution at position 454, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an D454C substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine at position 458, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a cysteine substitution at position 458, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an V458C substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises cysteine residues at two, three, four, five, or all six of the following positions relative to SEQ ID NO: 14: 84, 140, 147, 249, 454, and 458. In some embodiments, the stabilized prefusion hRSV- B F glycoprotein comprises two of the following substitutions: V84C, A140C, A147C, A249C, D454C, and V458C relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises three of the following substitutions: V84C, A140C, A147C, A249C, D454C, and V458C relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises four of the following substitutions: V84C, A140C, A147C, A249C, D454C, and V458C relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises five of the following substitutions: V84C, A140C, A147C, A249C, D454C, and V458C relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises all six of the following substitutions: V84C, A140C, A147C, A249C, D454C, and V458C relative to SEQ ID NO: 14. In some embodiments, a stabilized prefusion hMPV-A F glycoprotein comprises a replacement of one or more cavity-lining residues with a cavity-filling amino acid substitution (e.g., larger hydrophobic residues) which may stabilize the protein. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a threonine at position 114, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a threonine substitution at position 114, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an A114T substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a valine at position 449, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a valine substitution at position 449, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an I449V substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a threonine at position 114 and a valine at position 449, where the positions are numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a threonine at position 114 and a valine substitution at position 449, where the positions are numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a threonine substitution at position 114 and a valine at position 449, where the positions are numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a threonine substitution at position 114 and a valine substitution at position 449, where the positions are numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an A114T substitution and an I449V substitution relative to SEQ ID NO: 14. In some embodiments, a stabilized prefusion hMPV-A F glycoprotein comprises a replacement of one or more surface exposed residues with a hydrophilic amino acid (e.g., hydrophilic residues) which may stabilize the protein. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a lysine at position 138, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a lysine substitution at position 138, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an N138K substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a serine at position 232, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a serine substitution at position 232, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an P232S substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a lysine at position 138 and a serine at position 232, where the positions are numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a lysine substitution at position 138 and a serine substitution at position 232, where the positions are numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an N138K substitution and a P232S substitution relative to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an alanine at position 61, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an alanine substitution at position 61, where the position is numbered by alignment of the amino acid sequence of the hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an S61A substitution relative to SEQ ID NO: 14. In some embodiments, a stabilized prefusion hMPV-A F glycoprotein comprises a F1 / F2 cleavage site modification. In some embodiments, the F1 / F2 cleavage site modification comprises replacement of the F1 / F2 cleavage site with a linker. In some embodiments, the linker is a flexible linker. A flexible linker may comprise a glycine (G) and serine (S) linker. In some embodiments, the F1 / F2 cleavage site modification comprises substitution of amino acids 89-112 with a flexible linker (e.g., GS linker). In some embodiments, the F1 / F2 cleavage site modification comprises a substitution of amino acids 89-112 with GSGGSG (SEQ ID NO: 141). In some embodiments, a stabilized prefusion hMPV-A F glycoprotein comprises a modification of one or more amino acids, wherein the modification is a substitution, deletion, or insertion at a given position. In some embodiments, a residue is deleted. In some embodiments, a residue is substituted (e.g., replaced) by another amino acid residue. Exemplary Stabilized Prefusion hMPV-A F Glycoproteins In some embodiments, a stabilized prefusion form of a hMPV-A F glycoprotein comprises a truncated cytoplasmic tail and a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GS linker (e.g., GSGGSG (SEQ ID NO: 141)). In some embodiments, the stabilized hMPV-A F glycoprotein further comprises one or more substitutions of a non-cysteine residue to a cysteine residue. In some embodiments, the stabilized hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, replacement of the F1 / F2 cleavage site with a GS linker, and one or more substitutions of a non-cysteine residue to a cysteine residue. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, a cysteine at position 84, a cysteine at position 140, a cysteine at position 147, and a cysteine at position 249, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, a cysteine substitution at position 84, a cysteine substitution at position 140, a cysteine substitution at position 147, and a cysteine substitution at position 249, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89- 112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, and the following substitutions: V84C, A140C, A147C, and A249C, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises the amino acid sequence set forth in SEQ ID NO: 1. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein consists of the sequence set forth in SEQ ID NO: 1. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 18. In some embodiments, the stabilized prefusion hMPV- A F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 18. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an mRNA comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 31. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an mRNA comprising the nucleic acid sequence set forth in SEQ ID NO: 31. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, an alanine at position 61, a cysteine at position 84, a threonine at position 114, a lysine at position 138, a cysteine at position 140, a cysteine at position 147, a serine at position 232, a cysteine at position 249, a valine at position 449, a cysteine at position 454, and a cystine at position 458, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89- 112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, an alanine substitution at position 61, a cysteine substitution at position 84, a threonine substitution at position 114, a lysine substitution at position 138, a cysteine substitution at position 140, a cysteine substitution at position 147, a serine substitution at position 232, a cysteine substitution at position 249, a valine substitution at position 449, a cysteine substitution at position 454, and a cystine substitution at position 458, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, and the following substitutions: S61A, V84C, A114T, N138K, A140C, A147C, P232S, A249C, I449V, D454C, and V458C, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises the sequence set forth in SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein consists of the sequence set forth in SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 19. In some embodiments, the stabilized prefusion hMPV- A F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 19. In some embodiments, an mRNA encodes the stabilized prefusion hMPV-A F glycoprotein, wherein the mRNA comprises a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 32. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an mRNA comprising the sequence set forth in SEQ ID NO: 32. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, an alanine at position 61, a cysteine at position 84, a threonine at position 114, a lysine at position 138, a cysteine at position 140, a cysteine at position 147, a serine at position 232, a cysteine at position 249, and a valine at position 449, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, an alanine substitution at position 61, a cysteine substitution at position 84, a threonine substitution at position 114, a lysine substitution at position 138, a cysteine substitution at position 140, a cysteine substitution at position 147, a serine substitution at position 232, and a cysteine substitution at position 249, and a valine substitution at position 449, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 14) with a GSGGSG (SEQ ID NO: 141) linker, and the following substitutions: S61A, V84C, A114T, N138K, A140C, A147C, P232S, A249C, and I449V, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-A F glycoprotein to SEQ ID NO: 14. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 3. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises the amino acid sequence set forth in SEQ ID NO: 3. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein consists of the amino acid sequence set forth in SEQ ID NO: 3. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 20. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 20. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an mRNA comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 33. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an mRNA comprising the sequence set forth in SEQ ID NO: 33. It should be understood that any one of the hMPV-A proteins (e.g., antigens) may or may not comprise a signal sequence. Exemplary amino acid sequences of the stabilized prefusion hMPV-A F glycoproteins and nucleic acid sequences of the mRNA encoding the same are provided in Table 14. Stabilized Prefusion hMPV-B F Glycoproteins and Nucleic Acids In some aspects, a stabilized prefusion form of an hMPV-B F glycoprotein is provided. Stabilized prefusion forms of hMPV-B F glycoproteins may comprise one or more modifications (e.g., substitutions) relative to an amino acid sequence of an F glycoprotein of an hMPV-B isolate (e.g., relative to SEQ ID NO: 15). For example, a stabilized prefusion hMPV-B F glycoprotein may comprise one or more of the following modifications: (i) truncation of a cytoplasmic tail; (ii) substitution of one or more of a non-cysteine residue with a cysteine (C) residue (e.g., such that a disulfide bond forms and links neighboring protomer); and / or (iii) modification of a cleavage site. Such modifications may stabilize the hMPV-B F glycoprotein in a prefusion conformation (e.g., may result in a stabilized prefusion hMPV-B F glycoprotein). Those of ordinary skill in the art will appreciate that mutations disclosed in relation to a reference sequence of an hMPV-B F glycoprotein (e.g., SEQ ID NO: 15), may be applied to F glycoproteins of other hMPV-B isolates. For example, in applying a V84C substitution to an F glycoprotein of a given isolate of hMPV-B, the skilled artisan would align the amino acid sequence of that isolate’s F glycoprotein to the hMPV-B F glycoprotein sequence of SEQ ID NO: 15, and introduce a C at the residue of the isolate’s F glycoprotein amino acid sequence that aligns with V84 of SEQ ID NO: 15. In some embodiments, the stabilized prefusion form of an hMPV-B F glycoprotein comprises a truncated cytoplasmic tail. In some embodiments, the stabilized prefusion form of a hMPV-B F glycoprotein lacks a cytoplasmic tail. In some embodiments, the cytoplasmic tail comprises the C-terminal 20-30, 20-25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids of the of the hMPV-B F glycoprotein (that is, the hMPV-B F glycoprotein lacks the C-terminal 20-30, 20-25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids of the of the hMPV-B F glycoprotein). In some embodiments, the cytoplasmic tail comprises the C-terminal 25 amino acids (e.g., IKKTRKPTGAPPELNGVTNGGFIPH (SEQ ID NO: 44)) of the hMPV-B F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 20 amino acids (e.g., KPTGAPPELNGVTNGGFIPH (SEQ ID NO: 45)) of the hMPV-B F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 15 amino acids (e.g., PPELNGVTNGGFIPH (SEQ ID NO: 46)) of the hMPV-B F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 10 amino acids (e.g., GVTNGGFIPH (SEQ ID NO: 47)) of the hMPV-B F glycoprotein. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises substitution of a non-cysteine residue with a cysteine (C) residue. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 84, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 140, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 147, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 249, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 84, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine substitution at position 84, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises an V84C substitution relative to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 140, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine substitution at position 140, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises an A140C substitution relative to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 147, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine substitution at position 147, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises an A147C substitution relative to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine at position 249, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a cysteine substitution at position 249, where the position is numbered by alignment of the amino acid sequence of the hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises an A249C substitution relative to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises cysteine residues at two, three, or all four of the following positions relative to SEQ ID NO: 15: 84, 140, 147, and 249. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises two of the following substitutions: V84C, A140C, A147C, and A249C. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises three of the following substitutions: V84C, A140C, A147C, and A249C. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises the following substitutions: V84C, A140C, A147C, and A249C. In some embodiments, a stabilized prefusion hMPV-B F glycoprotein comprises a F1 / F2 cleavage site modification. In some embodiments, the F1 / F2 cleavage site modification comprises replacement of the F1 / F2 cleavage site with a linker. In some embodiments, the linker is a flexible linker. A flexible linker, in some embodiments, may comprise a glycine (G) and serine (S) linker. In some embodiments, the F1 / F2 cleavage site modification comprises substitution of amino acids 89-112 with a flexible linker (e.g., GS linker). In some embodiments, the F1 / F2 cleavage site modification comprises a substitution of amino acids 89-112 (relative to SEQ ID NO: 15) with GSGGSG (SEQ ID NO: 141). Exemplary Stabilized prefusion hMPV-B F Glycoproteins In some embodiments, a stabilized prefusion form of a hMPV-B F glycoprotein comprises a truncated cytoplasmic tail and a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 15) with a GSGGSG (SEQ ID NO: 141) linker. In some embodiments, the stabilized hMPV-B F glycoprotein further comprises one or more substitutions of a non- cysteine residue to a cysteine residue. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 15) with a GSGGSG (SEQ ID NO: 141) linker, a cysteine at position 84, a cysteine at position 140, a cysteine at position 147, and a cysteine at position 249, where the positions are numbered by alignment of the amino acid sequence of the stabilized prefusion hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a truncated cytoplasmic tail, a replacement of the F1 / F2 cleavage site (amino acids 89-112 of SEQ ID NO: 15) with a GSGGSG (SEQ ID NO: 141) linker, and the following substitutions: V84C, A147C, and A249C, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hMPV-B F glycoprotein to SEQ ID NO: 15. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises the amino acid sequence set forth in SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein consists of the amino acid sequence set forth in SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an ORF comprising a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 21. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 21. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an mRNA comprising a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 34. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an mRNA comprising the sequence set forth in SEQ ID NO: 34. It should be understood that any one of the hMPV-B proteins (e.g., antigens) may or may not comprise a signal sequence. Exemplary amino acid sequences of the stabilized prefusion hMPV-B F glycoproteins and nucleic acid sequences of the mRNA encoding the same are provided in Table 14. Human Respiratory Syncytial Virus Human respiratory syncytial virus (hRSV; also known as human orthopneumovirus) is a negative-sense, single-stranded ribonucleic acid (RNA) virus of the Pneumoviridae family, present in at least two antigenic subgroups, known as Group A and Group B (e.g., hRSV-A, hRSV-B), a categorization primarily resulting from differences in surface glycoproteins. The envelope of hRSV contains three surface glycoproteins: fusion (F), attachment (G), and small hydrophobic (SH). The attachment G glycoprotein and F glycoprotein, which are required for host cell infection, are protective antigens and targets of neutralizing antibodies. The hRSV G glycoprotein is responsible for attaching to primary human airway epithelial cells, while the hRSV F glycoprotein is responsible for mediating fusion between the viral membrane and the host cell membrane. Though these glycoproteins share some structural similarities, the G glycoprotein and F glycoprotein vary between clinical isolates, with the majority of variation occurring in the G glycoprotein. hRSV F glycoproteins are initially synthesized as inactive precursor monomers (F0), which, after further processing, assemble into metastable prefusion trimers. Host cell protease activity at an F1 / F2 cleavage site allows the formation of biologically fusion-active F1 and F2 domains, which are covalently linked via disulfide bonds (see, e.g., Cseke G. et al. Journal of Virology 2007;81(2):698-707, incorporated herein by reference in its entirety). The fusion-active F glycoprotein presents in two conformations: prefusion and post-fusion. The prefusion F glycoprotein is typically observed on the viral surface, where it interacts with receptors and attachment factors on the surface of a host cell. Once bound to a host cell surface, the prefusion F glycoprotein enters the post-fusion conformation, such that the viral and host cell membrane fuse and the viral nucleocapsid is released into the cytoplasm of the host cell. F glycoproteins also mediate coalescence of neighboring cells, resulting in the formation of syncytial cells. Although targets for neutralizing monoclonal antibodies exist on the post-fusion conformation of the F glycoprotein, the neutralizing antibody response primarily targets the F glycoprotein prefusion conformation in people naturally infected with hRSV (Magro M et al., Proc Natl Acad Sci USA 2012; 109(8): 3089-94; Ngwuta JO et al., Sci Transl Med 2015; 7(309): 309ra162). Accordingly, an hRSV F glycoprotein stabilized (e.g., mutated to exist in a labile, high-energy state) in a prefusion conformation produces a greater neutralizing immune response in animal models than that observed with hRSV F glycoprotein stabilized in the post-fusion conformation (McLellan et al., Science, 342: 592-598, 2013). hRSV infection commonly causes bronchiolitis. Most hRSV-infected adults develop mild cold-like symptoms such as congestion, low-grade fever, and wheezing. Infants, small children, and older adults may suffer more severe symptoms such as bronchiolitis and pneumonia. The disease may be transmitted among humans via contact with respiratory secretions. Both hRSV-A and hRSV-B are in global circulation at a given time; though cross-immunity to one hRSV antigenic subgroup may be observed after immunization with an antigen of another hRSV subgroup, multivalent compositions may more comprehensively address the disease burden caused by hRSV subgroups, and variants thereof. Thus, some aspects relate to stabilized prefusion hRSV F glycoproteins that comprise mutations to prevent the transition of the protein into its post-fusion conformation. Stabilized prefusion hRSV F glycoproteins are discussed below in the section entitled “hRSV F Glycoproteins.” Those skilled in the art will appreciate that hRSV proteins discussed therein are useful in multiple types of compositions (e.g., vaccine compositions). In some embodiments, a composition comprises one or more hRSV proteins. In some embodiments, the composition comprises one or more nucleic acids (e.g., mRNAs) encoding one or more hRSV proteins. Thus, discussion of hRSV proteins can also be applied to nucleic acids encoding said hRSV proteins and vice versa unless otherwise clear from context. Thus, disclosure related to particular polypeptide mutations is also relevant to nucleic acids encoding those polypeptides with those mutations, unless otherwise clear from context. Likewise, disclosure related to mRNA encoding mutated hRSV proteins may also be relevant to the mutated hRSV proteins. Thus, when a composition comprising an mRNA encoding an hRSV protein having a particular mutation is disclosed, the skilled artisan can infer that the hRSV protein per se, and compositions comprising the hRSV protein, are also disclosed. In some embodiments, an hRSV protein is a recombinant protein. A "recombinant protein” refers to a protein that is produced in a heterologous organism that does not naturally produce the protein or a variant thereof. Non- limiting examples of organisms in which recombinant proteins may be produced include bacteria (e.g., Escherichia coli), yeast (e.g., Saccharomyces cerevisiae), and mammalian cells. hRSV F Glycoproteins It is thought that hRSV F glycoproteins having one or more mutations (e.g., substitutions, deletions, insertions) relative to a wild-type hRSV F glycoprotein sequence may exhibit increase stability and / or immunogenicity as compared to the wild-type hRSV F glycoprotein. Without being bound by theory, it is believed that such mutations in hRSV F glycoproteins can stabilize the conformation of the hRSV F glycoprotein. For example, stabilizing mutations, in some embodiments, prevent the transition of the prefusion conformation hRSV F glycoprotein into its post-fusion conformation; such hRSV antigens are hereinafter referred to as “stabilized prefusion” antigens (e.g., stabilized prefusion hRSV F glycoproteins). Mutations that may be applied to hRSV F glycoproteins are described below. For clarity, mutations are described using amino acid numbering corresponding to a specific hRSV F glycoprotein reference amino acid sequence (e.g.¸ SEQ ID NO: 16). A person of ordinary skill in the art will appreciate that the mutations disclosed in reference to a given hRSV F glycoprotein amino acid sequence (e.g., SEQ ID NO: 16) may be applied to other hRSV F glycoproteins of the same antigenic subgroup (e.g., hRSV-B), or other hRSV F glycoproteins of different isolates. An hRSV F glycoprotein may comprise one or more substitutions relative to an F glycoprotein of an hRSV isolate. An “hRSV isolate” or “isolate of hRSV” refers to an hRSV that has been obtained from an infected host and grown in cell culture. Amino acid sequences of the F glycoproteins of a virus isolate may be determined by sequencing the isolate’s genome segments and / or viral mRNA from cells infected with the isolate. An hRSV F glycoprotein may comprise one or more substitutions relative to a reference F glycoprotein sequence. In the context of proteins having one or more mutations (e.g., substitutions), a “reference protein” (e.g., reference F glycoprotein) refers to a protein into which a mutation is introduced. For example, an hRSV F glycoprotein having the amino acid sequence of SEQ ID NO: 13 comprises A149C, S155C, S290C, and Y458C, and T529A substitutions relative to the reference hRSV F glycoprotein sequence of SEQ ID NO: 16. Mutations in the instant specification are numbered according to full-length amino acid sequences of hRSV F glycoproteins (e.g.¸SEQ ID NO: 16 (hRSV-B), SEQ ID NO: 17 (hRSV- A)), each of which includes the signal peptide of the hRSV F glycoprotein (i.e., residue 1 of each sequence is the methionine (M) encoded by the start codon of an ORF encoding the hRSV F glycoprotein). The skilled artisan will appreciate that mutations may be applied to any hRSV F glycoprotein amino acid sequence that is extant at the time this specification is filed. The skilled artisan will also appreciate that the mutations may be applied hRSV F glycoprotein amino acid sequences that do not yet exist at the time of filing this specification (e.g., an hRSV F glycoprotein amino acid sequence arising from the continued evolution of hRSV). When a given hRSV isolate is identified as circulating or as a variant of interest, the skilled artisan could apply a mutation described below to the F glycoprotein protein of that hRSV isolate, to produce a stabilized form of that isolate’s F glycoprotein. For example, the skilled artisan will appreciate that mutations described in the context of hRSV-B F glycoprotein may be applied to extant or later-arising hRSV-B F glycoprotein, as F glycoproteins within a given antigenic subgroup are more similar to each other than to F glycoprotein proteins of other antigenic subgroups (e.g., hRSV-A). Some embodiments relate to stabilized prefusion hRSV F glycoproteins comprising one or more mutations selected from: (i) truncation of a cytoplasmic tail; (ii) substitution of one or more non-cysteine residues with a cysteine (C) residue (e.g., such that a disulfide bond forms and links a neighboring protomer); (iii) substitution of one or more non-proline residues with a proline (P) residue; and / or (iv) modification of one or more domains with a flexible linker (e.g., GS linker). In some embodiments, a stabilized prefusion hRSV F glycoprotein comprises a truncated cytoplasmic tail. In some embodiments, a stabilized prefusion hRSV F glycoprotein lacks a cytoplasmic tail. In some embodiments, the cytoplasmic tail comprises the C-terminal 20-30, 20- 25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids of the of the hRSV F glycoprotein variant (e.g., the C-terminal 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 amino acids of the hRSV F glycoprotein variant). In some embodiments, a stabilized prefusion hRSV F glycoprotein comprises one or more cysteine substitutions, such that a disulfide bond is formed between the introduced cysteine residues. In some embodiments, the disulfide bond is an interprotomer disulfide bond, which covalently links two protomers of a stabilized prefusion hRSV F glycoprotein containing multiple protomers (e.g., three protomers). In some embodiments, the disulfide bond is an intraprotomer disulfide bond, which covalently links two residues of a single protomer of the stabilized prefusion hRSV F glycoprotein. In some embodiments, a stabilized prefusion hRSV F glycoprotein comprises one or more proline substitutions. hRSV F glycoproteins comprise multiple subdomains which enables the fusion of the viral envelope and endosomal membrane. Proline is among residues considered helix breakers, such that the introduction of proline residues can stabilize or destabilize proteins by altering their conformation (Lyu et al.1990. Science.250(4981), 669-673). In this way, without wishing to be bound by theory, it is thought that proline substitutions further stabilize subdomains of the hRSV F glycoprotein, resulting in a stabilized prefusion hRSV F glycoprotein. In some embodiments, a stabilized prefusion hRSV F glycoprotein comprises a modification of the p27 domain with a linker (e.g., a flexible linker). The p27 domain comprises an internal peptide which is typically released when hRSV F is cleaved into the F2 and F1 subunits (Rezende et al., Front Microbiol.2023; 14:1219846). A p27 domain may be modified (e.g., replaced) with a flexible linker, for example, a glycine (G) and serine (S) linker.In this way, without wishing to be bound by theory, it is thought that modification of the p27 domain prevent release of the fusion peptide, resulting in a stabilized prefusion hRSV F glycoprotein. Exemplary linkers are described in the section entitled “Linkers and Cleavable Peptides” below. In some embodiments, the stabilized prefusion hRSV F glycoprotein further comprises a foldon domain, such as a T4 foldon domain. Foldon domains are amino acid sequences that are able to form trimers. In some embodiments, the stabilized prefusion hRSV F glycoprotein further comprises a T4 foldon domain (i.e., GYIPEAPRDGQAYVRKDGEWVLLSTFL (SEQ ID NO: 148)). In some embodiments, the stabilized prefusion hRSV F glycoprotein does not comprise a foldon domain. Stabilized Prefusion hRSV-B F Glycoproteins Some embodiments relate to stabilized hRSV-B F glycoproteins. Stabilized prefusion forms of hRSV-B F glycoproteins may comprise one or more modifications (e.g., substitutions) relative to an amino acid sequence of an F glycoprotein of an hRSV-B isolate (e.g., relative to SEQ ID NO: 16). For example, a stabilized prefusion hRSV-B F glycoprotein may comprise one or more of the following modifications: : (i) truncation of a cytoplasmic tail; (ii) substitution of one or more non-cysteine residues with a cysteine residue (e.g., such that a disulfide bond forms and links neighboring protomer); (iii) substitution of one or more non-proline residues with a proline residue; and / or (iv) modification of one or more domains with a flexible linker (e.g., GS linker). Such modifications may stabilize the hRSV-B F glycoprotein in a prefusion conformation (e.g., may result in a stabilized prefusion hRSV-B F glycoprotein). Those of ordinary skill in the art will appreciate that mutations disclosed in relation to a reference sequence of an F glycoprotein, (e.g., SEQ ID NO: 16), may be applied to F glycoproteins of other hRSV isolates. For example, in applying an A149C substitution to an F glycoprotein of another isolate of hRSV-B, the skilled artisan would align the amino acid sequence of that isolate’s F glycoprotein to the hRSV-B F glycoprotein sequence of SEQ ID NO: 16, and introduce a C at the residue of the isolate’s F glycoprotein amino acid sequence that aligns to A149 of SEQ ID NO: 16. In some embodiments, the stabilized prefusion form of a hRSV-B F glycoprotein comprises a truncated cytoplasmic tail. In some embodiments, the stabilized prefusion form of a hRSV-B F glycoprotein lacks a cytoplasmic tail. In some embodiments, the cytoplasmic tail comprises the C-terminal 20-30, 20-25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids of a full length reference hRSV-B F glycoprotein (e.g., SEQ ID NO: 16). In some embodiments, the cytoplasmic tail comprises the C-terminal 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or more amino acids of a full length reference hRSV-B F glycoprotein (e.g., SEQ ID NO: 16). In some embodiments, the cytoplasmic tail comprises the C-terminal 25 amino acids (e.g., CKAKNTPVTLSKDQLSGINNIAFSK (SEQ ID NO: 53)) of a full length hRSV-B F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 20 amino acids (e.g., TPVTLSKDQLSGINNIAFSK (SEQ ID NO: 54)) of a full length hRSV-B F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 15 amino acids (e.g., SKDQLSGINNIAFSK (SEQ ID NO: 55)) of a full length hRSV-B F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 10 amino acids (e.g., SGINNIAFSK (SEQ ID NO: 56)) of a full length hRSV-B F glycoprotein. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises substitution of a non-cysteine residue with a cysteine (C) residue. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine at position 149, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine substitution at position 149, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a A149C substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine at position 155, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine substitution at position 155, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an S155C substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine at position 290, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine substitution at position 290, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an S290C substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine at position 458, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a cysteine substitution at position 458, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a Y458C substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises cysteine residues at two, three, or all four of the following positions relative to SEQ ID NO: 16: 149, 155, 290, and 458. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises two of the following substitutions: A149C, S155C, S290C, and Y458C. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises three of the following substitutions: A149C, S155C, S290C, and Y458C. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises all four of the following substitutions: A149C, S155C, S290C, and Y458C. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine at position 190, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine substitution at position 190, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an S190N substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine at position 207, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine substitution at position 190, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a V207L substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine at position 190 and a leucine at position 207, where the positions are numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine substitution at position 190 and a leucine substitution at position 207, where the positions are numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an S190N substitution and a V207L substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an isoleucine at position 17, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an isoleucine substitution at position 17, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a V17I substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a lysine at position 42, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a lysine substitution at position 42, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a R42K substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine at position 45, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine substitution at position 45, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a F45L substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine at position 99, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine substitution at position 99, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a N99S substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a threonine at position 103, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a threonine substitution at position 103, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a A103T substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a valine at position 103, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a valine substitution at position 103, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a A103V substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a glutamine at position 172, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a glutamine substitution at position 172, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a L172Q substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine at position 173, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine substitution at position 173, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a S173L substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine at position 173, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a leucine substitution at position 173, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a S173L substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an arginine at position 191, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an arginine substitution at position 191, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a K191R substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an arginine at position 202, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an arginine substitution at position 202, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a Q202R substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a methionine at position 206, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a methionine substitution at position 206, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a I206M substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an arginine at position 209, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an arginine substitution at position 209, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a Q209R substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine at position 211, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an asparagine substitution at position 211, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a S211N substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a methionine at position 226, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a methionine substitution at position 226, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a K226M substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a threonine at position 234, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a threonine substitution at position 234, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a N234T substitution relative to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an alanine at position 529, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises an alanine substitution at position 529, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a T529A substitution relative to SEQ ID NO: 16. In some embodiments, a stabilized prefusion hRSV-B F glycoprotein comprises a substitution of a non-proline residue with a proline residue In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a proline at position 389, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a proline substitution at position 389, where the position is numbered by alignment of the amino acid sequence of the hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a S389P substitution relative to SEQ ID NO: 16. In some embodiments, a stabilized prefusion hRSV-B F glycoprotein comprises a p27 domain modification. In some embodiments, the p27 domain modification comprises replacement of the p27 domain with a linker. In some embodiments, the p27 domain modification comprises replacement of the p27 domain with a flexible linker. A flexible linker may comprise a glycine (G) and serine (S) linker. In some embodiments, the p27 domain modification comprises substitution of amino acids 104-144 with a flexible linker (e.g., GS linker). In some embodiments, the p27 domain modification comprises a substitution of amino acids 104-144 with GS. In some embodiments, a stabilized prefusion hRSV-B F glycoprotein comprises a modification of one or more amino acids, wherein the modification is a substitution, deletion, or insertion at a given position. In some embodiments, a residue is deleted. In some embodiments, a residue is substituted (e.g., replaced) by another amino acid residue. Exemplary Stabilized Prefusion hRSV-B F Glycoproteins In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail and replacement of the p27 domain (amino acids 104-144 of SEQ ID NO: 16) with a GS linker. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein further comprises one or more substitutions of a non-cysteine residue to a cysteine residue. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain with a GS linker, one or more substitutions of a non-cysteine residue to a cysteine residue, and one or more cavity-filling amino acid substitutions. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein further comprises one or more additional amino acid residue substitutions. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine at position 17, a leucine at position 45, a valine at position 103, a cysteine at position 149, a cysteine at position 155, an asparagine at position 190, an arginine at position 191, a methionine at position 206, an asparagine at position 211, a threonine at position 234, a cysteine at position 290, a cysteine at position 458, and an alanine at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine substitution at position 17, a leucine substitution at position 45, a valine substitution at position 103, a cysteine substitution at position 149, a cysteine substitution at position 155, an substitution asparagine at position 190, an arginine substitution at position 191, a methionine substitution at position 206, an asparagine substitution at position 211, a threonine substitution at position 234, a cysteine substitution at position 290, a cysteine substitution at position 458, and an alanine substitution at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: V17I, F45L, A103V, A149C, S155C, S190N, K191R, I206M, S211N, N234T, S290C, Y458C, and T529A, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 5. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 5. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 5. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 22. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 22. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 35. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 35. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine at position 17, a leucine at position 45, a valine at position 103, a cysteine at position 149, a cysteine at position 155, a phenylalanine at position 190, an arginine at position 191, a methionine at position 206, an asparagine at position 211, a threonine at position 234, a cysteine at position 290, a cysteine at position 458, and an alanine at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine substitution at position 17, a leucine substitution at position 45, a valine substitution at position 103, a cysteine substitution at position 149, a cysteine substitution at position 155, a phenylalanine substitution at position 190, an arginine substitution at position 191, a methionine substitution at position 206, an asparagine substitution at position 211, a threonine substitution at position 234, a cysteine substitution at position 290, a cysteine substitution at position 458, and an alanine substitution at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: V17I, F45L, A103V, A149C, S155C, S190F, K191R, I206M, S211N, N234T, S290C, Y458C, and T529A, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 6. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 6. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 6. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 23. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 23. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 36. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 36. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine at position 17, a leucine at position 45, a valine at position 103, a glutamine at position 172, a leucine at position 173, an asparagine at position 190, an arginine at position 191, a methionine at position 206, an arginine at position 209, an asparagine at position 211, a threonine at position 234, a proline at position 389, and an alanine at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine substitution at position 17, a leucine substitution at position 45, a valine substitution at position 103, a glutamine substitution at position 172, a leucine substitution at position 173, an asparagine substitution at position 190, an arginine substitution at position 191, a methionine substitution at position 206, an arginine substitution at position 209, an asparagine substitution at position 211, a threonine substitution at position 234, a proline substitution at position 389, and an alanine substitution at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: V17I, F45L, A103V, L172Q, S173L, S190N, K191R, I206M, Q209R, S211N, N234T, S389P, and T529A, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 7. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 7. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 7. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 24. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 24. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 37. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 37. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine at position 17, a lysine at position 42, a leucine at position 45, a valine at position 103, a cysteine at position 149, a cysteine at position 155, an asparagine at position 190, an arginine at position 191, a methionine at position 206, an asparagine at position 211, a threonine at position 234, a cysteine at position 290, a proline at position 389, a cysteine at position 458, and an alanine at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine substitution at position 17, a lysine substitution at position 42, a leucine substitution at position 45, a valine substitution at position 103, a cysteine substitution at position 149, a cysteine substitution at position 155, an asparagine substitution at position 190, an arginine substitution at position 191, a methionine substitution at position 206, an asparagine substitution at position 211, a threonine substitution at position 234, a cysteine substitution at position 290, a proline substitution at position 389, a cysteine substitution at position 458, and an alanine substitution at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: V17I, R42K, F45L, A103V, A149C, S155C, S190N, K191R, I206M, S211N, N234T, S290C, S389P, Y458C, and T529A, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 8. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 8. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 8. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 25. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 25. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 38. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 38. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine at position 17, a lysine at position 42, a leucine at position 45, a valine at position 103, a glutamine at position 172, a leucine at position 173, an arginine at position 191, a methionine at position 206, an arginine at position 209, an asparagine at position 211, a threonine at position 234, a proline at position 389, and an alanine at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine substitution at position 17, a lysine at position 42, a leucine substitution at position 45, a valine at position 103, a glutamine substitution at position 172, a leucine substitution at position 173, an arginine substitution at position 191, a methionine substitution at position 206, an arginine substitution at position 209, an asparagine substitution at position 211, a threonine substitution at position 234, a proline substitution at position 389, and an alanine substitution at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: V17I, R42K, F45L, A103V, L172Q, S173L, K191R, I206M, Q209R, S211N, N234T, S389P, and T529A, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 9. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 9. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 9. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 26. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 26. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 39. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 39. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine at position 17, a lysine at position 42, a leucine at position 45, a valine at position 103, a cysteine at position 149, a cysteine at position 155, a phenylalanine at position 190, an arginine at position 191, a methionine at position 206, an asparagine at position 211, a threonine at position 234, a cysteine at position 290, a proline at position 389, a cysteine at position 458, and an alanine at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, an isoleucine substitution at position 17, a lysine substitution at position 42, a leucine substitution at position 45, a valine substitution at position 103, a cysteine substitution at position 149, a cysteine substitution at position 155, a phenylalanine substitution at position 190, an arginine substitution at position 191, a methionine substitution at position 206, an asparagine substitution at position 211, a threonine substitution at position 234, a cysteine substitution at position 290, a proline substitution at position 389, a cysteine substitution at position 458, and an alanine substitution at position 529, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16 In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: V17I, R42K, F45L, A103V, A149C, S155C, S190F, K191R, I206M, S211N, N234T, S290C, S389P, Y458C, and T529A, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 10. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 6. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 10. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 27. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 27. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 40. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 40. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, a cysteine at position 155, a phenylalanine at position 190, an arginine at position 191, an arginine at position 202, a leucine at position 207, a methionine at position 226, a threonine at position 234, and a cysteine at position 290, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, a cysteine substitution at position 155, a phenylalanine substitution at position 190, an arginine substitution at position 191, an arginine substitution at position 202, a leucine substitution at position 207, a methionine substitution at position 226, a threonine substitution at position 234, and a cysteine substitution at position 290, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: S155C, S190F, K191R, Q202R, V207L, K226M, N234T, and S290C, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 11. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 11. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 11. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 28. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 28. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 41. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 41. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, a serine at position 99, a threonine at position 103, a cysteine at position 149, a cysteine at position 155, a phenylalanine at position 190, an arginine at position 191, an arginine at position 202, a leucine in position 207, a methionine at position 226, a threonine at position 234, a cysteine at position 290, and a cysteine at position 458, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, a serine substitution at position 99, a threonine substitution at position 103, a cysteine substitution at position 149, a cysteine substitution at position 155, a phenylalanine substitution at position 190, an arginine substitution at position 191, an arginine substitution at position 202, a leucine substitution in position 207, a methionine substitution at position 226, a threonine substitution at position 234, a cysteine substitution at position 290, and a cysteine substitution at position 458, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: N99S, A103T, A149C, S155C, S190F, K191R, Q202R, V207L, K226M, N234T, S290C, and Y458C, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-B F glycoprotein to SEQ ID NO: 16. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 8. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein comprises the sequence set forth in SEQ ID NO: 12. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein consists of the sequence set forth in SEQ ID NO: 12. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 29. In some embodiments, the stabilized prefusion hRSV-B F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 29. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 42. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-B F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 42. It should be understood that any one of the hRSV-B proteins (e.g., antigens) may or may not comprise a signal sequence. Exemplary amino acid sequences of the stabilized prefusion hRSV-B F glycoproteins and nucleic acid sequences of the mRNA encoding the same are provided in Table 15. Stabilized Prefusion hRSV-A F Glycoproteins In some aspects, a stabilized prefusion form of an hRSV-A F glycoprotein is provided. Examples of stabilized prefusion forms of hRSV-A F glycoproteins are found in WO 2021 / 155243, the content of which is incorporated by reference herein to the extent it relates to RSV-A F glycoproteins and mRNA encoding the same. Some embodiments relate to stabilized hRSV-A F glycoproteins. Stabilized prefusion forms of hRSV-A F glycoproteins may comprise one or more modifications (e.g., substitutions) relative to an amino acid sequence of an F glycoprotein of an hRSV-A isolate (e.g., relative to SEQ ID NO: 17). For example, a stabilized prefusion hRSV-A F glycoprotein may comprise one or more of the following modifications: : (i) truncation of a cytoplasmic tail; (ii) substitution of one or more non-cysteine residues with a cysteine residue (e.g., such that a disulfide bond forms and links neighboring protomer); and / or (iii) substitution of one amino acid residue for another. Such modifications may stabilize the hRSV-A F glycoprotein in a prefusion conformation (e.g., may result in a stabilized prefusion hRSV-A F glycoprotein). Those of ordinary skill in the art will appreciate that mutations disclosed in relation to a reference sequence of an hRSV-A F glycoprotein, (e.g., SEQ ID NO: 17), may be applied to F glycoproteins of other hRSV-A isolates. For example, in applying an A149C substitution to an F glycoprotein of another isolate of hRSV-A, the skilled artisan would align the amino acid sequence of that isolate’s F glycoprotein to the hRSV-A F glycoprotein sequence of SEQ ID NO: 17, and introduce a C at the residue of the isolate’s F glycoprotein amino acid sequence that aligns to A149 of SEQ ID NO: 17. In some embodiments, the stabilized prefusion form of a hRSV-A F glycoprotein comprises a truncated cytoplasmic tail. In some embodiments, the stabilized prefusion form of a hRSV-A F glycoprotein lacks a cytoplasmic tail. In some embodiments, the cytoplasmic tail comprises the C-terminal 20-30, 20-25, 15-30, 15-25, 15-20, 10-30, 10-25, 10-20, 10-15, 5-30, 5-25, 5-20, or 5-15 amino acids (e.g., CKARSTPVTLSKDQLSGINNIAFSN (SEQ ID NO: 57)) of the stabilized prefusion hRSV-A F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 20 amino acids (e.g., TPVTLSKDQLSGINNIAFSN (SEQ ID NO: 58)) of the hRSV-A F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C- terminal 15 amino acids (e.g., SKDQLSGINNIAFSN (SEQ ID NO: 59)) of the hRSV-A F glycoprotein. In some embodiments, the cytoplasmic tail comprises the C-terminal 10 amino acids (e.g., SGINNIAFSN (SEQ ID NO: 60)) of the hRSV-A F glycoprotein. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises substitution of a non-cysteine residue with a cysteine (C) residue. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine at position 149, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine substitution at position 149, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a A149C substitution relative to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises substitution of a non-cysteine residue with a cysteine (C) residue. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine at position 155, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine substitution at position 155, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a S155C substitution relative to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises substitution of a non-cysteine residue with a cysteine (C) residue. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine at position 290, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine substitution at position 290, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a S290C substitution relative to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises substitution of a non-cysteine residue with a cysteine (C) residue. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine at position 458, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a cysteine substitution at position 458, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a Y458C substitution relative to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises cysteine residues at two, three, or all four of the following positions relative to SEQ ID NO: 17: 149, 155, 290, and 458. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises two of the following substitutions: A149C, S155C, S290C, and Y458C. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises three of the following substitutions: A149C, S155C, S290C, and Y458C. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises all four of the following substitutions: A149C, S155C, S290C, and Y458C. In some embodiments, a stabilized prefusion hRSV-A F glycoprotein comprises a p27 domain modification. In some embodiments, the p27 domain modification comprises replacement of the p27 domain with a linker. In some embodiments, the p27 domain modification comprises replacement of the p27 domain with a flexible linker. A flexible linker may comprise a glycine (G) and serine (S) linker. In some embodiments, the p27 domain modification comprises substitution of amino acids 104-144 with a flexible linker (e.g., GS linker). In some embodiments, the p27 domain modification comprises a substitution of amino acids 104-144 with GS. In some embodiments, a stabilized prefusion hRSV-A F glycoprotein comprises a modification of one or more amino acids, wherein the modification is a substitution, deletion, or insertion at a given position. In some embodiments, a residue is deleted. In some embodiments, a residue is substituted (e.g., replaced) by another amino acid residue. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a phenylalanine at position 190, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a phenylalanine substitution at position 190, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a S190F substitution relative to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a leucine at position 207, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a leucine substitution at position 207, where the position is numbered by alignment of the amino acid sequence of the hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a V207L substitution relative to SEQ ID NO: 17. Exemplary Stabilized Prefusion hRSV-A F Glycoproteins In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 103-145) with a GS linker, a cysteine at position 149, a cysteine at position 155, a phenylalanine at position 190, a leucine at position 207, a cysteine at position 290, and a cysteine at position 458, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a truncated cytoplasmic tail, a cysteine substitution at position 149, a cysteine substitution at position 155, a phenylalanine substitution at position 190, a leucine substitution at position 207, a cysteine substitution at position 290, and a cysteine substitution at position 458, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a truncated cytoplasmic tail, replacement of the p27 domain (amino acids 104-144) with a GS linker, and the following substitutions: A149C, S155C, S190F, V207L, S290C, and Y458C, where the positions are numbered by alignment of the amino acid sequences of the stabilized prefusion hRSV-A F glycoprotein to SEQ ID NO: 17. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 13. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein comprises the sequence set forth in SEQ ID NO: 13. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein consists of the sequence set forth in SEQ ID NO: 13. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 30. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 30. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 43. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 43. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 113. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 97% identity to the sequence of SEQ ID NO: 113. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 98% identity to the sequence of SEQ ID NO: 113. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 99% identity to the sequence of SEQ ID NO: 113. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 113. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 97% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 98% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 99% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 112. It should be understood that any one of the hRSV-A antigens encoded by the mRNA described herein may or may not comprise a signal sequence. Exemplary amino acid sequences of the hRSV-A antigens and nucleic acid sequences of the mRNA encoding the hRSV-A antigens are provided in Table 15. Glycoprotein Variants (Percent Identity) Some embodiments relate to proteins (e.g., hMPV and / or RSV glycoproteins) having one or more mutations (e.g., substitutions) relative to, and / or numbered according to, a reference amino acid sequence. Some embodiments relate to amino acid or nucleotide sequences having a specified percentage sequence identity to a reference amino acid or nucleotide sequence, respectively. The term “identity” refers to a relationship between the sequences of two or more polypeptides (e.g. antigens) or polynucleotides (nucleic acids), as determined by comparing the sequences. Identity also refers to the degree of sequence relatedness between or among sequences as determined by the number of matches between strings of two or more amino acid residues or nucleic acid residues. “Percent (%) identity” or “percent (%) sequence identity” as it applies to polypeptide or polynucleotide sequences is defined as the percentage of residues (amino acid residues or nucleic acid residues) in the candidate amino acid or nucleic acid sequence that are identical with the residues in the amino acid sequence or nucleic acid sequence of a second sequence after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent identity. The percent sequence identity that a candidate sequence (e.g., as recited in a claim) has to a reference sequence (e.g., having a SEQ ID NO: specified herein) is calculated by (i) aligning the candidate sequence to the reference sequence, (ii) determining the number of matching residues (amino acids or nucleotides) between the aligned candidate and reference sequences, and (iii) dividing the number of matching residues by the length of the reference sequence, including any gaps introduced by the alignment. The skilled artisan will appreciate that to determine whether a candidate protein or nucleic acid comprises an amino acid sequence or nucleotide sequence with a given percentage sequence identity to a reference sequence, the denominator (length of reference sequence plus internal gaps) in calculating sequence identity need not include gaps shown at the ends of the reference sequence in an alignment, as such gaps are added where a candidate sequence contains additional amino acids or nucleotides that extend beyond the portions that align to the N- terminus and / or C-terminus of the reference sequence. Where an alignment between two sequences is contemplated, the first sequence (e.g., candidate sequence) is aligned to the second sequence (e.g., reference sequence) using the Needleman-Wunsch algorithm for global alignment of the two sequences. Needleman & Wunsch, J Mol Biol.1970.48:443–453. Where two protein sequences are aligned, the Needleman-Wunsch algorithm uses a BLOSUM62 substitution scoring matrix, a Gap Open penalty of 10, a Gap Extend penalty of 0.5, and no End Gap penalties. Where two nucleotide sequences are aligned, the alignment uses an DNAFULL substitution scoring matrix, a Gap Open penalty of 10, a Gap Extend penalty of 0.5, and no End Gap penalties. The skilled artisan will appreciate that at the time of filing the instant specification, these parameters are the default parameters of the EMBOSS Needle pairwise comparison tool provided by European Bioinformatics Institute (see ebi.ac.uk). Linkers and Cleavable Peptides Some embodiments of proteins include a linker between at least one pair or portions of the protein. The linker may be, for example, a cleavable linker or protease-sensitive linker. In some embodiments, the linker is selected from the group consisting of F2A linker, P2A linker, T2A linker, E2A linker, and combinations thereof (see, e.g., WO 2017 / 127750). This family of self-cleaving peptide linkers, referred to as 2A peptides, has been described in the art (see, e.g., Kim, J.H. et al., PLoS ONE 2011;6:e18556). In some embodiments, the linker is an F2A linker. In some embodiments, the linker is a GS linker. GS linkers are polypeptide linkers that include glycine and serine amino acids repeats. They comprise flexible and hydrophilic residues and can be used to perform fusion of protein subunits without interfering in the folding and function of the protein domains, and without formation of secondary structures. In some embodiments, a protein comprises a GS linker that is 3 to 20 amino acids long. For example, the GS linker may have a length of (or have a length of at least) 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 amino acids. In some embodiments, a GS linker is (or is at least) 15 amino acids long (e.g., GGSGGSGGSGGSGGG (SEQ ID NO: 61)). In some embodiments, a GS linker is (or is at least) 8 amino acids long (e.g., GGGSGGGS (SEQ ID NO: 142)). In some embodiments, a GS linker is (or is at least) 7 amino acids long (e.g., GGGSGGG (SEQ ID NO: 143)). In some embodiments, a GS linker comprises the amino acid sequence GGGSGG (SEQ ID NO: 144). In some embodiments, a GS linker comprises the amino acid sequence GSGGSG (SEQ ID NO: 141). In some embodiments, a GS linker comprises the amino acid sequence GS. In some embodiments, a GS linker is (or is at least) 4 amino acid long (e.g., GGGS (SEQ ID NO: 145)). In some embodiments, the GS linker comprises (GGGS (SEQ ID NO: 145))n, where n is any integer from 1-5. In some embodiments, a GS linker is (or is at least) 4 amino acid long (e.g., GSGG (SEQ ID NO: 146)). In some embodiments, the GS linker comprises (GSGG (SEQ ID NO: 146))n, where n is any integer from 1-5. In some embodiments, a linker is a glycine linker, for example having a length of (or a length of at least) 3 amino acids (e.g., GGG). In some embodiments, a protein encoded by an RNA (e.g., mRNA) includes two or more linkers, which may be the same or different from each other. The skilled artisan will appreciate that other linkers may be suitable for use in proteins. Signal Peptides In some embodiments, a protein comprises a signal peptide. Signal peptides comprise the N-terminal 15-60 amino acids of proteins. In eukaryotes, the signal peptide of a nascent precursor protein (pre-protein) directs the ribosome to the rough endoplasmic reticulum (ER) membrane and initiates the transport of the growing peptide chain across it for processing. ER processing produces mature proteins, wherein the signal peptide is cleaved from precursor proteins, typically by an ER-resident signal peptidase of the host cell, or they remain uncleaved and function as a membrane anchor. A signal peptide may also facilitate the targeting of the protein to the cell membrane. A signal peptide may have a length of 15-60 amino acids. For example, a signal peptide may have a length of 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, or 60 amino acids. In some embodiments, a signal peptide has a length of 20-60, 25-60, 30-60, 35- 60, 40-60, 45- 60, 50-60, 55-60, 15-55, 20-55, 25-55, 30-55, 35-55, 40-55, 45-55, 50-55, 15-50, 20-50, 25-50, 30-50, 35-50, 40-50, 45-50, 15-45, 20-45, 25-45, 30-45, 35-45, 40-45, 15-40, 20- 40, 25-40, 30-40, 35-40, 15-35, 20-35, 25-35, 30-35, 15-30, 20-30, 25-30, 15-25, 20-25, or 15-20 amino acids. Signal peptides from heterologous genes (e.g., other than hMPV or RSV glycoproteins) may also be used in a protein. The native signal peptide of a protein may be determined using any suitable method, such as a signal peptide prediction tool. Signal peptide prediction tools use bioinformatic algorithms, such as neural network, machine learning, and / or language model-based approaches, in combination with annotated protein databases (e.g., UniProt) to predict the signal peptide sequence within a given amino acid sequence. See, e.g., Teufel et al., Nat Biotechnol.2022. 40(7):1023–1025 (SignalP 6.0); Armenteros et al., Nat Biotechnol.2019.37(4):420–423 (SignalP 5.0). As another example, a crystal structure of a protein in its post-signal peptide cleavage form may be analyzed to determine the N-terminal amino acids that are absent from the crystal structure, with the signal peptide being inferred as the N-terminal amino acids that are absent in the crystal structure. Seasonal Influenza Virus Compositions (e.g., mRNA vaccines) and vaccination methods may also elicit potent neutralizing antibodies against at least one influenza virus. In some embodiments, the compositions (e.g., mRNA vaccines) further comprise at least one mRNA comprising an ORF encoding at least one influenza virus antigen. Influenza viruses belong to the Orthomyxoviridae family and are categorized into types A, B, C, and D. Among these, influenza A and B viruses are of significant concern for human health. Influenza A viruses (IAVs) can be further classified based on two surface proteins, hemagglutinin (sometimes referred to as H or HA) and neuraminidase (sometimes referred to as N or NA). There are 18 known HA subtypes and 11 known N subtypes. However, only H1, H2, and H3, and N1 and N2 subtypes (e.g., A / (H1N1); A(H1N2); A(H2N2); and A(H3N2)) have caused widespread human disease. The genetic diversity of influenza A viruses, due to frequent mutation and reassortment, can result in novel strains with pandemic potential. Influenza B viruses (IBVs) are not divided into subtypes, but can be broken into lineages and strains within those lineages. Currently, two lineages circulate in humans: B / Yamagata (e.g., B / Yamagata / 16 / 1988-like) and B / Victoria (e.g., B / Victoria / 2 / 1987-like). An influenza virus protein may comprise one or more mutations relative to a reference influenza virus protein. In the context of proteins having one or more mutations (e.g., substitutions), a “reference protein” (e.g., reference HA protein) refers to a protein into which a mutation is introduced to yield a protein having the mutation. A reference protein may be a protein of an influenza virus isolate. An “influenza virus isolate” or “isolate of an influenza virus” refers to an influenza virus that has been obtained from an infected host and grown in cell culture. Amino acid sequences of proteins of a virus isolate (e.g., HA) may be determined by sequencing the isolate’s genome segments (e.g., segment 4 (HA)) and / or viral RNA from cells infected with the isolate. As used herein, a reference HA protein amino acid sequence of an influenza virus isolate is the HA protein amino acid sequence encoded by a consensus nucleotide sequence of segment 4 of the isolate. The skilled artisan will appreciate that a replicating virus (e.g., in cell culture) forms a population of virions, that each virion contains a genome that may have one or more mutations relative to a consensus nucleotide sequence (or set of consensus nucleotide sequences, for viruses with segmented genomes like influenza viruses), and that viral genomes may be defined in terms of that consensus nucleotide sequence (or set of consensus nucleotide sequences of genome segments, for segmented viral genomes). See, e.g., Domingo et al., Gene.1985.40(1):1-8; and Kuroda et al., PLoS ONE.2010. 5(4):e10256. A reference HA protein amino acid sequence of an isolate will be understood not to encompass HA protein amino sequences that are not encoded by the consensus nucleotide sequence of genome segment 4 of the isolate, even if such other HA protein amino acid sequences may be encoded by a minority of genomes in a virion population of the isolate. A reference HA protein may be an engineered HA protein that is not encoded by a consensus nucleotide sequence of genome segment 4 of a naturally occurring influenza virus isolate. The skilled artisan will appreciate that mutations may be applied to any B / Victoria lineage HA, B / Yamagata lineage HA, H1 HA protein, or H3 HA protein amino acid sequence that is extant at the time this specification is filed. The skilled artisan will also appreciate that the mutations may be applied to B / Victoria lineage HA, B / Yamagata lineage HA, H1 HA protein, or H3 HA protein amino acid sequences that do not yet exist at the time of filing this specification. Indeed, it is the continued evolution of influenza viruses that leads public health authorities to update the influenza virus isolates recommended for inclusion in seasonal vaccines each year. Thus, when a given influenza virus isolate is recommended for inclusion in a seasonal influenza vaccine, for instance, the skilled artisan could apply a mutation described below to the HA of that influenza virus isolate, to produce a mutated form of that recommended isolate’s HA protein. Sequence information for influenza viruses recommended for inclusion in seasonal influenza vaccines (e.g., HA protein amino acid sequences and corresponding genome segment nucleotide sequences) is typically available in publicly available databases, such as GenBank and GISAID. The skilled artisan will appreciate that mutations described in the context of H1 HA proteins may be applied to extant or later-arising H1 HA proteins, mutations described in the context of H3 HA proteins may be applied to extant or later-arising H3 HA proteins, mutations described in the context of B / Victoria lineage HA proteins may be applied to extant or later- arising B / Victoria lineage HA proteins, and mutations described in the context of B / Yamagata lineage HA proteins may be applied to extant or later-arising B / Yamagata lineage HA proteins, as HA proteins within a given subtype or lineage are more similar to each other than to HA proteins of other subtypes or lineages. The skilled artisan will appreciate that where mutations are disclosed in the context of H1 HAs of influenza A / (H1N1)pdm09 viruses (e.g., influenza A / Wisconsin / 67 / 2022(H1N1)pdm09 and A / Sydney / 5 / 2021(H1N1)pdm09 viruses), such mutations may also be applied to H1 HA proteins of other IAV subtypes containing H1 HAs (e.g., A / (H1N2). Mutations disclosed in the context of H3 HAs of influenza A / (H3N2) viruses (e.g., influenza A / Darwin / 6 / 2021(H3N2) virus) may similarly be applied to H3 HA proteins of other IAV subtypes containing H3 HAs (e.g., A / (H3N8)). Influenza virus proteins are discussed in more detail in WO / 2024 / 191860, which is incorporated herein by reference to the extent it discloses stabilizing influenza protein mutations. Discussion of influenza virus proteins can also be applied to nucleic acids encoding said influenza virus proteins and vice versa unless otherwise clear from context. Thus, disclosure related to particular polypeptide mutations is also relevant to nucleic acids encoding those polypeptides with those mutations, unless otherwise clear from context. Likewise, disclosure related to mRNA encoding mutated influenza virus proteins may also be relevant to the mutated influenza virus proteins. Thus, when a composition comprising an mRNA encoding an influenza virus protein having a particular mutation is disclosed, the skilled artisan can infer that the influenza virus protein per se, and compositions comprising the influenza virus protein, are also disclosed. In some embodiments, an influenza virus protein is a recombinant protein. A “recombinant protein” refers to a protein that is produced in a heterologous organism that does not naturally produce the protein or a variant thereof. Non-limiting examples of organisms in which recombinant proteins may be produced include bacteria (e.g., Escherichia coli), yeast (e.g., Saccharomyces cerevisiae), and mammalian cells. Severe Acute Respiratory Syndrome Coronavirus 2 Compositions (e.g., mRNA vaccines) and vaccination methods may elicit potent neutralizing antibodies against SARS-CoV-2. In some embodiments, the compositions (e.g., mRNA vaccines) further comprise at least one mRNA comprising an ORF encoding at least one SARS-CoV-2 antigen. The genome of SARS-CoV-2 is a single-stranded positive-sense mRNA (+ssRNA) with the size of 29.8–30 kb encoding about 9860 amino acids (Chan et al.2000, supra; Kim et al.2020 Cell, May 14; 181(4):914-921.e10.). SARS-CoV-2 is a polycistronic mRNA with 5′-cap and 3′-poly-A tail. The SARS-CoV-2 genome is organized into specific genes encoding both structural proteins and nonstructural proteins (Nsps). Coronavirus genomes include a variable number of open reading frames (ORFs) encoding accessory proteins, nonstructural proteins, and structural proteins (Song et al.2019 Viruses;11(1):p.59). Most SARS-CoV-2 peptides having antigenic properties are located in the structural proteins, including: Spike surface glycoprotein (S), a small envelope protein (E), matrix protein (M), and nucleocapsid protein (N) (Cui et al.2019 Nat. Rev. Microbiol.; 17(3):181–192). These structural proteins are ordered in the SARS-CoV-2 genome from 5’ to 3’ as follows: 5’-replicase (ORF1 / ab)-structural proteins [S-E-M-N]-3’. In addition to having a critical role in cell tropism and virus entry into host cells, the Spike protein of SARS-CoV-2 has also been demonstrated to induce neutralizing antibodies (Nab) and protective immunity in infected individuals. Moreover, amino acid sequence analysis has shown that while Spike proteins contain conserved regions among coronaviruses, the Spike protein also accumulate mutations as variant viral strains emerge (e.g.¸ during a pandemic, as a seasonally circulating virus). Accordingly, mRNAs comprising ORFs encoding Spike proteins may be effective in inducing immunity against SARS-CoV-2 infection. Coronavirus Spike proteins are trimers consisting of monomers which can be divided into two important functional subunits: the N-terminal S1 subunit, which forms of the globular head of the S protein, and the C-terminal S2 region, which forms the stalk of the protein and is embedded in the viral envelope. Upon interaction with a potential host cell, the S1 subunit recognizes and binds to receptors on the host cell (e.g., angiotensin-converting enzyme 2 (ACE2) receptors). The S2 subunit, which is the most conserved component of the S protein, then fuses the envelope of the virus with the host cell membrane. (See e.g., Shang et al., PLoS Pathog.2020 Mar; 16(3):e1008392.). As part of the infection process in vivo, the S1 and S2 subunits are enzymatically cleaved by furin-mediated cleavage (e.g., at the S1 / S2 cleavage site) and serine protease-mediated cleavage event (e.g., at the S2′ site within S1). The S1 and S2 subunits of the SARS-CoV-2 Spike protein each comprise additional structural and functional domains. S1 subunit domains include the receptor-binding domain (RBD), which binds to ACE2 receptors on the host cell, and the N-terminal domain (NTD), which may have a role in binding sugar moieties and facilitating the conformational transition of the Spike protein from prefusion to a post fusion conformation. The NTD and RBD have also been shown to be the targets of neutralizing antibodies in betacoronavirus-infected individuals. S2 subunit domains include the fusion peptide (FP), the fusion core region (including a heptad repeat 1 (HR1) and heptad repeat 2 (HR2)), transmembrane domain (TM), and cytoplasmic tail (CT) (Lu R. et al., supra; Wan et al., J. Virol. Mar 2020, 94 (7) e00127-20; Xia et al., 2020 Cell Mol Immunol. Jan; 17(1):1-12.). The fusion core region of the Spike protein contributes to the conformational change of the Spike protein from prefusion to postfusion. SARS-CoV-2 variants often accumulate amino acid mutations in the Spike protein; these mutations are believed to influence immune evasion, severity, and transmissibility. SARS-CoV-2 proteins (e.g., Spike proteins, sub-domains) may have one or more mutations compared to a reference SARS-CoV-2 protein, for example, the USA-WA1 / 2020 isolate reference strain (SEQ ID NO: 135). In exemplary embodiments, SARS-CoV-2 proteins may be designed to combat emerging and / or seasonal strains based on an understanding of circulating SARS-CoV-2 variants at a given point in time. For example, one SARS-CoV-2 sub-variant, JN.1 (also known as BA.2.86.1.1), is a subvariant (sublineage) of the SARS-CoV-2 “Omicron” variant, and was first observed in August 2023; this variant is closely related to BA.2.86. The mutations observed in this variant are believed to provide high potential for immune evasion, particularly the L455F “FLip" mutation- a mutation also observed in XBB lineage variants (e.g., HK.3 and EG.5.1). Mutations observed in JN.1 include A31D, V238L, K1155R, N1708S, A1892T, V24F, R252K, T35I, ins16MPLF, R21T, S50L, Δ69-70, V127F, F157S, R158G, N211del, L212I, V213G, L216F, H245N, A264D, I332V, K356T, R403K, V445H, N450D, L452W, L455S, N481K, V483del, E484K, E554K, A570V, P621S, P681H, S939F, and P1143L relative to SEQ ID NO: 135 (USA- WA1 / 2020 isolate reference strain). Several sub-variants of the XBB variant have also been identified recently: HV.1, JD.1.1, HK.3, and EG.5. These sub-variants share several mutations, including T19I, L24S, del25 / 27, V83A, G142D, del144 / 144, H146Q, Q183E, V213E, G252V, G339H, R346T, L368I, S371F, S373P, S375F, T376A, D405N, R408S, K417N, N440K, V445P, G446S, F456L, N460K, S477N, T478K, E484A, F486P, F490S, Q498R, N501Y, Y505H, D614G, H655Y, N679K, P681H, N764K, D796Y, Q954H, and N969K; some variants include additional mutations, such as: Q52H, F157L, L452R, L455F, and A475V; all relative to the USA-WA1 / 2020 isolate (SEQ ID NO: 123). As another example, one SARS-CoV-2 sub-variant, KP.2, has been observed. Mutations observed in the KP.2 S protein include ins16 / 17MPLF, T19I, R21T, del25 / 27, A28S, S50L, del69 / 70, V127F, G142D, del144, F157S, R158G, N211I, del212 / 212, V213G, L216F, H245N, A264D, I332V, G339H, K356T, S371F, S373P, S375F, T376A, R403K, D405N, R408S, K417N, N440K, V445H, G446S, N450D, L452W, L455S, N460K, S477N, T478K, N481K, del483 / 483, E484K, F486P, Q498R, N501Y, Y505H, E554K, A570V, D614G, P621S, H655Y, N679K, P681R, N764K, D796Y, S939F, Q954H, N969K, and P1143L, relative to the USA- WA1 / 2020 isolate (SEQ ID NO: 123). LP.8.1 is another example of a SARS-CoV variant. Mutations in the LP.8.1 variant can include S31del, F186L, R190S, R346T, V445R, F456L, Q493E, K1086R, and V1104L as compared to a JN.1 variant. A table depicting mutations of selected SARS-CoV-2 variants’ Spike proteins is provided below. Table 1. Spike mutations in SARS-CoV-2 variants These exemplary strains and other newly emerging strains are candidates for methods and compositions, such as combination mRNA vaccines. mRNA encoding antigens from these and other coronavirus strains have been designed for mRNA vaccines. Compositions Some aspects relate to compositions for use as a vaccine against hMPV. For instance, some embodiments comprise vaccines against hMPV comprising one or more mRNA(s) encoding one or more amino acid sequence(s) as set forth in Table 14 or an amino acid sequence having at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to one or more amino acid sequence(s) as set forth in Table 14. Some embodiments comprise mRNA encoding a stabilized prefusion hMPV-A F glycoprotein comprising an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises the sequence set forth in SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein consists of the sequence set forth in SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 19. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 19. In some embodiments, an mRNA encodes the stabilized prefusion hMPV-A F glycoprotein, wherein the mRNA comprises a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 32. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an mRNA comprising the sequence set forth in SEQ ID NO: 32. Some embodiments comprise mRNA encoding a stabilized prefusion hMPV-B F glycoprotein comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises the amino acid sequence set forth in SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein consists of the amino acid sequence set forth in SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an ORF comprising a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 21. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 21. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an mRNA comprising a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 34. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an mRNA comprising the sequence set forth in SEQ ID NO: 34. Some aspects relate to compositions for use as a vaccine against hRSV. For instance, some embodiments comprise vaccines against hRSV comprising a mRNA encoding an amino acid sequence as set forth in Table 15 or an amino acid sequence having at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to an amino acid sequence as set forth in Table 15. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 113. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 97% identity to the sequence of SEQ ID NO: 113. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 98% identity to the sequence of SEQ ID NO: 113. In some embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 99% identity to the sequence of SEQ ID NO: 113. In preferred embodiments, the stabilized prefusion hRSV-A F glycoprotein is encoded by an ORF comprising the sequence set forth in SEQ ID NO: 113. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 97% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 98% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises a sequence having at least 99% identity to the sequence of SEQ ID NO: 112. In some embodiments, an mRNA encodes the stabilized prefusion hRSV-A F glycoprotein, wherein the mRNA comprises the sequence set forth in SEQ ID NO: 112. Some aspects relate to compositions for use as a vaccine against a betacoronavirus. For instance, some embodiments comprise vaccines against a betacoronavirus comprising a mRNA encoding an amino acid sequence as set forth in Table 17 or an amino acid sequence having at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to an amino acid sequence as set forth in Table 17. As an example, some embodiments comprise vaccines against a betacoronavirus (e.g., SARS-CoV-2) comprising a mRNA encoding an amino acid sequence as set forth in SEQ ID NO: 136. Some embodiments comprise mRNA having an open reading frame comprising the nucleic acid sequence of SEQ ID NO: 134, or a nucleic acid sequence having at least 90%, at least 95%, at least 97%, at least 98%, or at least 99% identity to the nucleic acid sequence of SEQ ID NO: 134. Some embodiments comprise vaccines against a betacoronavirus comprising a mRNA encoding an amino acid sequence as set forth in SEQ ID NO: 140. Some embodiments comprise mRNA having an open reading frame comprising the nucleic acid sequence of SEQ ID NO: 138, or a nucleic acid sequence having at least 90%, at least 95%, at least 97%, at least 98%, or at least 99% identity to the nucleic acid sequence of SEQ ID NO: 138. Some aspects relate to combination vaccines for use against more than one infectious disease, such as hMPV and hRSV, betacoronavirus and influenza, or hRSV and betacoronavirus and / or influenza. Some embodiments relate to compositions comprising nucleic acids (e.g., RNAs, (e.g., mRNAs)) encoding two or more Pneumoviridae (e.g., hMPV, hRSV) antigens, where the nucleic acids encoding different antigens are present at certain ratios. Some aspects relate to multivalent vaccines that comprise components to protect a subject against more than one hRSV glycoprotein. A “ratio” of two nucleic acids (e.g., encoding proteins A and B) may refer to a molar ratio (the number of nucleic acid molecules encoding protein A, relative to the number of nucleic acid molecules encoding protein B), or a mass ratio (the mass of nucleic acids encoding protein A, relative to the mass of nucleic acids encoding protein B). Unless indicated otherwise or otherwise clear from context, reference to nucleic acids being present at a “ratio” refers to a mass ratio of the nucleic acids. Multivalent hMPV Compositions Some aspects relate to multivalent vaccines that comprise components to protect a subject against more than one hMPV subgroup. Multivalent vaccines can include two (bivalent), three (trivalent), four (quadrivalent), five (pentavalent), or more (such as octavalent) components that each are independently designed to protect against one of a variety of hMPV antigenic subgroups and / or isolates. For example, a bivalent vaccine can include an hMPV-A protein and an hMPV-B protein; a trivalent vaccine can include two hMPV-A proteins and one hMPV-B protein (or vice versa; and a quadrivalent vaccine can include two hMPV-A proteins and two hMPV-B proteins. As another example, a bivalent vaccine can include RNA(s) (e.g., mRNA(s)) encoding an hMPV-A protein and an hMPV-B protein; a trivalent vaccine can include RNA(s) encoding two hMPV-A proteins and one hMPV-B protein (or vice versa; a quadrivalent vaccine can include mRNA(s) encoding two hMPV-A proteins and two hMPV-B proteins. In some embodiments, mRNAs are present in an unequal amount (e.g., a 2:1, 3:1, 4:1, or 5:1 weight / weight ratio or a 2:1, 3:1, 4:1, or 5:1 molar ratio), for example, a 2:1, 3:1, 4:1, or 5:1 ratio of mRNAs encoding distinct antigens of the same hMPV subgroup, or a 2:1, 3:1, 4:1, or 5:1 ratio of mRNAs encoding hMPV-B and hMPV-A antigens. In some embodiments, the mRNAs are present in equal amounts (e.g., a 1:1 weight / weight ratio or a 1:1 molar ratio). For instance, some embodiments comprise vaccines against hMPV comprising one or more mRNA(s) encoding one or more amino acid sequence(s) associated with hMPV-A as set forth in Table 14 and one or more amino acid sequence(s) associated with hMPV-B as set forth in Table 14. Preferably the combination comprises mRNA associated with hMPV-A-2 and hMPV- B-1 in Table 14. That is, the composition comprises mRNA encoding a stabilized prefusion hMPV-A F glycoprotein comprising an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises the sequence set forth in SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein consists of the sequence set forth in SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 19. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 19. In some embodiments, an mRNA encodes the stabilized prefusion hMPV-A F glycoprotein, wherein the mRNA comprises a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 32. In some embodiments, the stabilized prefusion hMPV- A F glycoprotein is encoded by an mRNA comprising the sequence set forth in SEQ ID NO: 32. Additionally, the composition comprises mRNA encoding a stabilized prefusion hMPV- B F glycoprotein comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity to the amino acid sequence of SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises the amino acid sequence set forth in SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein consists of the amino acid sequence set forth in SEQ ID NO: 4. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an ORF comprising a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the nucleic acid sequence of SEQ ID NO: 21. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an ORF comprising the nucleic acid sequence set forth in SEQ ID NO: 21. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an mRNA comprising a nucleic acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the sequence of SEQ ID NO: 34. In some embodiments, the stabilized prefusion hMPV-B F glycoprotein is encoded by an mRNA comprising the sequence set forth in SEQ ID NO: 34. Multivalent hRSV Compositions Some aspects relate to multivalent vaccines that comprise components to protect a subject against more than one hRSV glycoprotein. Multivalent vaccines can include two (bivalent) or more (e.g., three (trivalent), four (quadrivalent), five (pentavalent), or more) components that each are independently designed to protect against one of a variety of hRSV antigenic subgroups and / or isolates. For instance, a bivalent vaccine can include RNA(s) (e.g., mRNA(s)) encoding an hRSV- B protein and an hRSV-A protein. In some aspects, a multivalent vaccine comprises (1) an mRNA comprising an ORF encoding a stabilized prefusion hRSV-B F glycoprotein and (2) an mRNA comprising an ORF encoding a stabilized prefusion hRSV-A F glycoprotein. In some embodiments, mRNAs are present in an unequal amount (e.g., a 2:1, 3:1, 4:1, or 5:1 weight / weight ratio or a 2:1, 3:1, 4:1, or 5:1 molar ratio), for example, a 2:1, 3:1, 4:1, or 5:1 ratio of mRNAs encoding distinct antigens of the same hRSV subgroup, or a 2:1, 3:1, 4:1, or 5:1 ratio of mRNAs encoding hRSV-B and hRSV- A antigens. In some embodiments, the mRNAs are present in equal amounts (e.g., a 1:1 weight / weight ratio or a 1:1 molar ratio). In some embodiments, a multivalent vaccine comprises (1) a stabilized prefusion hRSV-B F glycoprotein and (2) a stabilized prefusion hRSV-A F glycoprotein. The two components of the multivalent vaccines may be present in the composition in an equal amount (e.g., a 1:1 weight / weight ratio or a 1:1 molar ratio), for example, a 1:1 mass ratio of the hRSV-B antigen to the hRSV-A antigen. Combination vaccines Some embodiments of vaccines include combination vaccines. Combination mRNA vaccines comprise mRNA encoding at least two different viral antigens and can be administered to a subject (e.g., a mammalian subject, such as a human subject), such that the mRNA is translated and expressed in vivo to produce the antigens, which then stimulates an immune response in the subject. Combination vaccines include multiple mRNAs collectively encoding two or more different antigens of the same or different viruses. In some embodiments, a combination vaccine comprises an mRNA encoding one or more hRSV-A antigens, and an mRNA encoding one or more antigen(s) of a different organism (e.g., a coronavirus or influenza virus). In some embodiments, a combination vaccine comprises three or more mRNAs, each encoding an antigen of a different respiratory virus (e.g., an antigen of an hRSV-A antigen, an antigen of an influenza virus, and an antigen of a SARS-CoV-2 virus. Thus, the vaccines may be combination vaccines comprising antigens of different viruses (e.g., hRSV, coronavirus, and / or influenza virus), such that the antigens induce immunity to organisms which are found in the same geographic areas where the risk of respiratory virus infection is high or organisms to which an individual is likely to be exposed to when exposed to a respiratory virus. In some embodiments, a combination vaccine comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or more mRNAs encoding different respiratory virus antigens. In some embodiments, a combination vaccine comprises one or more mRNAs encoding one or more prefusion stabilized hRSV-A (e.g., F protein) variants. In some embodiments, a combination vaccine comprises one or more mRNAs encoding one or more influenza antigens (e.g., HA antigen, NA antigen). In some embodiments, a combination vaccine comprises two or more mRNAs encoding two or more distinct influenza antigens (e.g., HA antigen and NA antigen, distinct HA antigens, distinct NA antigens). In some embodiments, a combination vaccine comprises one or more mRNAs encoding one or more SARS-CoV-2 antigens (e.g., Spike protein domain antigens). In some embodiments, a combination vaccine comprises one or more mRNAs encoding two or more SARS-CoV-2 antigens (e.g., Spike protein domain antigens of different SARS-CoV-2 sub-variants). In some embodiments, a combination vaccine comprises two mRNAs each encoding a distinct antigen (e.g., of different respiratory viruses) are present in the composition in a 1:1 ratio. In some embodiments, the two mRNAs are present in the composition in a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10 ratio (e.g., wt / wt ratio or molar ratio). As used herein, a “weight / weight ratio” or wt / wt ratio or wt:wt ratio refers to the ratio between the weights (masses) of the different components. A “molar ratio” refers to the ratio between different components (e.g., the number of mRNA encoding each antigen) of compositions (e.g., combination mRNA vaccines). In some aspects, combination vaccines comprise a first set of mRNAs encoding two or more antigens of a first respiratory virus (e.g., hRSV-A) and second set of mRNAs encoding two or more antigens of a different respiratory virus (e.g., influenza). In some embodiments, these mRNAs are present in a “1:1 ratio” (e.g., wt / wt ratio or molar ratio) such that the composition comprises a 1:1 ratio of each mRNA in the first set of mRNA and a 1:1 ratio of each mRNA in the second set of mRNA. In some embodiments, these mRNAs are present in a 3:1 ratio, such that the composition comprises a 3:3 ratio of each mRNA of the first set of mRNA and a 1:1 ratio of each mRNA of the second set of mRNA. In some embodiments, a combination vaccine comprises a first, second, and third mRNA, wherein each mRNA encodes distinct respiratory virus antigens (e.g., from the same or different virus). In some embodiments, a combination vaccine comprises a first, second, and third mRNA wherein each mRNA encodes respiratory virus antigen from different respiratory virus antigens. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 1:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 4:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 3:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 5:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 4:2:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 1:2:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 1:2:2. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 8:2:2, 4:1:1, 4:2:2, 4:2:1, 4:3:2, 4:3:3, 4:3:2, or 4:2:2. In some embodiments, a combination vaccine comprises a first, second, third, and fourth mRNA, wherein each mRNA encodes distinct respiratory virus antigens (e.g., from the same or different virus). In some embodiments, a combination vaccine comprises a first, second, third, and fourth mRNA wherein each mRNA encodes respiratory virus antigen from different respiratory virus antigens. In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 1:1:1:1. In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 2:1:1:1. In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 3:1:1:1. In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 4:1:1:1. In some embodiments, each mRNA in the combination vaccine is complementary with and does not interfere with each other mRNA in the combination vaccine. That is, an antigen produced from administration of the combination vaccine does not significantly interfere with the immune response to any other of the antigens produced in response to the vaccine in such a way that would diminish the ability of the antigens to provoke a protective immune response in a subject. In some embodiments, the combination vaccine is additive with respect to neutralizing antibodies relative to each individual antigen in a vaccine. Thus, compositions (e.g., mRNA vaccines) may target one or more antigen(s) of the same viral subgroup, or one or more antigen(s) of different viral subgroups (e.g., subvariants), e.g., antigens which induce immunity to hRSV subgroups where the risk of hRSV infection is high. In each embodiment comprising one or more mRNA and one or more LNPs, it is understood that the mRNAs are encapsulated within LNPs. In some embodiments, each unique mRNA of the composition is encapsulated in its own LNP. However, mRNA vaccine technology also enjoys the significant technological advantage of being able to encapsulate several unique mRNAs in a single LNP product. In some embodiments, a single LNP comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 different mRNA polynucleotides. In other embodiments, the mRNAs are each formulated in unique LNPs (e.g., a composition comprises 8 LNPs, each LNP comprising 1 of 8 different mRNA polynucleotides). With respect to compositions comprising influenza proteins, in some embodiments, compositions comprise three (trivalent), four (quadrivalent), five (pentavalent), or more (such as octavalent) components that each independently are designed to protect against one of a variety of influenza virus strains. For instance, a trivalent influenza component can include RNA(s) encoding an influenza A / (H1N1) virus protein, an influenza A / (H3N2) virus protein, and an influenza B / Victoria lineage virus protein. Some trivalent influenza components comprise RNA(s) encoding two influenza A virus HA proteins and one influenza B virus HA proteins. Quadrivalent vaccines can include RNA(s) encoding three influenza A virus proteins (e.g., HA proteins) and one influenza B virus protein (e.g., HA protein). Some quadrivalent vaccines include mRNA encoding two influenza A virus proteins (e.g., HA proteins) and two influenza B virus proteins (e.g., HA proteins). In some embodiments, the influenza component comprises three mRNAs, a first encoding an IAV N1 NA protein, a second encoding an IAV N2 NA protein, and a third encoding an influenza B / Victoria lineage virus NA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the influenza component comprises four mRNAs, a first encoding an IAV N1 NA protein, a second encoding an IAV N2 NA protein, a third encoding an influenza B / Victoria lineage virus NA protein, and a fourth encoding an influenza B / Yamagata lineage virus NA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the influenza component comprises three mRNAs, a first encoding an influenza A / (H1N1) virus HA protein, a second encoding an influenza A / (H3N2) virus HA protein, and a third encoding an influenza B / Victoria lineage virus HA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the influenza component comprises four mRNAs, a first encoding an influenza A / (H1N1) virus HA protein, a second encoding an influenza A / (H3N2) virus HA protein, a third encoding an influenza B / Victoria lineage virus HA protein, and a fourth encoding influenza B / Yamagata lineage virus HA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the influenza component comprises eight mRNAs, a first encoding an IAV H1 HA protein, a second encoding an IAV H3 HA protein, a third encoding an influenza B / Victoria lineage virus HA protein, an influenza B / Yamagata lineage virus HA protein, a fifth encoding an IAV N1 NA protein, a sixth encoding an IAV N2 NA protein, a seventh encoding an influenza B / Victoria lineage virus NA protein, and an eighth encoding an influenza B / Yamagata lineage virus NA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the mRNAs are present at a 3:3:3:3:1:1:1:1 mass ratio (i.e., the mRNAs encoding the HA proteins are present at 3 times the amount (by mass) of mRNAs encoding the NA proteins). In some embodiments, the influenza component comprises five mRNAs, a first encoding an IAV H1 HA protein, a second encoding a first IAV H3 HA protein, a third encoding a second IAV H3 HA protein, a fourth encoding an influenza B / Victoria lineage virus HA protein, and a fifth encoding an influenza B / Yamagata lineage HA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the influenza component comprises four mRNAs, a first encoding an IAV H1 HA protein, a second encoding a first IAV H3 HA protein, a third encoding a second IAV H3 HA protein, and a fourth encoding an influenza B / Victoria lineage virus HA protein, where the vaccine does not comprise an mRNA encoding an influenza B / Yamagata lineage virus HA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the influenza component comprises six mRNAs, a first encoding an IAV H1 HA protein, a second encoding a first IAV H3 HA protein, a third encoding a second IAV H3 HA protein, a fourth encoding a third IAV H3 HA protein, a fifth encoding an influenza B / Victoria lineage virus HA protein, and a sixth encoding an influenza B / Yamagata lineage HA protein. In some embodiments, the mRNAs are present in substantially equal masses. In some embodiments, the influenza component comprises five mRNAs, a first encoding an IAV H1 HA protein, a second encoding a first IAV H3 HA protein, a third encoding a second IAV H3 HA protein, a fourth encoding a third IAV H3 HA protein, and a fifth encoding an influenza B / Victoria lineage virus HA protein, where the vaccine does not comprise an mRNA encoding an influenza B / Yamagata lineage HA protein. In some embodiments, the mRNAs are present in substantially equal masses. Some embodiments relate to influenza components comprising mRNAs encoding multiple H3 HA proteins derived from distinct influenza A / (H3N2) viruses. Separate H3 HA proteins may belong to different clades of the A / (H3N2) subtype. In some embodiments, each H3 HA protein encoded by an mRNA of a vaccine is derived from an influenza virus of a distinct clade within the A / (H3N2) subtype. In some embodiments, each H3 HA protein encoded by an mRNA of a vaccine differs from each other H3 HA protein encoded by other mRNAs of the vaccine by at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, at least 20 substitutions. With respect to compositions comprising SARS-CoV-2 proteins, in some embodiments, compositions comprises 1, 2, 3, 4, 5, or 6 mRNAs encoding different SARS-CoV-2 proteins. In some embodiments, different SARS-CoV-2 proteins are proteins derived from the same virus (e.g., variant), but having different structures, for example, a full-length Spike protein having a 2P mutation and an NTD-RBD-HATM fusion protein. In some embodiments, different SARS- CoV-2 proteins are proteins derived from different viruses (e.g., variants) but having similar structures, for example, two or more NTD-RBD-HATM fusion proteins, each having RBDs derived from different mutation from different SARS-CoV-2 variants. In some embodiments, different SARS-CoV-2 proteins are proteins differing by at least one mutation and / or at least one deletion. In some embodiments, compositions further comprise one or more mRNAs comprising an ORF encoding a wild-type SARS-CoV-2 Spike protein or fragment thereof. In some embodiments, a composition includes an mRNA encoding a fusion protein comprising at least two domains of a SARS-CoV-2 Spike (S) protein, and less than the full length spike protein. In some embodiments, a composition comprises a first messenger ribonucleic acid (mRNA) comprising a first open reading frame (ORF) that encodes a first fusion protein comprising at least two domains of a SARS-CoV-2 Spike (S) protein and less than the full length S protein, and a second mRNA comprising a second ORF that encodes a second fusion protein comprising at least two domains of a SARS-CoV-2 Spike (S) protein and less than the full length S protein; and a lipid nanoparticle. In some embodiments, the two RNAs are present in the composition in a 1:1 ratio. In some embodiments, the two RNAs are present in the composition in a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10 ratio. Exemplary sequences of the respiratory virus antigens and the mRNAs (or portions thereof) encoding the respiratory antigens of the compositions are provided in Tables 14-18. In some embodiments, the mRNA vaccines comprise a sequence that has at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to a sequence in Tables 14-18. In some embodiments, the mRNA vaccines encode a polypeptide that is 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or identical to an amino sequence in Tables 14-18. In some embodiments a combination vaccine comprises one or more components for eliciting an immune response (i) to one or more hMPV lineages, and (ii) to one or more hRSV strains. As noted previously, discussion of combination vaccines comprising one or more RNAs (e.g., mRNAs) encoding proteins of different viruses may also be applied, inter alia, to combination vaccines comprising the same proteins (e.g., as isolated proteins or proteins present in viral vectors). The skilled artisan will appreciate that for combination vaccines comprising one or more RNAs (e.g., mRNAs) encoding two or more proteins, the two or more proteins may be encoded a single RNA, or different RNAs of the combination vaccine. For example, a combination vaccine may include one or more RNAs, each encoding an antigen of hMPV and an antigen of hRSV. In some embodiments, a combination vaccine includes one or more RNAs (e.g., mRNAs) encoding at least one hMPV antigen, and at least one antigen of hRSV. In some embodiments, a combination vaccine includes one or more RNAs collectively encoding at least one hMPV virus antigen and at least one antigen of hRSV. With respect to the antigens of hRSV in the combination vaccine, in some embodiments, the combination vaccine comprises 1, 2, 3, 4, 5, or 6 RNAs (e.g., mRNAs) encoding different antigens of hRSV (e.g., hRSV-B, hRSV-A), wherein each antigen comprises at least one mutation and / or at least one deletion relative to a reference wild-type hRSV antigen. In some embodiments, the combination vaccine comprises an RNA encoding a wild-type hRSV F glycoprotein antigen or antigenic fragment thereof. In some embodiments, a combination vaccine includes an RNA encoding a hRSV F glycoprotein variant lacking a cytoplasmic tail. In some embodiments, a combination vaccine includes an RNA encoding a hRSV F glycoprotein variant lacking a cytoplasmic tail and further comprising one or more modifications relative to a wild-type hRSV F glycoprotein. In some embodiments, a combination vaccine comprises a first RNA encoding a first hRSV F glycoprotein variant lacking a cytoplasmic tail, and a second RNA encoding a second hRSV F glycoprotein variant lacking a cytoplasmic tail and further comprising one or more modifications relative to the wild-type hRSV F glycoprotein. In some embodiments, the two RNAs are present in the combination vaccine in a 1:1 ratio. In some embodiments, the two RNAs are present in the combination vaccine in a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10 ratio. With respect to the antigens of hMPV, in some embodiments, the combination vaccine comprises 1, 2, 3, 4, 5, or 6 RNAs (e.g., mRNAs) encoding different antigens of hMPV (e.g., hMPV-B, hMPV-A), wherein each antigen comprises at least one mutation and / or at least one deletion relative to a reference hMPV antigen. In some embodiments, the combination vaccine comprises an RNA encoding a wild-type hMPV F glycoprotein antigen or antigenic fragment thereof. In some embodiments, a combination vaccine includes an RNA encoding a hMPV F glycoprotein variant lacking a cytoplasmic tail. In some embodiments, a combination vaccine includes an RNA encoding a hMPV F glycoprotein variant lacking a cytoplasmic tail and further comprising one or more modifications relative to a wild-type hMPV F glycoprotein. In some embodiments, a combination vaccine comprises a first RNA encoding a first hMPV F glycoprotein variant lacking a cytoplasmic tail, and a second RNA encoding a second hMPV F glycoprotein variant lacking a cytoplasmic tail and further comprising one or more modifications relative to the wild-type hMPV F glycoprotein. In some embodiments, the two RNAs are present in the combination vaccine in a 1:1 ratio. In some embodiments, the two RNAs are present in the combination vaccine in a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10 ratio. In some embodiments, a combination vaccine comprises two mRNAs each encoding a distinct antigen (e.g., of different respiratory viruses) are present in the composition in a 1:1 ratio (e.g., RSV antigens : hMPV antigens). In some embodiments, the two mRNAs are present in the composition in a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10 ratio. In some embodiments, a combination vaccine comprises a first set of mRNAs encoding two or more antigens of a first respiratory virus (e.g., hRSV or hMPV) and a second set of mRNAs encoding an antigen of a different respiratory virus (e.g., hMPV or hRSV). In some embodiments, the two sets of mRNAs are present in the composition in a 1:1 ratio. In some embodiments, the two sets of mRNAs are present in the composition in a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10 ratio. In some embodiments, a combination vaccine comprises a first set of mRNAs encoding two or more antigens of a first respiratory virus (e.g., hRSV) and a second set of mRNAs encoding two or more antigens of a different respiratory virus (e.g., hMPV). In some embodiments, the two sets of mRNAs are present in the composition in a 1:1 ratio. In some embodiments, the two sets of mRNAs are present in the composition in a 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10 ratio. In some embodiments, a combination vaccine comprises a first, second, and third mRNA, wherein each mRNA encodes a distinct respiratory virus antigen (e.g., RSV-A, RSV-B, and hMPV-A; RSV-A, RSV-B, and hMPV-B; RSV-A, hMPV-A, and hMPV-B; or RSV-B, hMPV-A, and hMPV-B). In some embodiments, the first, second, and third mRNAs are present in the combination vaccine in a ratio of 1:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 4:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 3:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 5:1:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 4:2:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 1:2:1. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 1:2:2. In some embodiments, the first, second and third mRNAs are present in the combination vaccine in a ratio of 8:2:2, 4:1:1, 4:2:2, 4:2:1, 4:3:2, 4:3:3, 4:3:2, or 4:2:2. In some embodiments, a combination vaccine comprises a first, second, third, and fourth mRNA, wherein each mRNA encodes a distinct respiratory virus antigen (e.g., RSV-A, RSV-B, hMPV-A, and hMPV-B). In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 1:1:1:1. In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 2:1:1:1. In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 3:1:1:1. In some embodiments, the first, second, third, and fourth mRNAs are present in the combination vaccine in a ratio of 4:1:1:1. In some embodiments, each of the mRNA polynucleotides in the combination vaccine is complementary with and does not interfere with each other mRNA polynucleotide in the combination vaccine. That is, an antigen produced from administration of the combination vaccine do not significantly interfere with the immune response to any other of the antigens produced in response to the vaccine in such a way that would diminish the ability of the antigens to provoke a protective immune response in a subject. In some embodiments, the combination vaccine is additive with respect to neutralizing antibodies relative to each individual antigen in a vaccine. Thus, compositions (e.g., RNA vaccines (e.g., mRNA vaccines)) may target one or more antigen(s) of the same strain / species, or one or more antigen(s) of different strains / species, e.g., antigens which induce immunity to organisms which are found in the same geographic areas where the risk of respiratory virus (e.g. respiratory syncytial virus) infection is high. Combination vaccines comprising RNA (e.g., mRNA) polynucleotides encoding at least two antigenic polypeptides from Pneumoviridae viruses may be used for treating and / or preventing respiratory virus infections. In some embodiments, a combination vaccine comprises mRNA polynucleotides encoding antigens from hMPV and hRSV. In some embodiments, a composition comprises RNA (e.g., mRNA) polynucleotides encoding at least two antigenic polypeptides from at least two viruses. In some embodiments, one or more RNAs (e.g., mRNAs) encoding polypeptides from at least two different Pneumoviridae viruses are encapsulated in a single lipid nanoparticle. Some embodiments comprise one or more RNAs (e.g., mRNAs) encoding polypeptides from at least two different Pneumoviridae viruses, wherein the composition comprises lipid nanoparticles encapsulating one or more RNAs (e.g., mRNAs) encoding polypeptides from a single Pneumoviridae virus. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hMPV-A F glycoprotein and (ii) an hRSV-B F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hMPV-A F glycoprotein and (ii) an hRSV-A F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hMPV-B F glycoprotein and (ii) an hRSV-B F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hMPV-B F glycoprotein and (ii) an hRSV-A F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hMPV-A F glycoprotein, (ii) an hMPV-B F glycoprotein, and (iii) an hRSV-B F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hMPV-A F glycoprotein, (ii) an hMPV-B F glycoprotein, and (iii) an hRSV-A F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hRSV-A F glycoprotein, (ii) an hRSV-B F glycoprotein, and (iii) an hMPV-A F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hRSV-A F glycoprotein, (ii) an hRSV-B F glycoprotein, and (iii) an hMPV-B F glycoprotein or fragment thereof. In some embodiments, a combination vaccine comprises a combination of proteins or nucleic acids (e.g., RNAs (e.g., mRNAs)) collectively encoding the combination comprising: (i) an hMPV-A F glycoprotein, (ii) an hMPV-B F glycoprotein, (iii) an hRSV-B F glycoprotein, and (iv) an hRSV-A F glycoprotein, or fragment thereof. In some embodiments of combination vaccines, the stabilized prefusion hMPV-A F glycoprotein is a stabilized prefusion hMPV-A F glycoprotein described in the section entitled “Exemplary Stabilized Prefusion hMPV-A F Glycoproteins.” In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the stabilized prefusion hMPV-A F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 3. Some embodiments comprise a combination of mRNAs encoding two or more amino acid sequences as set forth in any of Tables 14-18 or encoding two or more amino acid sequences having at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to an amino acid sequence as set forth in any of Tables 14-18. As an example, some embodiments comprise vaccines comprising (i) mRNA encoding an amino acid of SEQ ID NO: 2, (ii) mRNA encoding an amino acid sequence of SEQ ID NO: 4, and (iii) mRNA encoding an amino acid sequence of SEQ ID NO: 13. Some embodiments comprise combinations of two or more mRNAs with nucleotide sequence as set forth in Tables 14-17, or with nucleotide sequences having at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity with nucleotide sequences (e.g., ORF sequences) as set forth in any of Tables 14-17. Some embodiments comprise a vaccine comprising (i) a first mRNA comprising the nucleotide sequence of SEQ ID NO: 32, (ii) a second mRNA comprising the nucleotide sequence of SEQ ID NO: 34, and (iii) a third mRNA comprising the nucleotide sequence of SEQ ID NO: 112. Some hMPV / hRSV combination vaccines comprise (i) a first mRNA comprising the nucleotide sequence of SEQ ID NO: 19, (ii) a second mRNA comprising the nucleotide sequence of SEQ ID NO: 21, and (iii) a third mRNA comprising the nucleotide sequence of SEQ ID NO: 30. Preferred hMPV / hRSV combination vaccines comprise (i) a first mRNA comprising the nucleotide sequence of SEQ ID NO: 19, (ii) a second mRNA comprising the nucleotide sequence of SEQ ID NO: 21, and (iii) a third mRNA comprising the nucleotide sequence of SEQ ID NO: 63. Some embodiments comprise a vaccine comprising (i) a first mRNA comprising the nucleotide sequence of SEQ ID NO: 63 and (ii) a second mRNA comprising a nucleotide sequence having at least 90% identity (such as at least 95% identity or 100% identity) to SEQ ID NO: 134. Some embodiments comprise a vaccine comprising (i) a first mRNA comprising the nucleotide sequence of SEQ ID NO: 63 and (ii) a second mRNA comprising a nucleotide sequence having at least 90% identity (such as at least 95% identity or 100% identity) to SEQ ID NO: 138. In some embodiments of combination vaccines, the stabilized prefusion hMPV-B F glycoprotein is a stabilized prefusion hMPV-B F glycoprotein described in the section entitled “Exemplary Stabilized Prefusion hMPV-B F Glycoproteins.” In some embodiments, the stabilized prefusion hMPV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 4. In some embodiments, the hRSV F protein or fragment thereof is an hRSV protein or fragment thereof described in the “Respiratory Syncytial Virus (hRSV)” section above. Nucleic Acids Provided are compositions comprising nucleic acids. In some embodiments, the nucleic acids comprise DNA. In some embodiments, the nucleic acids comprise RNA, such as self- amplifying RNA, circular RNA, or mRNA. Preferably, the nucleic acid comprises mRNA. Except where otherwise noted, nucleic acid sequences set forth in the instant application may recite “T”s in a representative DNA sequence but where the sequence represents RNA, the “T”s would be substituted for “U”s. Thus, any of the DNAs disclosed and identified by a particular sequence herein also discloses the corresponding RNA sequence where each “T” of the DNA sequence is substituted with “U.” Messenger RNA (mRNA) Messenger RNA (mRNA) is RNA that encodes a (at least one) protein or a fragment thereof and can be translated to produce the encoded protein or fragment in vitro, in vivo, in situ, or ex vivo. mRNA comprises an open reading frame (ORF) encoding the protein or fragment thereof. In some embodiments, the mRNA further comprises a 5^ untranslated region (UTR), 3^ UTR, a polyA tail, and / or a 5^ cap analog. The disclosed mRNA may encode a single protein or fragment or they may be polycistronic constructs, which encode more than one protein or fragment separately within the same mRNA molecule. Additionally or alternatively, the disclosed mRNA may encode a fusion protein or fragment thereof. i. Open Reading Frame (ORF) An open reading frame (ORF) is a continuous stretch of DNA or RNA beginning with a start codon (e.g., methionine (ATG or AUG)) and ending with a stop codon or codons (e.g., TAA, TAG, TGA, UAA, UAG, UGA, UGAUGA or UGAUAAUAG). For clarity: the stop codon itself is not considered a part of the ORF. An ORF typically encodes a protein or fragment thereof. ii. Untranslated Regions (UTRs) In some embodiments, mRNA comprises one or more regions or parts which act or function as an untranslated region. A 5′ untranslated region” (5′ UTR) is a region of an mRNA that is upstream (i.e., 5′) from the start codon and does not encode a polypeptide. A 3′ untranslated region” (3′UTR) is a region of an mRNA that is downstream (i.e., 3′) from the stop codon and also does not encode a polypeptide The 5′ UTR may start at the transcription start site and continues to the start codon but does not include the start codon. The 3′ UTR may start immediately following the stop codon and continue until a transcriptional termination signal. A variety of 5′ UTR and 3′ UTR sequences are known. Exemplary UTR sequences include SEQ ID NOs: 62-97 (5′ UTRs) and 98-110 (3′ UTRs), which are shown in Table 19 (5′ UTRs) and Table 20 (3′ UTRs) of the section “Exemplary Sequences.” In some embodiments, the 5′ UTR comprises a sequence provided in Table 19 or a sequence with at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or 100% identity to a 5′ UTR sequence provided in Table 19, or a variant or a fragment thereof. In some embodiments, the 3′ UTR comprises a sequence provided in Table 20 or a sequence with at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or 100% identity to a 3′ UTR sequence provided in Table 20, or a variant or a fragment thereof. Each RNA species in a multivalent RNA composition may comprise an IDR sequence that is not a sequence isomer of an IDR sequence of another RNA species in a multivalent RNA composition (e.g., the IDR sequence does not have the same number of adenosine nucleotides, the same number of cytosine nucleotides, the same number of guanine nucleotides, and the same number of uracil nucleotides (and consequently the same mass) as another IDR sequence in the composition, even if those sequences have different sequences). Each RNA species in a multivalent RNA composition may comprise an IDR (identity determining region) sequence having a mass that differs from the mass of IDR sequences of each other RNA species in a multivalent RNA composition. For example, the mass of each IDR sequence may differ from the mass of other IDR sequences by at least 9 Da, at least 25 Da, at least 25 Da, or at least 50 Da. Use of IDR sequences with distinct masses allows RNA fragments comprising different IDR sequences to be distinguished using mass-based analysis methods (e.g., mass spectrometry), which do not require reverse transcription, amplification, or sequencing of RNAs. Each RNA species in an RNA composition may comprise an IDR sequence with a different length. For example, each IDR sequence may have a length independently selected from 0 to 25 nucleotides. The length of a nucleic acid influences the rate at which the nucleic acid traverses a chromatography column, and so the use of IDR sequences of different lengths on different RNA species allows RNA fragments having different IDR sequences to be distinguished using chromatography-based methods (e.g., LC-UV). Combinations of features may be included in flanking regions and may be contained within other features. For example, the ORF may be flanked by a 5' UTR which may contain a strong Kozak translational initiation signal and / or a 3' UTR which may include an oligo(dT) sequence for templated addition of a poly-A tail.5′ UTR may comprise a first polynucleotide fragment and a second polynucleotide fragment from the same and / or different genes such as the 5′ UTRs described in US 2010 / 0293625 and WO 2015 / 085318, each incorporated herein by reference to the extent it discloses 5’ UTRs. In some embodiments, a double, triple or quadruple UTR such as a 5′ UTR or 3′ UTR may be used. As used herein, a “double” UTR is one in which two copies of the same UTR are encoded either in series or substantially in series. For example, a double beta-globin 3′ UTR may be used as described in US 2010 / 0129877, incorporated herein by reference to the extent it discloses 3’ UTRs. For the purposes of the present disclosure, a UTR may also include one or more translation enhancer elements (TEE). As a non-limiting example, the TEE may include those described in US 2009 / 0226470, herein incorporated by reference to the extent it discloses TEEs, and those known in the art. iii. PolyA Tail In some embodiments, the mRNA contains a 3′-polyA tail. A polyA tail may contain 10 to 300 adenosine monophosphates. It can, in some instances, comprise up to about 400 adenine nucleotides. For example, a polyA tail may contain 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290 or 300 adenosine nucleotides. In some embodiments, a polyA tail contains 50 to 250 adenosine nucleotides. In some embodiments, a polyA tail has a length of about 50, about 100, about 150, about 200, about 250, about 300, about 350, or about 400 nucleotides. In some embodiments, a polyA tail has a length of 100 nucleotides. In some embodiments, an mRNA may comprise two polyA sequences separated by an intervening nucleotide sequence. In some embodiments, the intervening nucleotide sequence comprises no more than 3, no more than two, no more than 1, or no adenosine nucleotides. In some embodiments, the intervening sequence comprises 3 adenosine nucleotides. In some embodiments, the intervening sequence is no more than 30, no more than 25, no more than 20, no more than 15, or no more than 10 nucleotides long. In some embodiments, the intervening sequence consists of 10 nucleotides. In some embodiments, the intervening sequence comprises the sequence of GCAUAUGACU (SEQ ID NO: 149). In some embodiments, the intervening sequence does not begin with an adenosine nucleotide, and does not end with an adenosine nucleotide. In some embodiments, the first polyA sequences comprises at least 15, at least 20, at least 25, or at least 30 consecutive adenosine nucleotides. In some embodiments, the second polyA sequences comprises at least 55, at least 60, at least 65, or at least 70 consecutive adenosine nucleotides. In some embodiments, the first polyA sequence comprises 30 consecutive adenosine nucleotides. In some embodiments, the second polyA sequence comprises 70 adenosine nucleotides. iv. 5′ Cap In some embodiments, mRNA comprises a 5′ end cap or a “5′ terminal cap.” A cap analog may be, for example, a dinucleotide cap, a trinucleotide cap, or a tetranucleotide cap. In some embodiments, a cap analog is a dinucleotide cap. In some embodiments, a cap analog is a trinucleotide cap. In some embodiments, a cap analog is a tetranucleotide cap. 5′-capping of polynucleotides may be completed concomitantly during an in vitro transcription reaction using, for example, the following chemical RNA cap analogs to generate the 5′- guanosine cap structure according to manufacturer protocols: 3´-O-Me-m7G(5')ppp(5') G [the ARCA cap];G(5')ppp(5')A; G(5')ppp(5')G; m7G(5')ppp(5')A; m7G(5')ppp(5')G (New England BioLabs, Ipswich, MA).5′-capping of modified mRNA may be completed post-transcriptionally using, for example, a Vaccinia Virus Capping Enzyme to generate the “Cap 0” structure: m7G(5')ppp(5')G (New England BioLabs, Ipswich, MA). A Cap 1 structure may be generated using both Vaccinia Virus Capping Enzyme and a 2′-O methyl-transferase to generate: m7G(5')ppp(5')G-2′-O-methyl. A Cap 2 structure may be generated from the Cap 1 structure followed by the 2′-O-methylation of the 5′-antepenultimate nucleotide using a 2′-O methyl- transferase. A Cap 3 structure may be generated from the Cap 2 structure followed by the 2′-O- methylation of the 5′-preantepenultimate nucleotide using a 2′-O methyl-transferase. Enzymes may be derived from a recombinant source. Other cap analogs, such as a 7mG(5′)ppp(5′)NlmpNp cap, may be used. Chemical Modifications An mRNA may include nucleotides that are not chemically modified (i.e., unmodified nucleotides), nucleotides that are chemically modified, or both. Nucleotides that are not chemically modified are the standard ribonucleotides consisting of adenosine, guanosine, cytidine, and uridine. Some embodiments of mRNAs comprise modified nucleosides and / or nucleotides. A “nucleoside” refers to a compound containing a sugar molecule (e.g., a pentose or ribose) or a derivative thereof in combination with an organic base (e.g., a purine or pyrimidine) or a derivative thereof (also referred to herein as “nucleobase”). A “nucleotide” refers to a nucleoside in combination with a phosphate group. Modifications to nucleotides or nucleosides can be at the sugar or nucleobase. Modified nucleotides may by synthesized by any useful method, such as, for example, chemically, enzymatically, or recombinantly. In some embodiments, modified nucleobases in nucleic acids (e.g., RNA, such as mRNA) comprise N1-methyl-pseudouridine (m1ψ), N1-ethyl-pseudouridine (e1ψ), 5-methoxy- uridine (mo5U), 5-methyl-uridine (m5U), 5-methyl-cytidine (m5C), and / or pseudouridine (ψ). In some embodiments, modified nucleobases in nucleic acids (e.g., RNA, such as mRNA) comprise 5-methoxymethyl uridine, 5-methylthio uridine, 1-methoxymethyl pseudouridine, 5-methyl cytidine, and / or 5-methoxy cytidine. In some embodiments, the RNA includes a combination of at least two (e.g., 2, 3, 4 or more) of any of the aforementioned modified nucleobases, including but not limited to chemical modifications. In some embodiments, a mRNA comprises 1-methyl-pseudouridine (m1ψ) substitutions at one or more or all uridine positions of the mRNA. In some embodiments, the ORF comprises 1-methyl-pseudouridine (m1ψ) substitutions at one or more or all uridine positions of the ORF. In some embodiments, the mRNA comprises nucleosides consisting of m1ψ, adenosine, guanosine, and cytidine. In some embodiments, the ORF comprises nucleosides consisting of m1ψ, adenosine, guanosine, and cytidine. In some embodiments, a mRNA comprises 1-methyl-pseudouridine (m1ψ) substitutions at one or more or all uridine positions of the nucleic acid and 5-methyl cytidine substitutions at one or more or all cytidine positions of the mRNA. In some embodiments, the ORF comprises 1-methyl-pseudouridine (m1ψ) substitutions at one or more or all uridine positions of the ORF and 5-methyl cytidine substitutions at one or more or all cytidine positions of the ORF. In some embodiments, the mRNA comprises nucleosides consisting of m1ψ, adenosine, guanosine, and 5-methyl cytidine. In some embodiments, the ORF comprises nucleosides consisting of m1ψ, adenosine, guanosine, and 5-methyl cytidine. In some embodiments, a mRNA comprises pseudouridine (ψ) substitutions at one or more or all uridine positions of the mRNA. In some embodiments, the ORF comprises pseudouridine (ψ) substitutions at one or more or all uridine positions of the ORF. In some embodiments, the mRNA comprises nucleosides consisting of ψ, adenosine, guanosine, and cytidine. In preferred embodiments, the ORF comprises nucleosides consisting of ψ, adenosine, guanosine, and cytidine. In some embodiments, a mRNA comprises pseudouridine (ψ) substitutions at one or more or all uridine positions of the nucleic acid and 5-methylcytidine substitutions at one or more or all cytidine positions of the mRNA. In some embodiments, the ORF comprises pseudouridine (ψ) substitutions at one or more or all uridine positions of the ORF and 5-methyl cytidine substitutions at one or more or all cytidine positions of the ORF. In some embodiments, the mRNA comprises nucleosides consisting of ψ, adenosine, guanosine, and 5-methyl cytidine. In some embodiments, the ORF comprises nucleosides consisting of ψ, adenosine, guanosine, and 5-methyl cytidine. In some embodiments, a mRNA comprises uridine at one or more or all uridine positions of the mRNA. In some embodiments, the ORF comprises uridine at one or more or all uridine positions of the ORF. In some embodiments, the mRNA comprises nucleosides consisting of uridine, adenosine, guanosine, and cytidine. In some embodiments, the ORF comprises nucleosides consisting of uridine, adenosine, guanosine, and cytidine. In some embodiments, a mRNA comprises 5-methyl-uridine and 5-methyl cytidine at one or more or all uridine and cytidine positions, respectively, of the mRNA. In some embodiments, the ORF comprises 5-methyl-uridine substitutions at one or more or all uridine positions of the ORF and 5-methyl cytidine substitutions at one or more or all cytidine positions of the ORF. In some embodiments, the mRNA comprises nucleosides consisting of 5-methyl-uridine, adenosine, guanosine, and 5-methyl cytidine. In some embodiments, the ORF comprises nucleosides consisting of 5-methyl-uridine, adenosine, guanosine, and 5-methyl cytidine. In some embodiments, a mRNA comprises 5-methyl-uridine and 5-methyl cytidine at one or more or all uridine and cytidine positions, respectively, of the mRNA. In some embodiments, mRNAs are uniformly modified (e.g., fully modified, modified throughout the entire sequence) for a particular modification. For example, a mRNA can be uniformly modified with 1-methyl-pseudouridine, meaning that all uridine residues in the mRNA sequence are replaced with 1-methyl-pseudouridine. Similarly, a nucleic acid can be uniformly modified for any type of nucleoside residue present in the sequence by replacement with a modified residue such as those set forth above. In some embodiments, the ORF is uniformly modified for a particular modification, such as 1-methyl-pseudouridine. In some embodiments, the uniform modification does not include the mRNA cap. For instance, a cap with different modifications from the remainder of the mRNA can be added co-transcriptionally or post- transcriptionally to the mRNA. Codon Optimization In some embodiments, an ORF encoding a protein or fragment thereof is codon optimized. Codon optimization methods are known in the art. For example, an ORF of any one or more of the sequences listed below may be codon optimized. Codon optimization, in some embodiments, may be used to match codon frequencies in target and host organisms to ensure proper folding; bias GC content to increase RNA stability or reduce secondary structures; minimize tandem repeat codons or base runs that may impair gene construction or expression; customize transcriptional and translational control regions; insert or remove protein trafficking sequences; remove / add post translation modification sites in encoded protein (e.g., glycosylation sites); add, remove or shuffle protein domains; insert or delete restriction sites; modify ribosome binding sites and RNA degradation sites; adjust translational rates to allow the various domains of the protein to fold properly; or reduce or eliminate problem secondary structures within the polynucleotide. Codon optimization tools, algorithms and services are known in the art – non- limiting examples include services from GeneArt (Life Technologies), DNA2.0 (Menlo Park CA) and / or proprietary methods. In some embodiments, the open reading frame (ORF) sequence is optimized using optimization algorithms. In some embodiments, a codon optimized sequence shares less than 95%, less than 90%, less than 85%, less than 80%, or less than 75% sequence identity to a naturally-occurring or wild-type sequence open reading frame (e.g., a naturally-occurring or wild-type mRNA sequence encoding a protein or fragment thereof). In some embodiments, a codon optimized sequence shares between 65% and 85% sequence identity to a naturally-occurring or wild-type sequence (e.g., a naturally-occurring or wild-type RNA or DNA sequence encoding a protein or fragment thereof). In some embodiments, a codon-optimized sequence encodes an antigen that is as immunogenic as, or more immunogenic than (e.g., at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 100%, or at least 200% more), than a protein or fragment thereof encoded by a non-codon-optimized sequence. Self-amplifying RNA In some embodiments, an RNA is a self-amplifying RNA. A self-amplifying RNA is an RNA encoding one or more proteins that, individually or in conjunction, are capable of replicating the self-amplifying RNA. In some embodiments, the proteins encoded by the self- amplifying RNA are non-structural proteins nsP1, nsP2, nsP3, and nsP4, which form an RNA- dependent RNA polymerase (RdRp), or replicase, that is capable of replicating the self- amplifying RNA. By encoding proteins that are capable of replicating the RNA, a self- amplifying RNA is capable of self-amplification in a cell, provided that the cell can translate the RNA and produce the encoded protein(s). A self-amplifying RNA may be referred to as an RNA replicon. When a self-amplifying RNA is translated, the one or more encoded viral non-structural proteins are translated. A “viral non-structural protein” is a protein encoded by a virus but that is not part of the virus particle. The viral non-structural proteins, in the context of self-amplifying RNA, replicate the nucleotide sequences encoding the desired protein from the self-amplifying RNA via the sub-genomic viral promoters. Such replication driven by the viral sub-genomic promoter using the viral non- structural proteins enhances the expression level of the encoded protein. In some embodiments, the viral non-structural proteins are from a single-strand positive- sense RNA viruses. In some embodiments, the viral non-structural proteins are from an Alphavirus, belonging to the Togaviridae family. In some embodiments, the alphavirus is Sindbis or Venezuelan equine encephalitis virus. In some embodiments, the viral non-structural protein is an RNA-dependent RNA polymerase (RdRp) polyprotein P1234 (also termed NSP1-4). Upon translation, P1234 is rapidly cleaved into P123 and nsP4 by autoproteolytic activity originating from the nsP2 (proteinase) portion of the polyprotein. Alphaviral RNA synthesis occurs at the plasma membrane of a cell, where the nsPs, together with alphaviral RNA, form membrane invaginations (or “spherules”). These spherules contain dsRNA created by replication of “+” strand viral genomic RNA into “–” strand anti-genomic RNA. The “–” strand serves as a template from which additional “+” strand genomic RNA (synthesized from the 5′ UTR) or a shorter subsequence of the genomic RNA (termed subgenomic RNA) is synthesized from the subgenomic viral promoter region located near the end of the nonstructural protein ORF. The “+” strand genomic RNA and the subgenomic RNA are exported out of the spherules into the cytoplasm where they are translated by endogenous ribosomes. The exported “+” strand genomic RNA can associate with nsPs and form additional spherules, thus resulting in exponential increase of replicon RNA. The viral non-structural proteins facilitate the replication of the nucleotide sequences encoding the desired protein via the subgenomic viral promoters (also referred to as “subgenomic promoters” herein). A “subgenomic viral promoter” refers to a promoter the drives the transcription of subgenomic mRNAs. Typically, an mRNA is transcribed from genomic DNAs and episomal DNAs (e.g., plasmids). Some viruses may transcribe subgenomic mRNAs from a RNA replicon that is produced from its genomic RNA. Many positive-sense RNA viruses produce subgenomic mRNAs as one of the common infection techniques used by these viruses and generally transcribe late viral genes. Subgenomic viral promoters range from 20 nucleotide (Sindbis virus) to over 100 nucleotides (Beet necrotic yellow vein virus) and are usually found upstream of the transcription start. In some embodiments, the subgenomic viral promoter is 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100 nucleotides long, or longer. Subgenomic viral promoters have been described in the art, e.g., in PCT Publication No. WO 2016 / 040359, and Wagner et al., Nature Chemical Biology, DOI: 10.1038 / s41589-018-0146-9 (2018). Circular RNA In some embodiments, an RNA is a circular RNA. A circular RNA is an RNA with no 5′ terminal nucleotide or 3′ terminal nucleotide. Every nucleotide in a circular RNA is covalently bonded to both (1) a 5′ adjacent nucleotide; and (2) a 3′ adjacent nucleotide. In a circular RNA with a nucleotide sequence comprising every nucleotide of the circular RNA in 5′-to-3′ order, the last nucleotide of the nucleotide sequence is covalently bonded to the first nucleotide of the nucleotide sequence. A circular RNA may be a circular mRNA, comprising one or more 5′ UTRs, an open reading frame, and one or more 3′ UTRs. A circular RNA may comprise a polyA region, as described in the section entitled “PolyA Tails”. The skilled artisan will appreciate that a polyA tail, when incorporated in a circular RNA, is referred to as a polyA region because a circular RNA does not have an end as a linear mRNA does. A circular RNA may comprise an internal ribosome entry site (IRES). Inclusion of an IRES permits the translation of one or more open reading frames from a circular RNA, as circular RNAs do not comprise a 5′ from which a ribosome may initiate translation as with capped linear mRNAs. The IRES element attracts a eukaryotic ribosomal translation initiation complex and promotes translation initiation. See, e.g., Kaufman et al., Nucleic Acid Res.1991 19:4485-4490; Gurtu et al., Biochem Biophys Res Commun.1996.229:295-298; Rees et al., BioTechniques.1996.20: 102-110; Kobayashi et al., BioTechniques.1996.21 :399-402; and Mosser et al., BioTechniques.1997.22:150-161. A multitude of IRES sequences are available and include sequences derived from a wide variety of viruses, such as from leader sequences of picornaviruses such as the encephalomyocarditis virus (EMCV) UTR (Jang et al., J Virol.1989. 63: 1651-1660), the polio leader sequence, the hepatitis A virus leader, the hepatitis C virus IRES, human rhinovirus type 2 IRES (Dobrikova et al., Proc Natl Acad Sci U S A.2003. 100(25): 15125- 15130), an IRES element from the foot and mouth disease virus (Ramesh et al., Nucleic Acid Res.1996.24:2697-2700), a giardiavirus IRES (Garlapati et al., J Biol Chem. 2004.279(5):3389-3397). Additionally or alternatively, a circular RNA may comprise any of a variety of nonviral IRES sequences, such as IRES sequences from yeast, as well as the human angiotensin II type 1 receptor IRES (Martin et al., Mol Cell Endocrinol. (2003) 212:51-61), fibroblast growth factor IRESs (FGF-1 IRES and FGF-2 IRES, Martineau et al., Mol Cell Biol. 2004.24(17):7622-7635), vascular endothelial growth factor (VEGF) IRES (Baranick et al., Proc Natl Acad Sci U S A.2008.105(12):4733-4738, Stein et al., Mol Cell Biol.1998. 18(6):3112-3119, Bert et al., RNA.2006.12(6):1074-1083), and insulin-like growth factor II (IGF-II) IRES (Pedersen et al., Biochem J.2002.363(Pt l):37-44). These elements are commercially available in plasmids sold, e.g., by Clontech (Mountain View, CA), Invivogen (San Diego, CA), Addgene (Cambridge, MA) and GeneCopoeia (Rockville, MD). See also IRESite: The database of experimentally verified IRES structures. In some embodiments, a circular RNA comprises a coxsackievirus B3 (CVB3) IRES. See Gharbi et al., PLoS One.2022. 17(10):e0274162. In some embodiments, a circular RNA comprises an EMCV IRES. In some embodiments, a circular RNA comprises a salivirus IRES. See Sweeney et al., J Virol.2012. 86(3):1468–1486. In some embodiments, the salivirus IRES is present in or derived from Salivirus FHB (SaliFHB). See GenBank Accession No. KM023140.1. Viral Vectors Some aspects relate to viral vectors comprising or encoding hMPV F glycoproteins and / or hRSV F glycoproteins. In some embodiments, the protein is comprised in a viral vector. In some embodiments, a viral vector comprises a nucleic acid encoding the protein. Any suitable virus may be used as a viral vector. Non-limiting examples of viruses that may be used as viral vectors include retrovirus (e.g., lentivirus), adenovirus, adeno-associated virus (AAV), vesicular stomatitis virus (VSV), herpesvirus, Rous sarcoma virus, measles virus, poxvirus, gammavirus, alphavirus, murine stem cell virus, Moloney murine leukemia virus, and bovine leukemia virus. In some embodiments, the viral vector is a VSV vector. In some embodiments, the viral vector is a measles virus vector. In some embodiments, the viral vector is an adenovirus vector. These and other viral vectors suitable for expression of heterologous proteins (i.e., proteins not naturally expressed by a virus from which the viral vector is derived) are known in the art. In some embodiments, a viral vector comprises an hMPV-A F glycoprotein, or a nucleic acid encoding the hMPV-B F glycoprotein. In some embodiments, the hMPV-A F glycoprotein is a hMPV-A F glycoprotein described in the section entitled “Exemplary Stabilized Prefusion hMPV-A Glycoproteins.” In some embodiments, the hMPV-A F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 1. In some embodiments, the hMPV-A F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 2. In some embodiments, the hMPV-A F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 3. In some embodiments, a viral vector comprises an hMPV-B F glycoprotein, or a nucleic acid encoding the hMPV-B F glycoprotein. In some embodiments, the hMPV-B F glycoprotein is a hMPV-B F glycoprotein described in the section entitled “Exemplary Stabilized Prefusion hMPV-B Glycoproteins.” In some embodiments, the hMPV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 4. In some embodiments, a viral vector comprises an hRSV-B F glycoprotein, or a nucleic acid encoding the hRSV-B F glycoprotein. In some embodiments, the hRSV-B F glycoprotein is a hRSV-B F glycoprotein described in the section entitled “Exemplary Stabilized Prefusion hRSV-B Glycoproteins.” In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 5. In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 6. In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 7. In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 8. In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 9. In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 10. In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 11. In some embodiments, the hRSV-B F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 12. In some embodiments, a viral vector comprises an hRSV-A F glycoprotein, or a nucleic acid encoding the hRSV-A F glycoprotein. In some embodiments, the hRSV-A F glycoprotein is a hRSV-A F glycoprotein described in the section entitled “Exemplary Stabilized Prefusion hRSV-A Glycoproteins.” In some embodiments, the hRSV-A F glycoprotein comprises an amino acid sequence with at least 90% sequence identity to the amino acid sequence of SEQ ID NO: 13. Nucleic Acid Production In vitro transcription (IVT) of RNA cDNA encoding RNA polynucleotides may be transcribed using an in vitro transcription (IVT) system. In vitro transcription of RNA is known in the art and is described in International Publication WO 2014 / 152027, which is incorporated by reference herein to the extent it discloses IVT methods. In some embodiments, the RNA is prepared in accordance with any one or more of the methods described in WO 2018 / 053209 and WO 2019 / 036682, each of which is incorporated by reference herein to the extent it discloses RNA production methods. In some embodiments, the RNA transcript is generated using a non-amplified, linearized DNA template in an in vitro transcription reaction to generate the RNA transcript. In some embodiments, the template DNA is isolated DNA. In some embodiments, the template DNA is cDNA. In some embodiments, the cDNA is formed by reverse transcription of an RNA polynucleotide, for example, but not limited to hMPV mRNA. In some embodiments, cells, e.g., bacterial cells, e.g., E. coli, e.g., DH-1 cells are transfected with the plasmid DNA template. In some embodiments, the transfected cells are cultured to replicate the plasmid DNA which is then isolated and purified. In some embodiments, the DNA template includes a RNA polymerase promoter, e.g., a T7 promoter located 5′ to and operably linked to the gene of interest. In some embodiments, an in vitro transcription template encodes a 5′ untranslated (UTR) region, contains an open reading frame, and encodes a 3′ UTR and a poly(A) tail. The particular nucleic acid sequence composition and length of an in vitro transcription template will depend on the mRNA encoded by the template. An in vitro transcription system typically comprises a transcription buffer, nucleotide triphosphates (NTPs), an RNase inhibitor, and a polymerase. The NTPs may be manufactured in house, may be selected from a supplier, or may be synthesized. The NTPs may be selected from natural and unnatural NTPs, and may be selected from unmodified (e.g., ATP, GTP, UTP, CTP) or modified NTPs. Any number of RNA polymerases or variants may be used to transcribe RNA. The polymerase may be selected from, but is not limited to, a phage RNA polymerase, e.g., a T7 RNA polymerase, a T3 RNA polymerase, a SP6 RNA polymerase, and / or mutant polymerases such as, but not limited to, polymerases able to incorporate modified nucleic acids and / or modified nucleotides, including chemically modified nucleic acids and / or nucleotides. Some embodiments exclude the use of DNase. In some embodiments, the RNA transcript is capped via enzymatic capping. In some embodiments, the RNA comprises 5' terminal cap, for example, 7mG(5’)ppp(5’)NlmpNp. In some embodiments the RNA polymerase is a wild-type RNA polymerase. In some embodiments, the RNA polymerase is an RNA polymerase variant, such as those described in WO 2020 / 172239, incorporated herein by reference to the extent it describes RNA polymerase variants. RNA polymerase variants may include at least one amino acid substitution, relative to the wild-type (WT) RNA polymerase. Purification Purification of the nucleic acids may include, but is not limited to, nucleic acid clean-up, quality assurance and quality control. Clean-up may be performed by methods known in the arts such as, but not limited to, AGENCOURT® beads (Beckman Coulter Genomics, Danvers, MA), poly-T beads, LNATM oligo-T capture probes (EXIQON® Inc, Vedbaek, Denmark); HPLC based purification methods such as, but not limited to, strong anion exchange HPLC, weak anion exchange HPLC, reverse phase HPLC (RP-HPLC), and hydrophobic interaction HPLC (HIC- HPLC); and / or tangential flow filtration. The term “purified” when used in relation to a nucleic acid such as a “purified nucleic acid” refers to one that is separated from at least one contaminant. A “contaminant” is any substance that makes another unfit, impure or inferior. Thus, a purified nucleic acid (e.g., DNA and RNA) is present in a form or setting different from that in which it is found in nature, or a form or setting different from that which existed prior to subjecting it to a treatment or purification method. Lipid Compositions In some embodiments, the nucleic acids are formulated as a lipid composition, such as a composition comprising a lipid nanoparticle, a liposome, and / or a lipoplex. In some embodiments, nucleic acids are formulated as lipid nanoparticle (LNP) compositions. Lipid nanoparticles typically comprise amino lipid, non-cationic lipid, structural lipid, and PEG lipid components along with the nucleic acid cargo of interest. The lipid nanoparticles can be generated using components, compositions, and methods as are generally known in the art, see for example PCT / US2016 / 052352; PCT / US2016 / 068300; PCT / US2017 / 037551; PCT / US2015 / 027400; PCT / US2016 / 047406; PCT / US2016000129; PCT / US2016 / 014280; PCT / US2017 / 038426; PCT / US2014 / 027077; PCT / US2014 / 055394; PCT / US2016 / 052117; PCT / US2012 / 069610; PCT / US2017 / 027492; PCT / US2016 / 059575; PCT / US2016 / 069491; PCT / US2016 / 069493; and PCT / US2014 / 066242, all of which are incorporated by reference herein to the extent they disclose lipid nanoparticles. In some embodiments, the lipid nanoparticle comprises at least one ionizable amino lipid, at least one non-cationic lipid, at least one sterol, and / or at least one polyethylene glycol (PEG)- modified lipid. In some embodiments, the lipid nanoparticle comprises a molar ratio of 20-60% ionizable amino lipid, 5-25% non-cationic lipid, 25-55% structural lipid, and 0.5-15% PEG-modified lipid. In some embodiments, the lipid nanoparticle comprises a molar ratio of 20-60% ionizable amino lipid, 5-30% non-cationic lipid, 10-55% structural lipid, and 0.5-15% PEG-modified lipid. In some embodiments, the lipid nanoparticle comprises 40-50 mol% ionizable lipid, optionally 45-50 mol%, for example, 45-46 mol%, 46-47 mol%, 47-48 mol%, 48-49 mol%, or 49-50 mol% for example about 45 mol%, 45.5 mol%, 46 mol%, 46.5 mol%, 47 mol%, 47.5 mol%, 48 mol%, 48.5 mol%, 49 mol%, or 49.5 mol%. In some embodiments, the lipid nanoparticle comprises 20-60 mol% ionizable amino lipid. For example, the lipid nanoparticle may comprise 20-50 mol%, 20-40 mol%, 20-30 mol%, 30-60 mol%, 30-50 mol%, 30-40 mol%, 40-60 mol%, 40-50 mol%, or 50-60 mol% ionizable amino lipid. In some embodiments, the lipid nanoparticle comprises 20 mol%, 30 mol%, 40 mol%, 50 mol%, or 60 mol% ionizable amino lipid. In some embodiments, the lipid nanoparticle comprises 35 mol%, 36 mol%, 37 mol%, 38 mol%, 39 mol%, 40 mol%, 41 mol%, 42 mol%, 43 mol%, 44 mol%, 45 mol%, 46 mol%, 47 mol%, 48 mol%, 49 mol%, 50 mol%, 51 mol%, 52 mol%, 53 mol%, 54 mol%, or 55 mol% ionizable amino lipid. In some embodiments, the lipid nanoparticle comprises 45 – 55 mole percent (mol%) ionizable amino lipid. For example, lipid nanoparticle may comprise 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, or 55 mol% ionizable amino lipid. Ionizable Amino Lipids In some embodiments, the ionizable lipid is a compound of Formula (IL*) (IL*) or a salt thereof, wherein: R1is -OH, -NRN-C4-10cycloalkenyl optionally substituted with one or more oxo or - N(RN’RN’’); RNis H or C1-6 alkyl; RN’is H or C1-6alkyl; RN’’is H or C1-6alkyl; o is 1, 2, 3, or 4; n is 4, 5, 6, 7, or 8; m is 4, 5, 6, 7, or 8; M is -C(=O)-O-* or -O-C(=O)-*, wherein * indicates attachment to R2; M’ is -C(=O)-O-* or -O-C(=O)-*, wherein * indicates attachment to R3; R2is or –(C1-6alkylene)-(C3-8cycloalkyl)-C1-6alkyl; R2ais -H or C1-10 alkyl; R2bis -H or C1-10 alkyl; R2cis C1-8alkyl or C2-8alkenyl; R3is ; R3ais H or C1-10 alkyl; R3bis H or C1-8alkyl; and R3cis C1-10 alkyl or C2-8 alkenyl. In some embodiments, the ionizable lipid is of Formula (IL**-I): (IL**-I) or a salt thereof, wherein: R1is -OH; o is 2, 3, or 4; n is 4, 5, 6, 7, or 8; M is -C(=O)-O-*, wherein * indicates attachment to R2; m is 6, 7, or 8; M’ is -C(=O)-O-*, wherein * indicates attachment to R3; R2cis C4-8 alkyl; R3ais C7-10 alkyl; and R3cis C3-5alkyl. In some embodiments, the ionizable lipid is of Formula (IL**-III): (IL**-III) or a salt thereof, wherein: R1is NRN-C4-10 cycloalkenyl optionally substituted with one or more oxo or - N(RN’RN’’); RNis H; RN’is C1-2alkyl; RN’’is H; o is 2, 3, or 4; n is 6, 7, or 8; M is -C(=O)-O-*, wherein * indicates attachment to R2; m is 6, 7, or 8; M’ is -C(=O)-O-*, wherein * indicates attachment to R3; R2ais C7-10 alkyl; R2cis C4-6 alkyl; R3ais C1-3alkyl; and R3cis C4-6alkyl. In some embodiments, the ionizable lipid is of Formula (IL**-IV): (IL**-IV) or a salt thereof, wherein: R1is OH; o is 2, 3, or 4; n is 6, 7, or 8; M is -C(=O)-O-*, wherein * indicates attachment to R2; m is 6, 7, or 8; M’ is -C(=O)-O-*, wherein * indicates attachment to R3; R2bis C3-5 alkyl; R2cis C2-4 alkyl; R3ais C7-10alkyl; and R3cis C4-6alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-I): (IL*-Ia) or a salt thereof, wherein: R1, o, m, n, M, M’, R2c, and R3care as defined for variable IL*; and R3ais C1-8alkyl. In some embodiments, ionizable lipid is of Formula (IL*-Ia): or a salt thereof, wherein: R1, o, m, n, M, M’, R2c, and R3care as defined for Formula IL*; and R3ais C1-8alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-Ia’): or a salt thereof, wherein: o, M, M’, R2cand R3care as defined for variable IL*; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IIa): (IL*-IIa) or a salt thereof, wherein: R1, o, m, n, M, M’, R2c, and R3care as defined for Formula IL*; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-II’): or a salt thereof, wherein: o, M, M’, R2cand R3care as defined for variable IL*; and R3ais C1-8alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-III): (IL*-III) or a salt thereof, wherein: R1, o, m, n, M, M’, R2c, and R3care as defined for variable IL*; R2ais a C1-8alkyl; and R3ais C1-8alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IIIa): (IL*-IIIa) or a salt thereof, wherein: R1, o, m, n, M, M’, R2c, and R3care as defined for variable IL*; R2bis a C1-8 alkyl; and R3ais C1-8alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IIIa): (IL*-IIIa) or a salt thereof, wherein: R1, o, M, M’, R2c, and R3care as defined for variable IL*; R2ais a C1-8alkyl; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IIIa’): (IL*-IIIa’) or a salt thereof, wherein: R1, o, M, M’, R2c, and R3care as defined for variable IL*; R2ais a C1-8alkyl; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IIIb): (IL*-IIIb) or a salt thereof, wherein: R1, o, M, M’, R2c, and R3care as defined for variable IL*; R2ais a C1-8alkyl; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IIIb’): (IL*-IIIb’) or a salt thereof, wherein: R1, o, M, M’, R2c, and R3care as defined for variable IL*; R2ais a C1-8alkyl; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IV): (IL*-IV) or a salt thereof, wherein: R1, o, m, n, M, M’, R2c, and R3care as defined for variable IL*; R2bis a C1-8alkyl; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IVa): (IL*-IVa) or a salt thereof, wherein: R1, o, m, n, M, M’, R2c, and R3care as defined for variable IL*; R2bis a C1-8alkyl; and R3ais C1-8 alkyl. In some embodiments, the ionizable lipid is of Formula (IL*-IVa’): (IL*-IVa) or a salt thereof, wherein: o, M, M’, R2c, and R3care as defined for variable IL*; R2ais a C1-8alkyl; and R3ais C1-8 alkyl. Variables o, R1, RN, RN’, RN’’of Ionizable Lipid In some embodiments of the ionizable lipid, o is 1. In some embodiments of the ionizable lipid, o is 2. In some embodiments of the ionizable lipid, o is 3. In some embodiments of the ionizable lipid, o is 4. In some embodiments of the ionizable lipid, R1is -OH. In some embodiments of the ionizable lipid, RNis H. In some embodiments of the ionizable lipid, RNis methyl. In some embodiments of the ionizable lipid, RNis ethyl. In some embodiments of the ionizable lipid, R1is -NRN-cyclobutenyl, wherein the cyclobutenyl is optionally substituted with one or more oxo or -N(RN’RN’’). In some embodiments of the ionizable lipid, RN’is H. In some embodiments of the ionizable lipid, RN’is methyl. In some embodiments of the ionizable lipid, RN’is ethyl. In some embodiments of the ionizable lipid, RN’’is H. In some embodiments of the ionizable lipid, RN’’is methyl. In some embodiments of the ionizable lipid, RN’’is ethyl. In some embodiments of the ionizable lipid, RN’is H and RN’’is methyl. In some embodiments of the ionizable lipid, In some embodiments of the ionizable lipid, Variables m and n of the Ionizable Lipid In some embodiments of the ionizable lipid, m is 4. In some embodiments of the ionizable lipid, m is 5. In some embodiments of the ionizable lipid, m is 6. In some embodiments of the ionizable lipid, m is 7. In some embodiments of the ionizable lipid, m is 8. In some embodiments of the ionizable lipid, m is 4. In some embodiments of the ionizable lipid, n is 5. In some embodiments of the ionizable lipid, n is 6. In some embodiments of the ionizable lipid, n is 7. In some embodiments of the ionizable lipid, n is 8. In some embodiments of the ionizable lipid, n is 5 and m is 7. In some embodiments of the ionizable lipid, n is 7 and m is 7. In some embodiments of the ionizable lipid, m is 6 and n is 6. Variables M and M’ In some embodiments of the ionizable lipid, M is -O-C(=O)-*, wherein * indicates attachment to R2. In some embodiments of the ionizable lipid, M is -C(=O)-O-* wherein * indicates attachment to R2. In some embodiments of the ionizable lipid, M’ is -O-C(=O)-*, wherein * indicates attachment to R3. In some embodiments of the ionizable lipid, M’ is -C(=O)-O-* wherein * indicates attachment to R3. In some embodiments of the ionizable lipid, M is -O-C(=O)-*, wherein * indicates attachment to R2, and M’ is -C(=O)-O-* wherein * indicates attachment to R3Variables R2, R2a, R2b, R2cIn some embodiments of the ionizable lipid, R2is . In some embodiments of the ionizable lipid, R2ais hydrogen. In some embodiments of the ionizable lipid, R2ais methyl. In some embodiments of the ionizable lipid, R2ais ethyl. In some embodiments of the ionizable lipid, R2ais propyl. In some embodiments of the ionizable lipid, R2ais butyl. In some embodiments of the ionizable lipid, R2ais pentyl. In some embodiments of the ionizable lipid, R2ais hexyl. In some embodiments of the ionizable lipid, R2ais heptyl. In some embodiments of the ionizable lipid, R2ais octyl. In some embodiments of the ionizable lipid, R2bis hydrogen. In some embodiments of the ionizable lipid, R2bis methyl. In some embodiments of the ionizable lipid, R2bis ethyl. In some embodiments of the ionizable lipid, R2bis propyl. In some embodiments of the ionizable lipid, R2bis butyl. In some embodiments of the ionizable lipid, R2bis pentyl. In some embodiments of the ionizable lipid, R2bis hexyl. In some embodiments of the ionizable lipid, R2bis heptyl. In some embodiments of the ionizable lipid, R2bis octyl. In some embodiments of the ionizable lipid, R2ais hydrogen and R2bis hydrogen. In some embodiments of the ionizable lipid, R2ais hexyl and R2bis hydrogen. In some embodiments of the ionizable lipid, R2ais octyl and R2bis hydrogen. In some embodiments of the ionizable lipid, R2ais hydrogen and R2bis butyl. In some embodiments of the ionizable lipid, R2cis methyl. In some embodiments of the ionizable lipid, R2cis ethyl. In some embodiments of the ionizable lipid, R2cis propyl. In some embodiments of the ionizable lipid, R2cis butyl. In some embodiments of the ionizable lipid, R2cis pentyl. In some embodiments of the ionizable lipid, R2cis hexyl. In some embodiments of the ionizable lipid, R2cis heptyl. In some embodiments of the ionizable lipid, R2cis octyl. In some embodiments of the ionizable lipid, R2is –(C1-6 alkylene)-(C3-8 cycloalkyl)-C1-6 alkyl. In some embodiments of the ionizable lipid, R2is –(C1-6alkylene)-(cyclohexyl)-C1-6alkyl. In some embodiments of the ionizable lipid, R2is –(C1-6 alkylene)-(cyclopentyl)-C1-6 alkyl. Variables R3, R3a, R3b, and R3c In some embodiments of the ionizable lipid, R3 is .In some embodiments of the ionizable lipid, R3ais hydrogen. In some embodiments of the ionizable lipid, R3ais methyl. In some embodiments of the ionizable lipid, R3ais ethyl. In some embodiments of the ionizable lipid, R3ais propyl. In some embodiments of the ionizable lipid, R3ais butyl. In some embodiments of the ionizable lipid, R3ais pentyl. In some embodiments of the ionizable lipid, R3ais hexyl. In some embodiments of the ionizable lipid, R3ais heptyl. In some embodiments of the ionizable lipid, R3ais octyl. In some embodiments of the ionizable lipid, R3bis hydrogen. In some embodiments of the ionizable lipid, R3bis methyl. In some embodiments of the ionizable lipid, R3bis ethyl. In some embodiments of the ionizable lipid, R3bis propyl. In some embodiments of the ionizable lipid, R3bis butyl. In some embodiments of the ionizable lipid, R3bis pentyl. In some embodiments of the ionizable lipid, R3bis hexyl. In some embodiments of the ionizable lipid, R3bis heptyl. In some embodiments of the ionizable lipid, R3bis octyl. In some embodiments of the ionizable lipid, R3ais octyl and R3bis hydrogen. In some embodiments of the ionizable lipid, R3ais ethyl and R3bis hydrogen. In some embodiments of the ionizable lipid, R3ais hexyl and R3bis hydrogen. In some embodiments of the ionizable lipid, R3cis methyl. In some embodiments of the ionizable lipid, R3cis ethyl. In some embodiments of the ionizable lipid, R3cis propyl. In some embodiments of the ionizable lipid, R3cis butyl. In some embodiments of the ionizable lipid, R3cis pentyl. In some embodiments of the ionizable lipid, R3cis hexyl. In some embodiments of the ionizable lipid, R3cis heptyl. In some embodiments of the ionizable lipid, R3cis octyl. It is understood that, for an ionizable lipid, variables o, R1, RN, RN’, RN’, m, n, M, M’, R2, R2a, R2b, R2c, R3, R3a, R3b, and R3ccan each be, where applicable, selected from the groups described herein, and any group described herein for any of variables o,.R1, RN, RN’, RN’, m, n, M, M’, R2, R2a, R2b, R2c, R3, R3a, R3b, and R3ccan be combined, where applicable, with any group described herein for one or more of the remainder of variables o, R1, RN, RN’, RN’, m, n, M, M’, R2, R2a, R2b, R2c, R3, R3a, R3b, and R3c. In some embodiments, the ionizable lipid is a compound selected from: In some embodiments, the ionizable lipid is In some embodiments, the ionizable lipid is In some embodiments, the ionizable lipid is In some embodiments, the ionizable lipid is Without wishing to be bound by theory, it is understood that an ionizable lipid may have a positive or partial positive charge at physiological pH. Such lipids may be referred to as cationic or ionizable (amino)lipids. Lipids may also be zwitterionic, i.e., neutral molecules having both a positive and a negative charge. Non-cationic lipids In certain embodiments, the lipid nanoparticles described herein comprise one or more non-cationic lipids. Non-cationic lipids may be phospholipids. In some embodiments, the lipid nanoparticle comprises 5-25 mol% non-cationic lipid. For example, the lipid nanoparticle may comprise 5-20 mol%, 5-15 mol%, 5-10 mol%, 10-25 mol%, 10-20 mol%, 10-25 mol%, 15-25 mol%, 15-20 mol%, or 20-25 mol% non-cationic lipid. In some embodiments, the lipid nanoparticle comprises 5 mol%, 10 mol%, 15 mol%, 20 mol%, or 25 mol% non-cationic lipid. In some embodiments, a non-cationic lipid of the disclosure comprises 1,2-distearoyl-sn- glycero-3-phosphocholine (DSPC), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC), 1,2-dimyristoyl-sn-gly cero- phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), l,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC), 1,2-diundecanoyl-sn-glycero-phosphocholine (DUPC), 1- palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-di-O-octadecenyl-sn-glycero-3- phosphocholine (18:0 Diether PC), 1-oleoyl-2 cholesterylhemisuccinoyl-sn-glycero-3- phosphocholine (OChemsPC), 1-hexadecyl-sn-glycero-3-phosphocholine (C16 Lyso PC), 1,2- dilinolenoyl-sn-glycero-3-phosphocholine,1,2-diarachidonoyl-sn-glycero-3-phosphocholine, 1,2- didocosahexaenoyl-sn-glycero-3-phosphocholine, 1,2-diphytanoyl-sn-glycero-3- phosphoethanolamine (ME 16.0 PE), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine, 1,2- dilinoleoyl-sn-glycero-3-phosphoethanolamine, 1,2-dilinolenoyl-sn-glycero-3- phosphoethanolamine, 1,2-diarachidonoyl-sn-glycero-3-phosphoethanolamine, 1,2- didocosahexaenoyl-sn-glycero-3-phosphoethanolamine, 1,2-dioleoyl-sn-glycero-3-phospho-rac- (1-glycerol) sodium salt (DOPG), sphingomyelin, or mixtures thereof. In some embodiments, the lipid nanoparticle comprises 5 – 15 mol%, 5 – 10 mol%, or 10 – 15 mol% DSPC. For example, the lipid nanoparticle may comprise 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 mol% DSPC. In certain embodiments, the lipid composition of the lipid nanoparticle composition disclosed herein can comprise one or more phospholipids, for example, one or more saturated or (poly)unsaturated phospholipids or a combination thereof. In general, phospholipids comprise a phospholipid moiety and one or more fatty acid moieties. A phospholipid moiety can be selected, for example, from the non-limiting group consisting of phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl glycerol, phosphatidyl serine, phosphatidic acid, 2-lysophosphatidyl choline, and a sphingomyelin. A fatty acid moiety can be selected, for example, from the non-limiting group consisting of lauric acid, myristic acid, myristoleic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, linoleic acid, alpha-linolenic acid, erucic acid, phytanoic acid, arachidic acid, arachidonic acid, eicosapentaenoic acid, behenic acid, docosapentaenoic acid, and docosahexaenoic acid. Particular phospholipids can facilitate fusion to a membrane. For example, a cationic phospholipid can interact with one or more negatively charged phospholipids of a membrane (e.g., a cellular or intracellular membrane). Fusion of a phospholipid to a membrane can allow one or more elements (e.g., a therapeutic agent) of a lipid-containing composition (e.g., LNPs) to pass through the membrane permitting, e.g., delivery of the one or more elements to a target tissue. Non-natural phospholipid species including natural species with modifications and substitutions including branching, oxidation, cyclization, and alkynes are also contemplated. For example, a phospholipid can be functionalized with or cross-linked to one or more alkynes (e.g., an alkenyl group in which one or more double bonds is replaced with a triple bond). Under appropriate reaction conditions, an alkyne group can undergo a copper-catalyzed cycloaddition upon exposure to an azide. Such reactions can be useful in functionalizing a lipid bilayer of a nanoparticle composition to facilitate membrane permeation or cellular recognition or in conjugating a nanoparticle composition to a useful component such as a targeting or imaging moiety (e.g., a dye). Phospholipids include, but are not limited to, glycerophospholipids such as phosphatidylcholines, phosphatidylethanolamines, phosphatidylserines, phosphatidylinositols, phosphatidy glycerols, and phosphatidic acids. Phospholipids also include phosphosphingolipid, such as sphingomyelin. In some embodiments, a phospholipid comprises 1,2-distearoyl-sn-glycero-3- phosphocholine (DSPC), 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine (DSPE), 1,2- dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), 1,2-dilinoleoyl-sn-glycero-3- phosphocholine (DLPC), 1,2-dimyristoyl-sn-gly cero-phosphocholine (DMPC), 1,2-dioleoyl-sn- glycero-3-phosphocholine (DOPC), l,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2- diundecanoyl-sn-glycero-phosphocholine (DUPC), 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphocholine (POPC), 1,2-di-O-octadecenyl-sn-glycero-3-phosphocholine (18:0 Diether PC), 1-oleoyl-2 cholesterylhemisuccinoyl-sn-glycero-3-phosphocholine (OChemsPC), 1-hexadecyl- sn-glycero-3-phosphocholine (C16 Lyso PC), 1,2-dilinolenoyl-sn-glycero-3-phosphocholine,1,2- diarachidonoyl-sn-glycero-3-phosphocholine, 1,2-didocosahexaenoyl-sn-glycero-3- phosphocholine, 1,2-diphytanoyl-sn-glycero-3-phosphoethanolamine (ME 16.0 PE), 1,2- distearoyl-sn-glycero-3-phosphoethanolamine, 1,2-dilinoleoyl-sn-glycero-3- phosphoethanolamine, 1,2-dilinolenoyl-sn-glycero-3-phosphoethanolamine, 1,2-diarachidonoyl- sn-glycero-3-phosphoethanolamine, 1,2-didocosahexaenoyl-sn-glycero-3-phosphoethanolamine, 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DOPG), sphingomyelin, or mixtures thereof. Formula (HI) In certain embodiments, a phospholipid is an analog or variant of DSPC. In certain embodiments, a phospholipid is a compound of Formula (HI): (HI), or a salt thereof, wherein: each R1is independently optionally substituted alkyl; or optionally two R1are joined together with the intervening atoms to form optionally substituted monocyclic carbocyclyl or optionally substituted monocyclic heterocyclyl; or optionally three R1are joined together with the intervening atoms to form optionally substituted bicyclic carbocyclyl or optionally substitute bicyclic heterocyclyl; n is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; m is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; A is of the formula: each instance of L2is independently a bond or optionally substituted C1-6 alkylene, wherein one methylene unit of the optionally substituted C1-6alkylene is optionally replaced with O, N(RN), S, C(O), C(O)N(RN), NRNC(O), C(O)O, OC(O), OC(O)O, OC(O)N(RN), NRNC(O)O, or NRNC(O)N(RN); each instance of R2is independently optionally substituted C1-30alkyl, optionally substituted C1-30 alkenyl, or optionally substituted C1-30 alkynyl; optionally wherein one or more methylene units of R2are independently replaced with optionally substituted carbocyclylene, optionally substituted heterocyclylene, optionally substituted arylene, optionally substituted heteroarylene, N(RN), O, S, C(O), C(O)N(RN), NRNC(O), NRNC(O)N(RN), C(O)O, OC(O), - OC(O)O, OC(O)N(RN), NRNC(O)O, C(O)S, SC(O), C(=NRN), C(=NRN)N(RN), NRNC(=NRN), NRNC(=NRN)N(RN), C(S), C(S)N(RN), NRNC(S), NRNC(S)N(RN), S(O), OS(O), S(O)O, - OS(O)O, OS(O)2, S(O)2O, OS(O)2O, N(RN)S(O), S(O)N(RN), N(RN)S(O)N(RN), OS(O)N(RN), N(RN)S(O)O, S(O)2, N(RN)S(O)2, S(O)2N(RN), N(RN)S(O)2N(RN), OS(O)2N(RN), or - N(RN)S(O)2O; each instance of RNis independently hydrogen, optionally substituted alkyl, or a nitrogen protecting group; Ring B is optionally substituted carbocyclyl, optionally substituted heterocyclyl, optionally substituted aryl, or optionally substituted heteroaryl; and p is 1 or 2. In certain embodiments, the compound is not of the formula: , wherein each instance of R2is independently unsubstituted alkyl, unsubstituted alkenyl, or unsubstituted alkynyl. In some embodiments, the phospholipids may be one or more of the phospholipids described in PCT Application No. PCT / US2018 / 037922. In some embodiments, the lipid nanoparticle comprises a molar ratio of 5-25% non- cationic lipid relative to the other lipid components. For example, the lipid nanoparticle may comprise a molar ratio of 5-30%, 5-15%, 5-10%, 10-25%, 10-20%, 10-25%, 15-25%, 15-20%, 20-25%, or 25-30% non-cationic lipid. In some embodiments, the lipid nanoparticle comprises a molar ratio of 5%, 10%, 15%, 20%, 25%, or 30% non-cationic lipid. In some embodiments, the lipid nanoparticle comprises a molar ratio of 5-25% phospholipid relative to the other lipid components. For example, the lipid nanoparticle may comprise a molar ratio of 5-30%, 5-15%, 5-10%, 10-25%, 10-20%, 10-25%, 15-25%, 15-20%, 20-25%, or 25-30% phospholipid. In some embodiments, the lipid nanoparticle comprises a molar ratio of 5%, 10%, 15%, 20%, 25%, or 30% phospholipid lipid. Structural lipids The lipid composition of a pharmaceutical composition disclosed herein can comprise one or more structural lipids. As used herein, the term “structural lipid” includes sterols and also to lipids containing sterol moieties. Incorporation of structural lipids in the lipid nanoparticle may help mitigate aggregation of other lipids in the particle. Structural lipids can be selected from the group including but not limited to, cholesterol, fecosterol, sitosterol, ergosterol, campesterol, stigmasterol, brassicasterol, tomatidine, tomatine, ursolic acid, alpha-tocopherol, hopanoids, phytosterols, steroids, and mixtures thereof. In some embodiments, the structural lipid is a sterol. As defined herein, “sterols” are a subgroup of steroids consisting of steroid alcohols. In certain embodiments, the structural lipid is a steroid. In certain embodiments, the structural lipid is cholesterol. In certain embodiments, the structural lipid is an analog of cholesterol. In certain embodiments, the structural lipid is alpha-tocopherol. In some embodiments, the structural lipids may be one or more of the structural lipids described in U.S. Application No.16 / 493,814. In some embodiments, the lipid nanoparticle comprises a molar ratio of 25-55% structural lipid relative to the other lipid components. For example, the lipid nanoparticle may comprise a molar ratio of 10- 55%, 25-50%, 25-45%, 25-40%, 25-35%, 25-30%, 30-55%, 30- 50%, 30-45%, 30-40%, 30-35%, 35-55%, 35-50%, 35-45%, 35-40%, 40-55%, 40-50%, 40-45%, 45-55%, 45-50%, or 50-55% structural lipid. In some embodiments, the lipid nanoparticle comprises a molar ratio of 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or 55% structural lipid. In some embodiments, the lipid nanoparticle comprises 30-45 mol% sterol, optionally 35- 40 mol%, for example, 30-31 mol%, 31-32 mol%, 32-33 mol%, 33-34 mol%, 34-35 mol%, 35- 36 mol%, 36-37 mol%, 37-38 mol%, 38-39 mol%, or 39-40 mol%. In some embodiments, the lipid nanoparticle comprises 25-55 mol% sterol. For example, the lipid nanoparticle may comprise 25-50 mol%, 25-45 mol%, 25-40 mol%, 25-35 mol%, 25-30 mol%, 30-55 mol%, 30- 50 mol%, 30-45 mol%, 30-40 mol%, 30-35 mol%, 35-55 mol%, 35-50 mol%, 35-45 mol%, 35- 40 mol%, 40-55 mol%, 40-50 mol%, 40-45 mol%, 45-55 mol%, 45-50 mol%, or 50-55 mol% sterol. In some embodiments, the lipid nanoparticle comprises 25 mol%, 30 mol%, 35 mol%, 40 mol%, 45 mol%, 50 mol%, or 55 mol% sterol. In some embodiments, the lipid nanoparticle comprises 35 – 40 mol% cholesterol. For example, the lipid nanoparticle may comprise 35, 35.5, 36, 36.5, 37, 37.5, 38, 38.5, 39, 39.5, or 40 mol% cholesterol. Polyethylene glycol (PEG)-Lipids The lipid composition of a pharmaceutical composition disclosed herein can comprise one or more polyethylene glycol (PEG) lipids. As used herein, the term “PEG-lipid” or “PEG-modified lipid” refers to polyethylene glycol (PEG)-modified lipids. Non-limiting examples of PEG-lipids include PEG-modified phosphatidylethanolamine and phosphatidic acid, PEG-ceramide conjugates (e.g., PEG-CerC14 or PEG-CerC20), PEG-modified dialkylamines, and PEG-modified 1,2-diacyloxypropan-3- amines. Such lipids are also referred to as PEGylated lipids. For example, a PEG lipid can be PEG-c-DOMG, PEG-DMG, PEG-DLPE, PEG-DMPE, PEG-DPPC, or a PEG-DSPE lipid. In some embodiments, the PEG-lipid includes, but not limited to 1,2-dimyristoyl-sn- glycerol methoxypolyethylene glycol (PEG-DMG), 1,2-distearoyl-sn-glycero-3- phosphoethanolamine-N-[amino(polyethylene glycol)] (PEG-DSPE), PEG-disteryl glycerol (PEG-DSG), PEG-dipalmetoleyl, PEG-dioleyl, PEG-distearyl, PEG-diacylglycamide (PEG- DAG), PEG-dipalmitoyl phosphatidylethanolamine (PEG-DPPE), or PEG-l,2- dimyristyloxlpropyl-3-amine (PEG-c-DMA). In some embodiments, the PEG-lipid is selected from the group consisting of a PEG- modified phosphatidylethanolamine, a PEG-modified phosphatidic acid, a PEG-modified ceramide, a PEG-modified dialkylamine, a PEG-modified diacylglycerol, a PEG-modified dialkylglycerol, and mixtures thereof. In some embodiments, the PEG-modified lipid is PEG- DMG, PEG-c-DOMG (also referred to as PEG-DOMG), PEG-DSG, and / or PEG-DPG. In some embodiments, the lipid moiety of the PEG-lipids includes those having lengths of from about C14to about C22, preferably from about C14to about C16. In some embodiments, a PEG moiety, for example an mPEG-NH2, has a size of about 1000, 2000, 5000, 10,000, 15,000 or 20,000 daltons. In some embodiments, the PEG-lipid is PEG2k-DMG. In some embodiments, the lipid nanoparticles described herein can comprise a PEG lipid which is a non-diffusible PEG. Non-limiting examples of non-diffusible PEGs include PEG- DSG and PEG-DSPE. PEG-lipids are known in the art, such as those described in U.S. Patent No.8158601 and International Publ. No. WO 2015 / 130584 A2, which are incorporated herein by reference in their entirety. In general, some of the other lipid components (e.g., PEG lipids) of various formulae described herein may be synthesized as described International Patent Application No. PCT / US2016 / 000129, filed December 10, 2016, entitled “Compositions and Methods for Delivery of Therapeutic Agents,” which is incorporated by reference in its entirety. The lipid component of a lipid nanoparticle composition may include one or more molecules comprising polyethylene glycol, such as PEG or PEG-modified lipids. Such species may be alternately referred to as PEGylated lipids. A PEG lipid is a lipid modified with polyethylene glycol. A PEG lipid may be selected from the non-limiting group including PEG- modified phosphatidylethanolamines, PEG-modified phosphatidic acids, PEG-modified ceramides, PEG-modified dialkylamines, PEG-modified diacylglycerols, PEG-modified dialkylglycerols, and mixtures thereof. For example, a PEG lipid may be PEG-c-DOMG, PEG- DMG, PEG-DLPE, PEG-DMPE, PEG-DPPC, or a PEG-DSPE lipid. In some embodiments the PEG-modified lipids are a modified form of PEG DMG. PEG- DMG has the following structure: In some embodiments, PEG lipids can be PEGylated lipids described in International Publication No. WO2012099755, the contents of which is herein incorporated by reference in its entirety. Any of these exemplary PEG lipids described herein may be modified to comprise a hydroxyl group on the PEG chain. In certain embodiments, the PEG lipid is a PEG-OH lipid. As generally defined herein, a “PEG-OH lipid” (also referred to herein as “hydroxy-PEGylated lipid”) is a PEGylated lipid having one or more hydroxyl (–OH) groups on the lipid. In certain embodiments, the PEG-OH lipid includes one or more hydroxyl groups on the PEG chain. In certain embodiments, a PEG-OH or hydroxy-PEGylated lipid comprises an –OH group at the terminus of the PEG chain. Each possibility represents a separate embodiment. Formula (PI) In certain embodiments, a PEG lipid is a compound of Formula (PI): (PI), or salts thereof, wherein: R3is –ORO; ROis hydrogen, optionally substituted alkyl, or an oxygen protecting group; r is an integer between 1 and 100, inclusive; L1is optionally substituted C1-10 alkylene, wherein at least one methylene of the optionally substituted C1-10 alkylene is independently replaced with optionally substituted carbocyclylene, optionally substituted heterocyclylene, optionally substituted arylene, optionally substituted heteroarylene, O, N(RN), S, C(O), C(O)N(RN), NRNC(O), C(O)O, OC(O), OC(O)O, OC(O)N(RN), NRNC(O)O, or NRNC(O)N(RN); D is a moiety obtained by click chemistry or a moiety cleavable under physiological conditions; m is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; A is of the formula: each instance of L2is independently a bond or optionally substituted C1-6 alkylene, wherein one methylene unit of the optionally substituted C1-6alkylene is optionally replaced with O, N(RN), S, C(O), C(O)N(RN), NRNC(O), C(O)O, OC(O), OC(O)O, OC(O)N(RN), NRNC(O)O, or NRNC(O)N(RN); each instance of R2is independently optionally substituted C1-30 alkyl, optionally substituted C1-30alkenyl, or optionally substituted C1-30alkynyl; optionally wherein one or more methylene units of R2are independently replaced with optionally substituted carbocyclylene, optionally substituted heterocyclylene, optionally substituted arylene, optionally substituted heteroarylene, N(RN), O, S, C(O), C(O)N(RN), NRNC(O), NRNC(O)N(RN), C(O)O, OC(O), - OC(O)O, OC(O)N(RN), NRNC(O)O, C(O)S, SC(O), C(=NRN), C(=NRN)N(RN), NRNC(=NRN), NRNC(=NRN)N(RN), C(S), C(S)N(RN), NRNC(S), NRNC(S)N(RN), S(O) , OS(O), S(O)O, - OS(O)O, OS(O)2, S(O)2O, OS(O)2O, N(RN)S(O), S(O)N(RN), N(RN)S(O)N(RN), OS(O)N(RN), N(RN)S(O)O, S(O)2, N(RN)S(O)2, S(O)2N(RN), N(RN)S(O)2N(RN), OS(O)2N(RN), or - N(RN)S(O)2O; each instance of RNis independently hydrogen, optionally substituted alkyl, or a nitrogen protecting group; Ring B is optionally substituted carbocyclyl, optionally substituted heterocyclyl, optionally substituted aryl, or optionally substituted heteroaryl; and p is 1 or 2. In certain embodiments, the compound of Fomula (PI) is a PEG-OH lipid (i.e., R3is – ORO, and ROis hydrogen). In certain embodiments, the compound of Formula (PI) is of Formula (PI-OH): (PI-OH), or a salt thereof. Formula (PII) In certain embodiments, a PEG lipid is a PEGylated fatty acid. In certain embodiments, a PEG lipid is a compound of Formula (PII). In some embodiments, compounds of Formula (PII) have the following formula: (PII), or a salts thereof, wherein: R3is–ORO; ROis hydrogen, optionally substituted alkyl or an oxygen protecting group; r is an integer between 1 and 100, inclusive; R5is optionally substituted C10-40alkyl, optionally substituted C10-40alkenyl, or optionally substituted C10-40 alkynyl; and optionally one or more methylene groups of R5are replaced with optionally substituted carbocyclylene, optionally substituted heterocyclylene, optionally substituted arylene, optionally substituted heteroarylene, N(RN), O, S, C(O), C(O)N(RN), - NRNC(O), NRNC(O)N(RN), C(O)O, OC(O), OC(O)O, OC(O)N(RN), NRNC(O)O, C(O)S, SC(O), C(=NRN), C(=NRN)N(RN), NRNC(=NRN), NRNC(=NRN)N(RN), C(S), C(S)N(RN), NRNC(S), - NRNC(S)N(RN), S(O), OS(O), S(O)O, OS(O)O, OS(O)2, S(O)2O, OS(O)2O, N(RN)S(O), - S(O)N(RN), N(RN)S(O)N(RN), OS(O)N(RN), N(RN)S(O)O, S(O)2, N(RN)S(O)2, S(O)2N(RN), - N(RN)S(O)2N(RN), OS(O)2N(RN), or N(RN)S(O)2O; and each instance of RNis independently hydrogen, optionally substituted alkyl, or a nitrogen protecting group. In certain embodiments, the compound of Formula (PII) is of Formula (PII-OH): or a salt thereof. In some embodiments, r is 40-50. . In some embodiments, the lipid composition of the pharmaceutical compositions disclosed herein does not comprise a PEG-lipid. In some embodiments, the PEG-lipids may be one or more of the PEG lipids described in U.S. Application No. US15 / 674,872. In some embodiments, the lipid nanoparticle comprises a molar ratio of 0.5-15% PEG lipid relative to the other lipid components. For example, the lipid nanoparticle may comprise a molar ratio of 0.5-10%, 0.5-5%, 1-15%, 1-10%, 1-5%, 2-15%, 2-10%, 2-5%, 5-15%, 5-10%, or 10-15% PEG lipid. In some embodiments, the lipid nanoparticle comprises a molar ratio of 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15% PEG- lipid. In some embodiments, the lipid nanoparticle comprises 1-5% PEG-modified lipid, optionally 1-3 mol%, for example 1.5 to 2.5 mol%, 1-2 mol%, 2-3 mol%, 3-4 mol%, or 4-5 mol%. In some embodiments, the lipid nanoparticle comprises 0.5-15 mol% PEG-modified lipid. For example, the lipid nanoparticle may comprise 0.5-10 mol%, 0.5-5 mol%, 1-15 mol%, 1-10 mol%, 1-5 mol%, 2-15 mol%, 2-10 mol%, 2-5 mol%, 5-15 mol%, 5-10 mol%, or 10-15 mol%. In some embodiments, the lipid nanoparticle comprises 0.5 mol%, 1 mol%, 2 mol%, 3 mol%, 4 mol%, 5 mol%, 6 mol%, 7 mol%, 8 mol%, 9 mol%, 10 mol%, 11 mol%, 12 mol%, 13 mol%, 14 mol%, or 15 mol% PEG-modified lipid. Some embodiments comprise adding PEG to a composition comprising an LNP encapsulating a nucleic acid (e.g., which already includes PEG in the amounts listed above). In embodiments comprise adding about 0.5mo% or more PEG to an LNP composition, such as about 1mol%, about 1.5mol%, about 2mol%, about 2.5mol%, about 3mol%, about 3.5mol%, about 4mol%, about 5mol%, or more after formation of an LNP composition (e.g., which already contains PEG in amount listed elsewhere herein). In some embodiments, the lipid nanoparticle comprises 20-60 mol% ionizable amino lipid, 5-25 mol% non-cationic lipid, 25-55 mol% sterol, and 0.5-15 mol% PEG-modified lipid. In some embodiments, a LNP of the disclosure comprises an ionizable amino lipid of Compound 1, wherein the non-cationic lipid is DSPC, the structural lipid that is cholesterol, and the PEG lipid is DMG-PEG. In some embodiments, a LNP of the disclosure comprises an ionizable amino lipid of Compound 2, wherein the non-cationic lipid is DSPC, the structural lipid that is cholesterol, and the PEG lipid is DMG-PEG. In some embodiments, a LNP comprises an ionizable amino lipid of any of Formula (AIII), (AIV), or (AV), a phospholipid comprising DSPC, a structural lipid, and a PEG lipid comprising PEG-DMG. In some embodiments, a LNP comprises an ionizable amino lipid of any of Formula (AIII), (AIV), or (AV), a phospholipid comprising DSPC, a structural lipid, and a PEG lipid comprising a compound having Formula (PII). In some embodiments, a LNP comprises an ionizable amino lipid of Formula (AIII), (AIV), or (AV), a phospholipid comprising a compound having Formula (HI), a structural lipid, and the PEG lipid comprising a compound having Formula (PI) or (PII). In some embodiments, a LNP comprises an ionizable amino lipid of Formula (AIII), (AIV), or (AV), a phospholipid comprising a compound having Formula (HI), a structural lipid, and the PEG lipid comprising a compound having Formula (PI) or (PII). In some embodiments, a LNP comprises an ionizable amino lipid of Formula (AIII), (AIV), or (AV), a phospholipid having Formula (HI), a structural lipid, and a PEG lipid comprising a compound having Formula (PII). In some embodiments, the lipid nanoparticle comprises 49 mol% ionizable amino lipid, 10 mol% DSPC, 38.5 mol% cholesterol, and 2.5 mol% DMG-PEG. In some embodiments, the lipid nanoparticle comprises 49 mol% ionizable amino lipid, 11 mol% DSPC, 38.5 mol% cholesterol, and 1.5 mol% DMG-PEG. In some embodiments, the lipid nanoparticle comprises 48 mol% ionizable amino lipid, 11 mol% DSPC, 38.5 mol% cholesterol, and 2.5 mol% DMG-PEG. In some embodiments, a LNP comprises an N:P ratio of from about 2:1 to about 30:1. In some embodiments, a LNP comprises an N:P ratio of about 6:1. In some embodiments, a LNP comprises an N:P ratio of about 3:1, 4:1, or 5:1. In some embodiments, a LNP comprises a wt / wt ratio of the ionizable amino lipid component to the RNA (e.g., mRNA) of from about 10:1 to about 100:1. In some embodiments, a LNP comprises a wt / wt ratio of the ionizable amino lipid component to the RNA (e.g., mRNA) of about 20:1. In some embodiments, a LNP comprises a wt / wt ratio of the ionizable amino lipid component to the RNA (e.g., mRNA) of about 10:1. Some embodiments comprise a composition having one or more LNPs having a diameter of about 150 nm or less, such as about 140 nm, 130 nm, 120 nm, 110 nm, 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm or less. Some embodiments comprise a composition having a mean LNP diameter of about 150 nm or less, such as about 140 nm, 130 nm, 120 nm, 110 nm, 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, or 20 nm or less. In some embodiments, the composition has a mean LNP diameter from about 30nm to about 150nm, or a mean diameter from about 60nm to about 120nm. A LNP may comprise or one or more types of lipids, including but not limited to amino lipids (e.g., ionizable amino lipids), neutral lipids, non-cationic lipids, charged lipids, PEG- modified lipids, phospholipids, structural lipids and sterols. In some embodiments, a LNP may further comprise one or more cargo molecules, including but not limited to nucleic acids (e.g., mRNA, plasmid DNA, DNA or RNA (e.g., mRNA) oligonucleotides, siRNA, shRNA, snRNA, snoRNA, lncRNA, etc.), small molecules, proteins and peptides. In some embodiments, the composition comprises a liposome. A liposome is a lipid particle comprising lipids arranged into one or more concentric lipid bilayers around a central region. The central region of a liposome may comprise an aqueous solution, suspension, or other aqueous composition. In some embodiments, a lipid nanoparticle may comprise two or more components (e.g., amino lipid and nucleic acid, PEG-lipid, phospholipid, structural lipid). For instance, a lipid nanoparticle may comprise an amino lipid and a nucleic acid. Compositions comprising the lipid nanoparticles, such as those described herein, may be used for a wide variety of applications, including the stealth delivery of therapeutic payloads with minimal adverse innate immune response. Effective in vivo delivery of nucleic acids represents a continuing medical challenge. Exogenous nucleic acids (i.e., originating from outside of a cell or organism) are readily degraded in the body, e.g., by the immune system. Accordingly, effective delivery of nucleic acids to cells often requires the use of a particulate carrier (e.g., lipid nanoparticles). The particulate carrier should be formulated to have minimal particle aggregation, be relatively stable prior to intracellular delivery, effectively deliver nucleic acids intracellularly, and illicit no or minimal immune response. To achieve minimal particle aggregation and pre-delivery stability, many conventional particulate carriers have relied on the presence and / or concentration of certain components (e.g., PEG-lipid). However, it has been discovered that certain components may decrease the stability of encapsulated nucleic acids (e.g., mRNA molecules). The reduced stability may limit the broad applicability of the particulate carriers. As such, there remains a need for methods by which to improve the stability of nucleic acid (e.g., mRNA) encapsulated within lipid nanoparticles. In some embodiments, the lipid nanoparticles comprise one or more of ionizable molecules, polynucleotides, and optional components, such as structural lipids, sterols, neutral lipids, phospholipids and a molecule capable of reducing particle aggregation (e.g., polyethylene glycol (PEG), PEG-modified lipid), such as those described above. In some embodiments, a LNP described herein may include one or more ionizable molecules (e.g., amino lipids or ionizable lipids). The ionizable molecule may comprise a charged group and may have a certain pKa. In certain embodiments, the pKa of the ionizable molecule may be greater than or equal to about 6, greater than or equal to about 6.2, greater than or equal to about 6.5, greater than or equal to about 6.8, greater than or equal to about 7, greater than or equal to about 7.2, greater than or equal to about 7.5, greater than or equal to about 7.8, greater than or equal to about 8. In some embodiments, the pKa of the ionizable molecule may be less than or equal to about 10, less than or equal to about 9.8, less than or equal to about 9.5, less than or equal to about 9.2, less than or equal to about 9.0, less than or equal to about 8.8, or less than or equal to about 8.5. Combinations of the above referenced ranges are also possible (e.g., greater than or equal to 6 and less than or equal to about 8.5). Other ranges are also possible. In embodiments in which more than one type of ionizable molecule are present in a particle, each type of ionizable molecule may independently have a pKa in one or more of the ranges described above. In general, an ionizable molecule comprises one or more charged groups. In some embodiments, an ionizable molecule may be positively charged or negatively charged. For instance, an ionizable molecule may be positively charged. For example, an ionizable molecule may comprise an amine group. As used herein, the term “ionizable molecule” has its ordinary meaning in the art and may refer to a molecule or matrix comprising one or more charged moiety. As used herein, a “charged moiety” is a chemical moiety that carries a formal electronic charge, e.g., monovalent (+1, or -1), divalent (+2, or -2), trivalent (+3, or -3), etc. The charged moiety may be anionic (i.e., negatively charged) or cationic (i.e., positively charged). Examples of positively-charged moieties include amine groups (e.g., primary, secondary, and / or tertiary amines), ammonium groups, pyridinium group, guanidine groups, and imidizolium groups. In a particular embodiment, the charged moieties comprise amine groups. Examples of negatively- charged groups or precursors thereof, include carboxylate groups, sulfonate groups, sulfate groups, phosphonate groups, phosphate groups, hydroxyl groups, and the like. The charge of the charged moiety may vary, in some cases, with the environmental conditions, for example, changes in pH may alter the charge of the moiety, and / or cause the moiety to become charged or uncharged. In general, the charge density of the molecule and / or matrix may be selected as desired. In some cases, an ionizable molecule (e.g., an amino lipid or ionizable lipid) may include one or more precursor moieties that can be converted to charged moieties. For instance, the ionizable molecule may include a neutral moiety that can be hydrolyzed to form a charged moiety, such as those described above. As a non-limiting specific example, the molecule or matrix may include an amide, which can be hydrolyzed to form an amine, respectively. Those of ordinary skill in the art will be able to determine whether a given chemical moiety carries a formal electronic charge (for example, by inspection, pH titration, ionic conductivity measurements, etc.), and / or whether a given chemical moiety can be reacted (e.g., hydrolyzed) to form a chemical moiety that carries a formal electronic charge. The ionizable molecule (e.g., amino lipid or ionizable lipid) may have any suitable molecular weight. In certain embodiments, the molecular weight of an ionizable molecule is less than or equal to about 2,500 g / mol, less than or equal to about 2,000 g / mol, less than or equal to about 1,500 g / mol, less than or equal to about 1,250 g / mol, less than or equal to about 1,000 g / mol, less than or equal to about 900 g / mol, less than or equal to about 800 g / mol, less than or equal to about 700 g / mol, less than or equal to about 600 g / mol, less than or equal to about 500 g / mol, less than or equal to about 400 g / mol, less than or equal to about 300 g / mol, less than or equal to about 200 g / mol, or less than or equal to about 100 g / mol. In some instances, the molecular weight of an ionizable molecule is greater than or equal to about 100 g / mol, greater than or equal to about 200 g / mol, greater than or equal to about 300 g / mol, greater than or equal to about 400 g / mol, greater than or equal to about 500 g / mol, greater than or equal to about 600 g / mol, greater than or equal to about 700 g / mol, greater than or equal to about 1000 g / mol, greater than or equal to about 1,250 g / mol, greater than or equal to about 1,500 g / mol, greater than or equal to about 1,750 g / mol, greater than or equal to about 2,000 g / mol, or greater than or equal to about 2,250 g / mol. Combinations of the above ranges (e.g., at least about 200 g / mol and less than or equal to about 2,500 g / mol) are also possible. In embodiments in which more than one type of ionizable molecules are present in a particle, each type of ionizable molecule may independently have a molecular weight in one or more of the ranges described above. In some embodiments, the percentage (e.g., by weight, or by mole) of a single type of ionizable molecule (e.g., amino lipid or ionizable lipid) and / or of all the ionizable molecules within a particle may be greater than or equal to about 15%, greater than or equal to about 16%, greater than or equal to about 17%, greater than or equal to about 18%, greater than or equal to about 19%, greater than or equal to about 20%, greater than or equal to about 21%, greater than or equal to about 22%, greater than or equal to about 23%, greater than or equal to about 24%, greater than or equal to about 25%, greater than or equal to about 30%, greater than or equal to about 35%, greater than or equal to about 40%, greater than or equal to about 42%, greater than or equal to about 45%, greater than or equal to about 48%, greater than or equal to about 50%, greater than or equal to about 52%, greater than or equal to about 55%, greater than or equal to about 58%, greater than or equal to about 60%, greater than or equal to about 62%, greater than or equal to about 65%, or greater than or equal to about 68%. In some instances, the percentage (e.g., by weight, or by mole) may be less than or equal to about 70%, less than or equal to about 68%, less than or equal to about 65%, less than or equal to about 62%, less than or equal to about 60%, less than or equal to about 58%, less than or equal to about 55%, less than or equal to about 52%, le...

Claims

CLAIMS What is claimed is:

1. A human metapneumovirus type A (hMPV-A) fusion (F) protomer, wherein the hMPV-A F protomer comprises at least one mutation selected from the group consisting of: (i) truncation of a cytoplasmic tail; (ii) one or more substitutions of a non-cysteine residue to a cysteine (C) residue; (iii) one or more substitutions of a surface exposed residue to a serine (S) residue or lysine (K) residue; (iv) one or more substitutions of an internal residue to a valine (V) residue or threonine (T) residue; and (v) modification of a F1 / F2 cleavage site with a flexible linker.

2. The hMPV-A F protomer of claim 1, wherein the hMPV-A F protomer does not comprise a cytoplasmic tail sequence of SEQ ID NO: 49 (KKTKKPTGAPPELSGVTNNGFIPHN).

3. The hMPV-A F protomer of claim 1 or 2, wherein the one or more substitutions of a non- cysteine residue to a C residue are present at one or more of positions of 84, 140, 147, 249, 454, and 458; wherein the positions are numbered by alignment to SEQ ID NO:

14.

4. The hMPV-A F protomer of claim 3, wherein the one or more substitutions of a non- cysteine to a C residue are present at positions 84 and 249; optionally wherein the substitutions comprise V84C and A249C.

5. The hMPV-A F protomer of claim 3, wherein the one or more substitutions of a non- cysteine to a C residue are present at positions 140 and 147; optionally wherein the substitutions comprise A140C and A147C.

6. The hMPV-A F protomer of claim 3, wherein the one or more substitutions of a non- cysteine to a C residue are present at positions 454 and 458; optionally wherein the substitutions comprise D454C and V458C.

7. The hMPV-A F protomer of any one of claims 1-6, wherein the one or more substitutions of a surface exposed residue to a S residue or K residue are present at positions 61, 138, or 232; wherein the positions are numbered by alignment to SEQ ID NO: 14.

8. The hMPV-A F protomer of claim 7, wherein the one or more substitutions of a surface exposed residue are selected from the group consisting of: A61S, N138K, and P232S.

9. The hMPV-A F protomer of any one of claims 1-8, wherein the one or more substitutions of an internal residue to a T residue or V residue are present at positions 114 and / or 449; wherein the positions are numbered by alignment to SEQ ID NO:

14.

10. The hMPV-A F protomer of claim 9, wherein the one or more substitutions of an internal residue are selected from the group consisting of: A114T, A114V, I449T, and I449V.

11. The hMPV-A F protomer of any one of claims 1-10, wherein the flexible linker replaces amino acid residues 89-112; wherein the positions are numbered by alignment to SEQ ID NO:

14.

12. The hMPV-A F protomer of claim 11, wherein the flexible linker is GSGGSG (SEQ ID NO: 141).

13. The hMPV-A F protomer of any one of claims 1-12, wherein the hMPV F protomer comprises an amino acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a sequence selected from the group consisting of SEQ ID NOs: 1-3.

14. The hMPV-A F protomer of claim 13, wherein the hMPV F protomer comprises an amino acid sequence selected from the group consisting of SEQ ID NOs: 1-3.

15. A messenger ribonucleic acid (mRNA) encoding the hMPV-A F protomer of any one of claims 1-14.

16. The mRNA of claim 15, wherein the mRNA comprises an open reading frame (ORF) encoding the hMPV-A F protomer, and wherein the ORF comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a sequence selected from the group consisting of SEQ ID NOs: 18-20.

17. The mRNA of claim 16, wherein the ORF comprises a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 18-20.

18. The mRNA of any one of claims 15-17, wherein the mRNA comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 31-33.

19. The mRNA of claim 18, wherein the mRNA comprises a nucleic acid sequence selected from the group consisting of SEQ ID NOs: 31-33.

20. A human metapneumovirus type B (hMPV-B) fusion (F) protomer, wherein the hMPV-B F protomer comprises at least one mutation selected from the group consisting of: (i) truncation of a cytoplasmic tail; (ii) one or more substitutions of a non-cysteine residue to a cysteine (C) residue; and (iii) modification of a F1 / F2 cleavage site with a flexible linker.

21. The hMPV-B F protomer of claim 20, wherein the hMPV-B F protomer does not comprise a cytoplasmic tail sequence of SEQ ID NO: 49 (KKTKKPTGAPPELSGVTNNGFIPHN).

22. The hMPV-B F protomer of claim 20 or 21, wherein the one or more substitutions of a non-cysteine residue to a C residue are present at one or more of positions of 84, 140, 147, and 249; wherein the positions are numbered by alignment to SEQ ID NO:

15.

23. The hMPV-B F protomer of claim 22, wherein the one or more substitutions of a non- cysteine to a C residue are present at positions 84 and 249; optionally wherein the substitutions comprise V84C and A249C.

24. The hMPV-B F protomer of claim 22, wherein the one or more substitutions of a non- cysteine to a C residue are present at positions 140 and 147; optionally wherein the substitutions comprise A140C and A147C.

25. The hMPV-B F protomer of any one of claims 20-24, wherein the flexible linker replaces amino acid residues 89-112; wherein the positions are numbered by alignment to SEQ ID NO: 15.

26. The hMPV-B F protomer of claim 25, wherein the flexible linker is GSGGSG (SEQ ID NO: 141).

27. The hMPV-B F protomer of any one of claims 20-26, wherein the hMPV F protomer comprises an amino acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to the amino acid sequence of SEQ ID NO:

4.

28. The hMPV-B F protomer of claim 27, wherein the hMPV F protomer comprises an amino acid sequence of SEQ ID NO:

4.

29. A messenger ribonucleic acid (mRNA) encoding the hMPV-B F protomer of any one of claims 20-28.

30. The mRNA of claim 29, wherein the mRNA comprises an open reading frame (ORF) encoding the hMPV-B F protomer, and wherein the ORF comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a sequence of SEQ ID NO:

21.

31. The mRNA of claim 30, wherein the ORF comprises a nucleic acid sequence of SEQ ID NO:

21.

32. The mRNA of any one of claims 29-31, wherein the mRNA comprises a nucleic acid sequence with at least 80%, at least 85%, at least 90%, at least 95%, or at least 98% identity to a nucleic acid sequence selected of SEQ ID NO:

34.

33. The mRNA of claim 32, wherein the mRNA comprises a nucleic acid sequence of SEQ ID NO:

34.

34. A composition comprising: (a) the mRNA of any one of claims 15-19; and (b) an mRNA comprising an ORF encoding a human respiratory syncytial virus (hRSV) fusion (F) protein.

35. A composition comprising: (a) the mRNA of any one of claims 29-33; and(b) an mRNA comprising an ORF encoding an hRSV F protein.

36. The composition of claim 34 or 35, wherein the hRSV F protein comprises: (a) an hRSV-A F protein; (b) an hRSV-B F protein; or (c) an hRSV-A F protein and an hRSV-B F protein.

37. The composition of claim 36, wherein the composition comprises: (a) the mRNA of any one of claims 15-19, (b) the mRNA of any one of claims 29-33, and (c) the mRNA comprising an ORF encoding an hRSV-A F protein.

38. The composition of claim 36 or 37, wherein: (a) the hRSV-A protein comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to SEQ ID NO: 13; and / or (b) the hRSV-B protein comprises an amino acid sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to any one of SEQ ID NOs: 5-12.

39. The composition of claim 37 or 38, wherein (a) the hRSV-A protein comprises SEQ ID NO: 13; and / or (b) the hRSV-B protein comprises any one of SEQ ID NOs: 5-12.

40. The composition of any one of claims 36-39, wherein: (a) the ORF encoding the hRSV-A protein comprises a nucleic acid sequence having at 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to SEQ ID NO: 30 or SEQ ID NO: 113; and / or (b) the ORF encoding the hRSV-B protein comprises a nucleic acid sequence having at 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to any one of SEQ ID NOs: 22-29.

41. The composition of claim 40, wherein: (a) the ORF encoding the hRSV-A protein comprises SEQ ID NO: 30 or SEQ ID NO: 113; and / or(b) the ORF encoding the hRSV-B protein comprises any one of SEQ ID NOs: 22-29.

42. The composition of claim 41, wherein the mRNA encoding the hRSV-B protein comprises a nucleic acid sequence having at 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identity to any one of SEQ ID NOs: 35-42.

43. The composition of claim 42, wherein the mRNA encoding the hRSV-B protein comprises any one of SEQ ID NOs: 35-42.

44. A composition comprising: (a) a messenger ribonucleic acid (mRNA) comprising an open reading frame (ORF) encoding a human metapneumovirus-A (hMPV-A) fusion (F) protomer, wherein the hMPV-A F protomer comprises at least one stabilizing mutation relative to SEQ ID NO: 14; and (b) a mRNA comprising an ORF encoding an hMPV-B F protomer, wherein the hMPV-B F protomer comprises at least one stabilizing mutation relative to SEQ ID NO:

15.

45. The composition of claim 44, wherein (a) the mRNA comprising an ORF encoding an hMPV-A F protomer comprises the mRNA of any one of claims 15-19; and (b) the mRNA comprising an ORF encoding an hMPV-B F protomer comprises the mRNA of any one of claims 29-33.

46. The composition of claim 44 or 45, wherein the composition further comprises an mRNA comprising an ORF encoding a hRSV F glycoprotein.

47. The composition of claim 46, wherein (a) the hRSV F glycoprotein is an hRSV-A F glycoprotein; or (b) the hRSV F glycoprotein is an hRSV-B F glycoprotein; or (c) the hRSV F glycoprotein is an hRSV-A F glycoprotein and wherein the composition further comprises an hRSV-B F glycoprotein.

48. The composition of claim 46 or 47, wherein the composition comprises (a) the mRNA of any one of claims 15-19, (b) the mRNA of any one of claims 29-33, and(c) the mRNA comprising an ORF encoding an hRSV-A F protein.

49. A combination vaccine composition comprising messenger ribonucleic acid (mRNA) formulated in a lipid nanoparticle, wherein the combination vaccine composition comprises: (a) a first mRNA comprising a first open reading frame (ORF) encoding a human metapneumovirus-A (hMPV-A) fusion (F) glycoprotein, wherein the first ORF comprises a nucleotide sequence at least 90%, at least 95%, at least 98%, or at least 99% identical to SEQ ID NO: 19 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine; (b) a second mRNA comprising a second ORF encoding a human metapneumovirus-B (hMPV-B) F glycoprotein, wherein the second ORF comprises a nucleotide sequence at least 90%, at least 95%, at least 98%, or at least 99% identical to SEQ ID NO: 21 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine; and (c) a third mRNA comprising a third ORF encoding a human respiratory syncytial virus A (hRSV-A) F protein, wherein the third ORF comprises a nucleotide sequence at least 90%, at least 95%, at least 98%, or at least 99% identical to SEQ ID NO: 113 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine.

50. A combination vaccine composition comprising messenger ribonucleic acid (mRNA) formulated in a lipid nanoparticle, wherein the combination vaccine composition comprises: (a) a first mRNA comprising a first open reading frame (ORF) encoding a human metapneumovirus-A (hMPV-A) fusion (F) glycoprotein, wherein the first ORF comprises a nucleotide sequence of SEQ ID NO: 19 having nucleosides consisting of N1- methylpseudouridine, adenosine, guanosine, and cytidine; (b) a second mRNA comprising a second ORF encoding a human metapneumovirus-B (hMPV-B) F glycoprotein, wherein the second ORF comprises a nucleotide sequence of SEQ ID NO: 21 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine; and (c) a third mRNA comprising a third ORF encoding a human respiratory syncytial virus A (hRSV-A) F protein, wherein the third ORF comprises a nucleotide sequence of SEQ ID NO: 113 having nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine.

51. The composition of any one of claims 34-35, wherein the ratio of the mRNA of (a) to the mRNA of (b) is 1:1, 1:2, or 2:1.

52. The composition of any one of claims 37 or 48-50, wherein the mass ratio of (a):(b):(c) is 1:1:

2.

53. The composition of any one of claims 34-48, wherein each mRNA comprises one or more chemically modified nucleotides.

54. The composition of claim 53, wherein each mRNA comprises N1-methylpseudouridine.

55. The composition of claim 54, wherein the ORF of each mRNA comprises nucleosides consisting of N1-methylpseudouridine, adenosine, guanosine, and cytidine.

56. The composition of any one of claims 34 to 48, 51, and 53-54, wherein each mRNA is formulated in a lipid nanoparticle.

57. A composition comprising the mRNA of any one of claims 15-19 formulated in a lipid nanoparticle.

58. A composition comprising the mRNA of any one of claims 29-33 formulated in a lipid nanoparticle.

59. The composition of any one of claims 49-50, 52, and 56-58, wherein the lipid nanoparticle comprises an ionizable amino lipid, a non-cationic lipid, a sterol, and a PEG- modified lipid.

60. The composition of claim 59, wherein the lipid nanoparticle comprises 20-60 mol% of the ionizable amino lipid, 5-25 mol% DSPC, 25-55 mol% cholesterol, and 0.5-15 mol% PEG- DMG.

61. A method comprising administering to a subject the composition of any one of claims 34- 60.

62. A method of preventing hMPV infection comprising administering to a human subject the composition of any one of claims 1-60 to thereby prevent hMPV infection.

62. A method of preventing hMPV and / or hRSV infection comprising administering to a human subject the composition of any one of claims 34-40 and 46-60 to thereby prevent hMPV and / or hRSV infection.

63. Use of the composition of any one of claims 1-60 in the manufacture of a medicament for prevention of hMPV infection.

64. Use of the composition of any one of claims 34-40 and 46-60 in the manufacture of a medicament for prevention of hMPV and / or hRSV infection.

65. A method of preventing lower respiratory tract disease (LRTD) caused by hMPV and / or hRSV comprising administering to a human subject the composition of any one of claims 34-40 and 46-60 to thereby prevent LRTD caused by hMPV and / or hRSV.

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