Chimeric fusion proteins as human metapneumovirus (HMPV) vaccines

Abstract

The present invention provides chimeric polypeptides comprising a human metapneumovirus (hMPV) fusion protein (F) and a structure-stabilizing moiety (SSM). Further provided are nucleic acids and nucleic acid vectors encoding such chimeric polypeptides as well as compositions and kits comprising such chimeric polypeptides or such nucleic acids. The present invention further provides methods and uses employing the herein provided chimeric polypeptides, the herein provided nucleic acids, the herein provided compositions, or the herein provided kits in the immunization and / or vaccination of a subject. Further provided are methods of production of antigen-binding molecules that bind to the herein provided chimeric polypeptides as well as the corresponding antigen-binding molecules.

Description

[0001] New PCT patent application The University of Queensland Vossius Ref.: AJ2850 PCT BS Chimeric fusion proteins as human metapneumovirus (hMPV) vaccines This application claims the benefit of priority of European patent application EP24193729.1 filed on August 08, 2024 and of European patent application EP25183881.9 filed on June 18, 2025, each of which is incorporated herein by reference in its entirety. The present invention provides chimeric polypeptides comprising a human metapneumovirus (hMPV) fusion protein (F) and a structure-stabilizing moiety (SSM). Further provided are nucleic acids and nucleic acid vectors encoding such chimeric polypeptides as well as compositions and kits comprising such chimeric polypeptides or such nucleic acids. The present invention further provides methods and uses employing the herein provided chimeric polypeptides, the herein provided nucleic acids, the herein provided compositions, or the herein provided kits in the immunization and / or vaccination of a subject. Further provided are methods of production of antigen-binding molecules that bind to the herein provided chimeric polypeptides as well as the corresponding antigen-binding molecules. Lower respiratory infections caused by hMPV pose a significant health burden, particularly amongst infants, young children, the elderly, and individuals with underlying medical conditions. There are no licensed vaccines currently available to prevent hMPV. Vaccines recently shown to be effective for the related respiratory syncytial virus (RSV) rely on antigens comprising the RSV fusion protein. The RSV fusion protein (and likewise the hMPV fusion protein) occurs in the metastable pre-fusion conformation and the thermodynamically more stable post-fusion conformation. Efficient vaccination requires the presentation of the corresponding viral fusion protein in the pre-fusion conformation. The stabilization of the pre-fusion conformation, however, particularly over extended periods of storage without freezing, poses significant technical challenges. Vaccines for hMPV are at present not commercially available or approved. There is hence an urgent and unmet need for novel and improved vaccines against hMPV, particularly vaccines exhibiting an advantageous stabilization of the pre-fusion conformation of the hMPV fusion protein, extended storage stability, potent vaccine immunogenicity and / or potent vaccine efficacy. The present invention addresses the above-discussed shortcomings in the state of the art and solves the technical problem of providing novel and improved means and methods for the immunization and / or vaccination of a subject against human metapneumovirus. Accordingly, in a first aspect, the present invention provides a chimeric polypeptide comprising a human metapneumovirus (hMPV) fusion protein and a structure-stabilizing moiety, wherein said hMPV fusion protein comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 28, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI or NKI (preferably NRI), wherein said structure-stabilizing moiety comprises a first heptad repeat region (FHRR) and a second heptad repeat region (SHRR), wherein said FHRR comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32, and wherein said SHRR comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 33. The herein provided chimeric polypeptides comprise an hMPV fusion protein (or a fragment thereof) which is to be stabilized in the pre-fusion conformation by a structure-stabilizing moiety. Such chimericpolypeptides are particularly useful as antigens, inter alia, in the immunization and / or vaccination of asubject against hMPV. In the context of the present invention, it has surprisingly been found that the chimeric polypeptides according to the invention exhibit a range of properties that render them outstandingly advantageous for medical use, particularly for the vaccination / immunization againsthMPV. Thus, as also described in the appended examples and figures (see also Table 5 in Example 2), ithas been surprisingly found that the herein provided chimeric polypeptides: -can be efficiently produced (i.e., their production results in advantageously high yield) andpurified (see also Figures 1 and 9, and Examples 1 to 3),- are advantageously stable over several months (such as, e.g., for at least 6 months at 2-8 °C orfor at least 12 months at 2-8 °C) without the need for lyophilization thereof and / or at high temperatures (such as, e.g., at exposure to 25 °C or 40 °C for at least one week) (see also Figures 5, 6, 9, and 16 and Examples 2 and 3),- form a (desired) trimer conformation that is particularly effective, inter alia, for theimmunization of a subject against hMPV (see also Figures 2 and 6, and Examples 1 and 2),- comprise the hMPV fusion protein in the (desired) pre-fusion conformation, which is particularlyadvantageous for medical applications such as the immunization and / or vaccination of a subject against hMPV (see also Figure 8 and Example 2),- have an advantageously high binding affinity to known anti-hMPV fusion protein antibodies (seealso Figures 3 and 7, and Examples 1 and 2),- allow the formation of advantageously high antibody titers when administered to a subject, inparticular higher than the best-in-class comparator DsCavES2 (see also Figure 10 andExample 3),- have exposed internal epitopes in the hMPV fusion proteins that likely contribute to theinduction of high neutralizing antibody titers (see also Figures 13, 17, and 18, and Examples 5and 8), and- demonstrated in a phase 1 clinical trial:o desirable safety and reactogenicity (Tables 8 to 11, and Example 9),o particularly desirable induction of neutralizing antibodies against hMPV A and B strains(Figure 20 and Example 9),o surprisingly high titers of neutralizing antibodies that exceeded those of RSV-hMPVbivalent vaccine candidate IVX-A12 in phase 1 clinical trial (NCT05664334; Shapiro et al., 2025; see Example 9).These advantageous properties were unexpected and render the chimeric polypeptides according to the present invention particularly well suited for pharmaceutical use, especially for the vaccination / immunization against hMPV as well as the prevention (or prophylactic treatment) of an hMPV infection. Accordingly, a vaccine comprising the herein provided antigen, in particular a chimericpolypeptide according to SEQ ID NO: 29, is currently being probed in clinical trials (see also Example 9).Moreover, the chimeric polypeptides provided herein have been found to exhibit remarkable advantages over antigens known from the state of the art, and even in comparison to hitherto best-in-class comparators, as also described and demonstrated in Example 3. In particular, as also shown inFigure 10, it has been found that a chimeric polypeptide in accordance with the present invention (as exemplified by T3 PD) resulted in an approximately 2.5-fold higher neutralizing immune response as compared to the best-in-class comparator DSCavEs2, this was unexpected given that DSCavEs2 has been highly optimized for stability of the pre-fusion conformation.Further, the present inventors could surprisingly find that, inter alia, the length of the C-terminus of thehMPV fusion protein comprised in the chimeric polypeptide of the present invention can influence the above-mentioned characteristics. This is particularly remarkable as the C-terminus of the hMPV fusion protein is covalently bound (or, in other words, connected / linked / coupled / fused), optionally via a linker, to the structure-stabilizing moiety. As such, the C-terminus is likely not comprised in an epitope to be bound by an antigen-binding molecule (such as an antibody) with specificity to an hMPV fusion protein. The present inventors surprisingly found that a C-terminal amino acid sequence consisting of NRI (see, for example, the last three / the three C-terminal amino acids of the hMPV fusion protein according to SEQ ID NO: 28, as comprised in T3 PD) or NKI is particularly advantageous in the context of the present invention. The length and / or sequence of the C-terminal amino acids of the hMPV fusion protein in accordance with the present invention differ from previously known hMPV antigens (such as the best-in-class comparator DSCavEs2; Hsieh, Rush et al. 2022) and contribute to the advantageouscharacteristics (such as the stabilization of the pre-fusion conformation) of the chimeric polypeptides provided herein. In particular, a chimeric polypeptide comprising the above-mentioned C-terminal amino acid sequence NRI (such as the chimeric polypeptide T3) or NKI shows drastically improved binding to all assessed antibodies, including all tested pre-fusion specific antibodies, as compared to a reference polypeptide having a C-terminus that is extended by a single amino acid residue (i.e., thereference polypeptide T2, which comprises the C-terminal amino acid sequence RIL; see Example 1 andFigure 3). The chimeric polypeptide provided in the first aspect of the present invention may comprise a Human metapneumovirus (hMPV) fusion protein and a structure-stabilizing moiety, wherein said hMPV fusion protein comprises an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 28, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI or NKI (preferably NRI), wherein said structure-stabilizing moiety comprises a first heptad repeat region (FHRR) and a second heptad repeat region (SHRR), wherein said FHRR comprises an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32, and wherein said SHRR comprises an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 33. The hMPV fusion protein comprised in the chimeric polypeptide according to the invention may also be referred to as hMPV fusion protein moiety. In the context of the present invention, higher percentages of sequence identities are preferred. For example: “about 100% sequence identity” is preferred over “at least about 99% sequence identity”, which, in turn, is preferred over “at least about 98% sequence identity”, etc. In this context, the disclosure of a certain sequence identity percentage (such as, e.g., “at least about 90% sequence identity”) also herein implies the disclosure of any higher (i.e., more preferred) sequence identity percentage (such as, e.g., “at least about 90% sequence identity”, “at least about 91% sequence identity”, “at least about 92% sequence identity”, “at least about 93% sequence identity”, “at least about 94% sequence identity”, “at least about 95% sequence identity”, “at least about 96% sequence identity”, “at least about 97% sequence identity”, “at least about 98% sequence identity”, “at least about 99% sequence identity”, or “about 100% sequence identity”). The same applies to other ranges (such as in the context of trimer formation) detailed herein below. The hMPV fusion protein as well as the FHRR and the SHRR are each defined herein above by a percent sequence identity to a specific reference sequence. While the sequence identity for each one of these moieties (the hMPV fusion protein, the FHRR, and the SHRR) can, in principle, be selected independently from the sequence identity for any of the other moieties, it is generally preferred that the same percent values / thresholds are selected for the sequence identity of the hMPV fusion protein, the sequence identity of the FHRR, and the sequence identity of the SHRR. When an amino acid sequence is indicated herein to allow for a certain percentage of sequence variability (such as, e.g., “an amino acid sequence having at least about 90% sequence identity”, which allows for about 10% sequence variability), it is preferred that such sequence variability arises through conservative amino acid substitutions. In a preferred embodiment, the herein provided chimeric polypeptide comprises a human metapneumovirus (hMPV) fusion protein and a structure-stabilizing moiety, wherein said hMPV fusion protein consists of an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 28, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI or NKI (preferably NRI), wherein said structure-stabilizing moiety comprises a first heptad repeat region (FHRR) and a second heptad repeat region (SHRR), wherein said FHRR consists of an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32, and wherein said SHRR consists of an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 33. It is furthermore preferred that the chimeric polypeptide comprises, in N- to C-terminal order (i.e., from the N-terminus to the C-terminus of the chimeric polypeptide), the hMPV fusion protein and thereafter the structure-stabilizing moiety. Accordingly, it is preferred that the C-terminus of the hMPV fusion protein is connected, optionally via a linker, to the N-terminus of the structure-stabilizing moiety. Moreover, it is preferred that the structure-stabilizing moiety (which is comprised in the chimeric polypeptide) comprises, in N- to C-terminal order, the FHRR and the SHRR. Thus, it is preferred that the C-terminus of the FHRR is connected, optionally via a linker, to the N-terminus of the SHRR. In general, unless explicitly stated otherwise, the order in which the different moieties comprised in the chimeric polypeptide according to the invention are mentioned (within any sentence in the present specification) corresponds to the specific N- to C-terminal order in which these moieties are preferably present. The inventors have introduced various modifications / amino acid substitutions / mutations (apart from the above-mentioned modification of the C-terminus) in the wildtype (backbone of the) hMPV fusion protein of hMPV strain Arg / 1 / 98 (the respective wildtype fusion protein is shown in SEQ ID NO: 1;Galiano et al., 2006). Such modifications include, for example, the deletion of a transmembrane domainand a cytoplasmic domain (see SEQ ID NO: 1), thereby resulting in the production of a non-membrane- bound hMPV fusion protein. Such ‘non-membrane-bound’ hMPV fusion proteins may also be termed ‘soluble’ hMPV fusion proteins, or the like. The present inventors have further substituted / replaced several amino acid residues in the hMPV fusion protein comprised in the chimeric polypeptide. Such amino acid substitutions are further summarized in Example 2. It has been shown in the appended examples that such amino acid substitutions (particularly the presence of a proline residue at position 167 of SEQ ID NO: 28 and of two cysteine residues at positions 122 and 129 of SEQ ID NO: 28), when used in the chimeric polypeptides according to the invention, contribute to the above-mentioned advantageous effects (such as, for example, theattainment of the desired pre-fusion conformation; see also Figure 8).Accordingly, in the context of the first aspect of the invention, the hMPV fusion protein (comprised in the chimeric polypeptide) may comprise any one or more, preferably all, of the following amino acid residues: (i) a cysteine residue at position 122 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position; (ii) a cysteine residue in position 129 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position; and / or (iii) a proline residue in position 167 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position. Said cysteine residues (of (i) and (ii), above) may preferably form a disulfide bridge. In particular, it is preferred that the cysteine residue in position 122 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position forms / is capable of forming a disulfide bridge with the cysteine residue in position 129 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position. The skilled person is aware of, e.g., conditions / buffers / buffer systems / the like that allow for the formation of disulfide bridges. It will be understood that, in principle, disulfide bridges may be intramolecular disulfide bridges or intermolecular disulfide bridges. Intramolecular / intrachain disulfide bridges are formed between two cysteine residues comprised in the same molecule (i.e., the same polypeptide chain) whereas intermolecular / interchain disulfide bridges are typically formed between two cysteine residues comprised in two different molecules (i.e., two different polypeptide chains), such as, in particular, between two identical polypeptides (i.e., two polypeptides with identical amino acid sequence). The above-mentioned disulfide bridge between the cysteine residues in positions 122 and 129 (of, e.g., SEQ ID NO: 28), or between cysteine residues in positions corresponding to these positions, is preferably an intramolecular / intrachain disulfide bridge. Surprisingly, the present inventors have found that the combination of amino acid substitution “P” (i.e., the proline residue at position 167 of SEQ ID NO: 28) and “D” (i.e., two cysteine residues at positions 122 and 129 of SEQ ID NO: 28) into the chimeric polypeptide T3 resulted in particularly desired characteristicsof the resulting chimeric polypeptide (i.e., “T3-PD”; see also Example 2). In particular, the affinity of thechimeric polypeptide T3-P to pre-fusion specific antibody MPE8 was decreased as compared to T3 (i.e., lacking the “P” amino acid substitution), indicated slightly reduced stability of the prefusion conformation of T3-P (even though the kD was still below the desired threshold of 1 nM). The addition of the amino acid substitution “D” to T3 (i.e., resulting in the chimeric polypeptide “T3-D”) did not substantially affect binding to MPE8. However, when combining these proline and cysteine amino acid substitutions in chimeric polypeptide T3-PD, surprisingly, the affinity to MPE8 was further increased (see Table 5, Screen 1). This indicates that the combination of the proline residue at position 167 of SEQ ID NO: 28 and the two cysteine residues at positions 122 and 129 of SEQ ID NO: 28 contributes to the particularly desired characteristics of the herein provided chimeric polypeptides. Similarly and notably, the highly advantageous characteristics of the combination of amino acid substitutions “P” and “D” was also observed after incubations of the antigens at 25 °C or 40 °C. In particular, while T3, T3-P, and T3-D showed reduced affinity to MPE8 after incubation at 25 °C or 40 °C, surprisingly, binding of T3-PD toMPE8 was not affected (see Example 2 and Table 5)Preferably, the chimeric polypeptide comprises an hMPV fusion protein comprising the amino acid sequence set forth in SEQ ID NO: 28. More preferably, the chimeric polypeptide comprises an hMPV fusion protein consisting of the amino acid sequence set forth in SEQ ID NO: 28. The FHRR and the SHRR comprised in the structure-stabilizing moiety are preferably linked via a linker, wherein said linker is preferably a peptide linker. Such linkers (or peptide linkers) are not particularly limited in the context of the present invention, as long as they allow for the flexible linkage / coupling of said FHRR and said SHRR. Such linkers may comprise (or, preferably, consist of) an amino acid sequence selected from GGSGG (SEQ ID NO: 34), GSG, GS, GGSG (SEQ ID NO: 35), GSGS (SEQ ID NO: 36), AS, GGGS (SEQ ID NO: 37), G4S (SEQ ID NO: 38), (G4S)2 (SEQ ID NO: 39), (G4S)3 (SEQ ID NO: 40), (G4S)4 (SEQ ID NO: 41), G4SG (SEQ ID NO: 42), GSGG (SEQ ID NO: 43), GSGGS (SEQ ID NO: 44), and GSGGSG (SEQ ID NO: 45). As mentioned above, also any combination of such linkers (e.g., a combination of any two or three of the aforementioned linkers) is herein envisaged. It is further envisaged herein that such linkers may be absent (e.g., that the C-terminus of the FHRR is directly coupled / linked to the N-terminus of the SHRR). Preferably, the C-terminus of the FHRR is linked to the N-terminus of the SHRR via a linker comprising (or, preferably, consisting of) the amino acid sequence GGSGG (SEQ ID NO: 34). Accordingly, it is preferred that the structure-stabilizing moiety comprises: (i) an FHRR comprising (or, preferably, consisting of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32; (ii) an SHRR comprising (or, preferably, consisting of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 33; and (iii) a linker linking the C-terminus of said FHRR to the N-terminus of said SHRR, wherein said linker preferably comprises (or, more preferably, consists of) the amino acid sequence GGSGG (SEQ ID NO: 34). Accordingly, it is particularly preferred that the structure-stabilizing moiety comprises (or, more preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 31. Even more preferably, the structure-stabilizing moiety comprises the amino acid sequence set forth in SEQ ID NO: 31. Yet even more preferably, the structure-stabilizing moiety consists of the amino acid sequence set forth in SEQ ID NO: 31. A structure-stabilizing moiety consisting of the amino acid sequence set forth in SEQ ID NO: 31 is also referred to herein as “molecular clamp 2 silenced”, “MC2S”, or “MC2S SSM”. This structure-stabilizing moiety is further described in WO 2023 / 187743 (where it is referred to as “clamp2s”), which is incorporated herein by reference in its entirety. The structure-stabilizing moiety may also be post-translationally modified, for example by glycosylation, as also described, e.g., in WO 2023 / 187743. Such glycosylation may reduce the immunogenicity (i.e., the reactivity to an immune system / non-self-recognition / elicitation of an immune response) of the SSM. Accordingly, in the context of the present invention, the structure-stabilizing moiety preferably comprises at least one immune-silencing moiety that reduces or inhibits elicitation of an immune response to the structure-stabilizing moiety, preferably wherein the immune silencing moiety is a glycosylation site. Thus, the structure-stabilizing moiety may comprise at least one glycosylation site, preferably wherein the at least one glycosylation site is an N-linked glycosylation site selected from:(i) -Asn-Xaa-Ser-; and(ii) -Asn-Xaa-Thr-;wherein Xaa is an amino acid other than Pro; preferably wherein the glycosylation site is glycosylated with an occupancy level of at least 50%. In particular, the structure-stabilizing moiety may comprise one or more N-linked glycosylation sites, e.g., at the amino acid positions corresponding to:(i) positions 5-7 of SEQ ID NO: 32;(ii) positions 1-3 of SEQ ID NO: 33; (iii) positions 6-8 of SEQ ID NO: 33; (iv) positions 13-15 of SEQ ID NO: 33; (v) positions 17-19 of SEQ ID NO: 33; and / or (vi) positions 27-29 of SEQ ID NO: 33; wherein preferably each N-linked glycosylation site is independently -Asn-Xaa-Thr-, wherein Xaa is an amino acid other than Pro. The structure-stabilizing moiety may comprise N-linked glycosylation sites at the amino acid positions corresponding to:(i) (i-a) positions 5-7 of SEQ ID NO: 32;(i-b) positions 1-3 of SEQ ID NO: 33; and (i-c) positions 17-19 of SEQ ID NO: 33; or (ii) (ii-a) positions 5-7 of SEQ ID NO: 32; (ii-b) positions 1-3 of SEQ ID NO: 33; (ii-c) positions 17-19 of SEQ ID NO: 33; and (ii-d) positions 27-29 of SEQ ID NO: 33; or (iii) (iii-a) positions 5-7 of SEQ ID NO: 32; (iii-b) positions 1-3 of SEQ ID NO: 33; (iii-c) positions 13-15 of SEQ ID NO: 33; (iii-d) positions 17-19 of SEQ ID NO: 33; and (iii-e) positions 27-29 of SEQ ID NO: 33; or (iv) (iv-a) positions 5-7 of SEQ ID NO: 32; (iv-b) positions 1-3 of SEQ ID NO: 33; (iv-c) positions 6-8 of SEQ ID NO: 33; (iv-d) positions 13-15 of SEQ ID NO: 33; (iv-e) positions 17-19 of SEQ ID NO: 33; and (iv-f) positions 27-29 of SEQ ID NO: 33; wherein preferably each N-linked glycosylation site is independently -Asn-Xaa-Thr-, wherein Xaa is an amino acid other than Pro. If the SSM comprises one or more glycosylation site(s), it is preferred that said one or more glycosylation site(s) are comprised in said SHRR. Accordingly, it is preferred that the SHRR (as comprised in the SSM) comprises one or more N-linked glycosylation site(s) at any one or more of amino acid positions corresponding to: (i) positions 1-3 of SEQ ID NO: 33; (ii) positions 13-15 of SEQ ID NO: 33; (iv) positions 17-19 of SEQ ID NO: 33; and (v) positions 27-29 of SEQ ID NO: 33; wherein preferably each N-linked glycosylation site is independently -Asn-Xaa-Thr-, wherein Xaa is an amino acid other than Pro. More preferably, the SHRR (as comprised in the SSM) comprises an N-linked glycosylation site at amino acid positions corresponding to: (i) positions 1-3 of SEQ ID NO: 33; (ii) positions 13-15 of SEQ ID NO: 33; (iv) positions 17-19 of SEQ ID NO: 33; and (v) positions 27-29 of SEQ ID NO: 33; wherein preferably each N-linked glycosylation site is independently -Asn-Xaa-Thr-, wherein Xaa is an amino acid other than Pro. Even more preferably, the SHRR (as comprised in the SSM) comprises an N-linked glycosylation site at amino acid positions corresponding to: (i) positions 1-3 of SEQ ID NO: 33, with said N-linked glycosylation site consisting of NHT; (ii) positions 13-15 of SEQ ID NO: 33, with said N-linked glycosylation site consisting of NHT; (iv) positions 17-19 of SEQ ID NO: 33, with said N-linked glycosylation site consisting of NLT; and (v) positions 27-29 of SEQ ID NO: 33, with said N-linked glycosylation site consisting of NQT. The present invention also envisages non-silenced structure-stabilizing moieties, i.e., structure stabilizing moieties not comprising immune-silencing moieties (e.g., glycosylation sites), however, herein preferred are silenced structure-stabilizing moieties as detailed herein above. Accordingly, the herein provided chimeric polypeptide may also comprise a structure-stabilizing moiety comprising: (i) an FHRR comprising (or, preferably, consisting of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 48; (ii) an SHRR comprising (or, preferably, consisting of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 49; and (iii) a linker linking the C-terminus of said FHRR to the N-terminus of said SHRR, wherein said linker preferably comprises (or, more preferably, consists of) the amino acid sequence GGSGG (SEQ ID NO: 34). Accordingly, the structure-stabilizing moiety may comprise (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 50. Even preferably, the structure-stabilizing moiety comprises the amino acid sequence set forth in SEQ ID NO: 50. Yet more preferably, the structure-stabilizing moiety consists of the amino acid sequence set forth in SEQ ID NO: 50. The structure stabilizing moiety according to SEQ ID NO: 50 may herein also be referred to as “MC2” or “MC2 SSM” or the like. This structure-stabilizing moiety is further described in WO 2023 / 187743 (where it is referred to as “clamp2”). Accordingly, the chimeric polypeptide may preferably comprise (or, more preferably, consist of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 51. Accordingly, the chimeric polypeptide may preferably consist of an amino acid sequence set forth in SEQ ID NO: 51. Accordingly, the chimeric polypeptide may preferably comprise (or, more preferably, consist of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 52. Accordingly, the chimeric polypeptide may preferably consist of an amino acid sequence set forth in SEQ ID NO: 52. In the context of the chimeric polypeptide provided herein, the hMPV fusion protein and the structure- stabilizing moiety may be linked via a linker, preferably wherein said linker is a peptide linker. Said linker may comprise (or, preferably, consists of) an amino acid sequence selected from GSG, GS, GGSGG (SEQ ID NO: 34), GGSG (SEQ ID NO: 35), GSGS (SEQ ID NO: 36), AS, GGGS (SEQ ID NO: 37), G4S (SEQ ID NO: 38), (G4S)2 (SEQ ID NO: 39), (G4S)3 (SEQ ID NO: 40), (G4S)4 (SEQ ID NO: 41), G4SG (SEQ ID NO: 42), GSGG (SEQ ID NO: 43), GSGGS (SEQ ID NO: 44), and GSGGSG (SEQ ID NO: 45). However, also any combination of such flexible linkers is herein envisaged. It is further envisaged herein that such flexible linkers may be absent (e.g., that the C-terminus of the hMPV fusion protein is linked / coupled directly to the N-terminus of the structure-stabilizing moiety). Preferably, the C-terminus of said hMPV fusion protein is linked / coupled to the N-terminus of said structure-stabilizing moiety via a linker, wherein said linker comprises (or, more preferably, consists of) the amino acid sequence GSG. In a preferred embodiment, the first aspect of the present invention provides a chimeric polypeptide comprising a human metapneumovirus (hMPV) fusion protein and a structure-stabilizing moiety, wherein said hMPV fusion protein comprises (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 28, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI (i.e., Asn-Arg-Ile) or NKI (i.e., Asn-Lys-Ile), preferably NRI, wherein said structure-stabilizing moiety comprises a first heptad repeat region (FHRR) and a second heptad repeat region (SHRR), wherein said FHRR comprises (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32, wherein said SHRR comprises (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 33, wherein said FHRR is linked to said SHRR via a linker, and wherein said hMPV fusion protein is linked to said structure-stabilizing moiety via a linker. In a preferred embodiment, the first aspect of the present invention provides a chimeric polypeptide comprising a human metapneumovirus (hMPV) fusion protein and a structure-stabilizing moiety, wherein said hMPV fusion protein comprises (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 28, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI or NKI (preferably NRI), wherein said structure-stabilizing moiety comprises a first heptad repeat region (FHRR) and a second heptad repeat region (SHRR), wherein said FHRR comprises (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32, wherein said SHRR comprises (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 33, wherein said FHRR is linked to said SHRR via a linker comprising (or, preferably, consisting of) the amino acid sequence GGSGG (SEQ ID NO: 34), and wherein said hMPV fusion protein is linked to said structure-stabilizing moiety via a linker comprising (or, preferably, consisting of) the amino acid sequence GSG. In a preferred embodiment, the herein provided chimeric polypeptide comprises an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 29. In a particularly preferred embodiment, the chimeric polypeptide comprises (or consists of) the amino acid sequence set forth in SEQ ID NO: 29.As also shown in Example 2 and Figure 8, the chimeric polypeptides according to the invention (asexemplified by ‘MPV T3 PD’) are primarily forming / adopting a (desired) (homo-) trimeric polypeptide complex (as opposed to monomers or aggregates). A “homotrimeric polypeptide complex” consists of three identical polypeptide subunits, such as of three identical chimeric polypeptides. In this context, it is preferred that the FHRRs and SHRRs of the three SSMs are associated in the form of a six-helix bundle. The term “helix bundle” refers to a plurality of peptide helices that fold such that the helices are substantially parallel or anti-parallel to one another. A two-helix bundle has two helices folded such that they are substantially parallel or anti-parallel to one another. Likewise, a six-helix bundle has six helices folded such that they are substantially parallel or anti-parallel to one another. By “substantially parallel or anti-parallel” it is meant that the helices are folded such that the side chains of the helices are able to interact with one another. For example, the hydrophobic side chains of the helices are able to interact with one another to form a hydrophobic core. As explained above, viral fusion proteins can be present in a pre-fusion and a post-fusion state / conformation. It is desired in the context of the present invention that the chimeric fusionpolypeptides (largely) remain in the pre-fusion state / conformation. As illustrated in Example 2 andFigure 7, the chimeric polypeptide of the invention (as exemplified by hMPV-T3PD) has an advantageously low dissociation constant (kD) when contacted with an anti-hMPV fusion protein antibody that is specific / has substrate specificity for an hMPV fusion protein in pre-fusion conformation (such as the antibody MPE8). This indicates that the herein provided chimeric polypeptides adopt the desired pre-fusion conformation. Accordingly, it is preferred that the hMPV fusion protein comprised in the chimeric polypeptide adopts / is present in the pre-fusion conformation. When present in the prefusion conformation antigens are particularly suitable for eliciting neutralizing immune responses / inducing production of neutralizing antibodies. The enclosed examples illustrate that the induction of such neutralizing immune responses after administration of T3PD (see in particularFigure 10). Further, Figure 14 illustratively demonstrates that the herein provided chimeric polypeptidesare highly efficient in depleting neutralizing antibodies. Accordingly, it is preferred that the herein provided chimeric polypeptides are capable of inducing a neutralizing immune response.Examples 5 and 8 illustrate that the herein provided chimeric polypeptides (in particular the hMPV fusionprotein comprised therein) can adopt an open conformation with internal, yet surface exposed epitopes. Accordingly, it is preferred that the hMPV fusion protein comprised in the herein provided chimeric polypeptides has / comprises internal surface exposed epitopes. As used herein, such 'internal surface exposed epitopes' refers to regions of the chimeric polypeptide (in particular of the fusion protein comprised therein) that, while located at the interface between subunits in a homotrimeric complex, become accessible to antibodies or solvents. The skilled person can assess the presence of such epitopes using methods such as hydrogen / deuterium exchange mass spectrometry (HDX-MS), as described in Example 8. It will be understood that the chimeric polypeptides may further comprise an N-terminal signal sequence or signal peptide that may determine the localization of the chimeric polypeptide when expressed in a host cell. Suitable signal peptides are known in the art. In the context of the present invention, a preferred signal peptide comprises (or, preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to SEQ ID NO: 30. More preferably, the signal peptide comprises (or, even more preferably, consists of) the amino acid sequence set forth in SEQ ID NO: 30. Accordingly, the hMPV fusion protein comprised in the chimeric polypeptide may further comprise an N-terminal signal peptide, preferably wherein said signal peptide comprises (or, more preferably, consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to SEQ ID NO: 30. More preferably, the signal peptide comprises (or, even morepreferably, consists of) the amino acid sequence set forth in SEQ ID NO: 30. As is illustrated in Example 4,the herein provided chimeric polypeptides can comprise various signal peptides for efficient production thereof. Accordingly, the sequence of the signal peptide is not particularly limited. Preferred sequences are illustrated in SEQ ID NO: 30 and 46. Accordingly, in the context of the present invention, the signal peptide may comprise (or, preferably, consist of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to SEQ ID NO: 30 or 46. Preferably, the signal peptide comprises (or, even more preferably, consists of) the amino acid sequence set forth in SEQ ID NO: 30 or 46. SEQ ID NO: 47 and 53 to 75 exemplify herein provided chimeric polypeptides comprising a signal peptide according to SEQ ID NO: 46. A particularly preferred chimeric polypeptide comprising such a signal peptide is shown in SEQ ID NO: 47. Accordingly, the chimeric polypeptide may comprise (or consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 47. In a preferred embodiment, the chimeric polypeptide comprises (or, more preferably, consists of) the amino acid sequence set forth in SEQ ID NO: 47. SEQ ID NO: 29 illustrates an exemplary, particularly preferred embodiment of the chimeric polypeptide lacking a signal peptide. SEQ ID NO: 25 comprises an N-terminal signal peptide according to SEQ ID NO: 30 which is linked to the exemplary chimeric polypeptide of SEQ ID NO: 29. Accordingly, the chimeric polypeptide may comprise (or consists of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 25. In a preferred embodiment, the chimeric polypeptide comprises (or, more preferably, consists of) the amino acid sequence set forth in SEQ ID NO: 25. As explained herein, the chimeric polypeptides are particularly advantageous for use in the immunization and / or vaccination of a subject (against hMPV). It will be understood that the chimeric polypeptides preferably do not comprise a signal peptide when employed for medical use, particularly in the immunization and / or vaccination of a subject. In contrast, when a nucleic acid encoding the chimeric polypeptide is employed for medical use, e.g., for the immunization and / or vaccination of a subject, or for expression in a host cell, the presence of an N-terminal signal peptide (such as, e.g., the signal peptide exemplified in SEQ ID NO: 30) in the encoded polypeptide is preferred. As explained above, the hMPV fusion protein (comprised in the chimeric polypeptide according to the first aspect of the invention) comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI or NKI, preferably consisting of the amino acid sequence NRI. In the present specification, various specific / exemplary sequences comprising the aforementioned amino acid sequence NRI are described, including, e.g., the sequence set forth in SEQ ID NO: 28, the sequence set forth in SEQ ID NO: 29, and the sequence set forth in SEQ ID NO: 25. It is to be understood that the present invention also relates to sequences corresponding to SEQ ID NO: 28, SEQ ID NO: 29 and SEQ ID NO: 25, respectively, but differing from these sequences in that they comprise the sequence NKI instead of the sequence NRI. In line with this, all passages of the present specification that make reference to any of SEQ ID NO: 28, SEQ ID NO: 29 or SEQ ID NO: 25 are to be understood as also making reference to the respective sequence wherein the amino acid sequence NRI is replaced by NKI, and preferably as making reference to the corresponding explicitly mentioned sequence (comprising the amino acid sequence NRI). Accordingly, the present invention relates, in particular, to (i) a chimeric polypeptide comprising an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 29, (ii) a chimeric polypeptide comprising an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 29 wherein the sequence NRI comprised in SEQ ID NO: 29 is replaced by NKI, or (iii) a composition comprising both the aforementioned chimeric polypeptides (i) and (ii). The invention further relates to (i) a chimeric polypeptide comprising (or, preferably, consisting of) the amino acid sequence set forth in SEQ ID NO: 29, (ii) a chimeric polypeptide comprising (or, preferably, consisting of) the amino acid sequence set forth in SEQ ID NO: 29 wherein the sequence NRI comprised in SEQ ID NO: 29 is replaced by NKI, or (iii) a composition comprising both the aforementioned chimeric polypeptides (i) and (ii). In a second aspect, the present invention provides a nucleic acid comprising a nucleic acid sequence encoding the chimeric polypeptide provided in the first aspect of the invention. The term “nucleic acid”, as used herein, encompasses any molecule containing two or more nucleotides, particularly a polymer of nucleotides, such as, e.g., a deoxyribonucleic acid (DNA) or a ribonucleic acid (RNA). The nucleotides may be, e.g., deoxyribonucleotides, ribonucleotides or nucleotide analogues, and they may optionally be substituted or modified. The nucleotides can be linked by phosphodiester bonds / linkages or, e.g., by phosphorothioate linkages, methylphosphonate linkages or boranophosphate linkages. The term “nucleic acid” particularly relates to DNA or RNA, such as, e.g., mRNA, cRNA, or cDNA. The term typically refers to polymeric forms of nucleotides of, e.g., at least 10 bases in length, either ribonucleotides or deoxynucleotides or a modified form of either type of nucleotide. A “nucleic acid” can be single stranded or double stranded. Accordingly, the herein provided nucleic acid may be a ribonucleic acid (RNA), preferably a messenger RNA (mRNA). As will be detailed herein below, such nucleic acids (particularly mRNA) encoding the chimeric polypeptides of the invention are also particularly useful in the immunization and / or vaccination of a subject. As indicated herein above and below, the present invention provides for a complex comprising three subunits, wherein each subunit is a herein provided chimeric polypeptide. Preferably, the complex is characterized by a six-helix bundle formed by homo-trimerization of the structure-stabilizing moieties of the three chimeric polypeptides. Accordingly, it is preferred that the herein provided complex comprises three identical subunits (e.g., wherein each subunit is a chimeric polypeptide having an amino acid sequence set forth in SEQ ID NO: 29). The present invention further provides for a method of producing a chimeric polypeptide complex, wherein the method comprises: combining chimeric polypeptides as defined in accordance with the first aspect of the invention under conditions suitable for the formation of a chimeric polypeptide complex, whereby a chimeric polypeptide complex is produced that comprises three chimeric polypeptide subunits and is preferably characterized by a six-helix bundle formed by homo-trimerization of the structure-stabilizing moieties of the three chimeric polypeptides. In a third aspect, the present invention provides a composition comprising the chimeric polypeptide (according to the first aspect) or the nucleic acid encoding the chimeric polypeptide (according to the second aspect), preferably wherein said composition is a pharmaceutical composition. The composition according to the third aspect may also (i.e., additionally, or alternatively to the chimeric polypeptide or the nucleic acid) comprise the herein provided complex. Optionally, said composition may further comprise a pharmaceutically acceptable carrier, diluent and / or adjuvant. In the context of a composition comprising an mRNA encoding a chimeric polypeptide, it is preferred that the pharmaceutically acceptable carrier is a lipid nanoparticle (LNP) or that the composition is formulated as an LNP encapsulating the mRNA. The composition (or pharmaceutical composition) according to the third aspect may be an aqueous pharmaceutical composition, particularly an aqueous pharmaceutical composition comprising a buffer (or buffer system) and optionally a surfactant. Preferably, said buffer (or buffer system) is phosphate buffered saline (PBS). The surfactant may be, e.g., polysorbate 80. The composition (or pharmaceutical composition) can also be provided as a powder for reconstitution (e.g., to form an aqueous pharmaceutical composition, as described above, after reconstitution). A single dose of the composition may contain, e.g., an amount of about 10 µg to about 500 µg, preferably about 20 µg to about 300 µg, more preferably about 30 µg to about 240 µg (such as, in particular, about 30 µg, about 60 µg, about 120 µg, or about 240 µg), of the chimeric polypeptide. The composition (or pharmaceutical composition) may further comprise one or more other antigens (e.g., other viral antigens), particularly one or more antigens of (an)other respiratory virus(es) (i.e., other than hMPV). For example, the composition may further comprise any one or more (e.g., one, two, three, four, five, six, or seven) antigen(s) selected from one or more antigen(s) of respiratory syncytial virus (RSV), one or more antigen(s) of human parainfluenza virus (hPIV; e.g., hPIV-1, hPIV-2, hPIV-3, and / or hPIV-4), one or more antigen(s) of an influenza virus (particularly a human influenza virus; e.g., influenza A H1N1, influenza A H3N2, and / or influenza B), one or more antigen(s) of a coronavirus (particularly a human coronavirus; e.g., SARS-CoV-2), and / or one or more antigen(s) of other human respiratory viruses. Preferably, said one or more antigen(s) is / are antigen(s) comprising a viral fusion protein (i.e., a fusion protein antigen) of the respective virus. In particular, the composition may further comprise an antigen of RSV and / or an antigen of hPIV, particularly an RSV fusion protein antigen and / or an hPIV (particularly hPIV-3) fusion protein antigen. A corresponding composition comprising one further antigen may be used as a bivalent vaccine (e.g., a bivalent vaccine against hMPV and RSV). A corresponding composition comprising two further antigens may be used as a trivalent vaccine (e.g., a trivalent vaccine against hMPV, RSV and hPIV). It is also envisaged herein that compositions comprising more than two (e.g., three, four, five, six, or seven) further antigens may be used as polyvalent vaccine (e.g., a polyvalent vaccine against hMPV and three or more, preferably all, of RSV, hPIV, influenza A H1N1, influenza A H3N2, influenza B, and SARS-CoV-2). Said composition may for example further comprise an RSV-antigen comprising (or preferably consisting of) an amino acid sequence having, with an increasing degree of preference, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, about 100%, or 100% sequence identity to the amino acid sequence set forth in SEQ ID NO: 77. In a fourth aspect, the present invention provides a kit comprising the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), or the composition (according to the third aspect). The kit according to the fourth aspect may also (i.e., additionally, or alternatively to the chimeric polypeptide, the nucleic acid, or the composition) comprise the herein provided complex. In a fifth aspect, the present invention provides the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), the composition (according to the third aspect), or the kit (according to the fourth aspect), for use in the immunization and / or the vaccination of a subject. The present invention also provides for the herein provided complex for use in the immunization and / or the vaccination of a subject. Thus, the present invention provides the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), the composition (according to the third aspect), or the kit (according to the fourth aspect), for use in the immunization and / or the vaccination of a subject against a virus, particularly against hMPV. The present invention also provides for the herein provided complex for use in the immunization and / or the vaccination of a subject against a virus, particularly against hMPV. Accordingly, the present invention also provides the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), the composition (according to the third aspect), or the kit (according to the fourth aspect), for use as a vaccine against hMPV or for use in the prevention (or prophylactic treatment) of an hMPV infection (or a disease caused by an hMPV infection). The present invention also provides for the herein provided complex for use as a vaccine against hMPV or for use in the prevention (or prophylactic treatment) of an hMPV infection (or a disease caused by an hMPV infection). The present invention further provides a method for the immunization and / or the vaccination of a subject (preferably a human), particularly against hMPV, wherein the method comprises administering the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), the composition (according to the third aspect), or the kit (according to the fourth aspect), to said subject. The present invention further provides a method for the immunization and / or the vaccination of a subject (preferably a human), particularly against hMPV, wherein the method comprises administering the herein provided complex to said subject. It will be understood that an effective amount (particularly a pharmacologically effective amount) is to be administered to the subject in any such methods. The present invention further provides a method for the prevention of an hMPV infection (or a disease caused by an hMPV infection) in a subject (preferably a human), wherein the method comprises administering the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), the composition (according to the third aspect), or the kit (according to the fourth aspect), to said subject. The present invention also provides a method for the prevention of an hMPV infection (or a disease caused by an hMPV infection) in a subject (preferably a human), wherein the method comprises administering the herein provided complex to said subject. The present invention further provides the use of the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), or the composition (according to the third aspect) in the manufacture of a vaccine against hMPV. The present invention also provides for the use of the herein provided complex in the manufacture of a vaccine against hMPV. The present invention further provides the use of the chimeric polypeptide (according to the first aspect), the nucleic acid (according to the second aspect), or the composition (according to the third aspect) in the manufacture of a medicament for the prevention (or prophylactic treatment) of an hMPV infection (or a disease caused by an hMPV infection). The present invention further provides the use of the herein provided complex in the manufacture of a medicament for the prevention (or prophylactic treatment) of an hMPV infection (or a disease caused by an hMPV infection). In the context of the fifth aspect of the present invention, the subject is preferably a human (e.g., a male human or a female human). The subject may be, in particular, an adult human (e.g., a human at the age of 18 years or older), preferably a human of about 50 years or older, more preferably a human of about 55 years or older (e.g., about 56 years or older), even more preferably a human of about 60 years or older (including, e.g., about 65 years or older; about 66 years or older; about 70 years or older; about 75 years or older; about 76 years or older; or about 80 years or older). The subject may also be a pregnant (or gestating) female human (e.g., between weeks 24 and 36 of gestation). The immunization of pregnant women may be beneficial for the passive protection of newborn infants against hMPV infections. Moreover, the subject may also be a human child (e.g., a human at the age of less than 18 years), particularly a human child of about 4 years or younger, or about 2 years or younger, or a human infant of about 1 year or younger, or about 6 months or younger. Further, the hMPV infection may be, e.g., an infection with an hMPV A strain (such as, e.g., the hMPV strain A1, A2a, or A2b) or an hMPV B strain (such as, e.g., the hMPV strain B1 or B2). Accordingly, said hMPV may be, e.g., hMPV A or hMPV B, particularly hMPV A1, hMPV A2a, hMPV A2b, hMPV B1, or hMPVB2 (see, e.g., Papenburg J et al., J Clin Virol, 2013, 58(3):541-547, doi: 10.1016 / j.jcv.2013.08.029). Seealso Figure 20 and Example 9, illustrating induction of desirable neutralizing antibody titers against bothhMPV A and hMPV B strains. In the context of the fifth aspect, it is preferred that an effective amount of the chimeric polypeptide, the nucleic acid, or the composition is to be administered to the subject. When the herein provided complex is to be administered to the subject in the context of the fifth aspect, it is also preferred that an effective amount thereof is to be administered. Typically, a physician will determine the specific amount / dosage that will be most suitable for an individual subject. The specific dose level and frequency of dosage for any particular subject may be varied and will depend upon a variety of factors, including the efficacy of the active agent employed, the metabolic stability and length of action of that agent, the mode and time of administration, and the age, body weight, general health and sex of the individual subject. A proposed, yet non-limiting dose of the chimeric polypeptide according to the invention for administration (e.g., for parenteral administration, particularly for intramuscular administration) to a human subject may be, e.g., about 10 µg to about 500 µg, preferably about 20 µg to about 300 µg, more preferably about 30 µg to about 240 µg (such as, in particular, about 30 µg, about 60 µg, about 120 µg, or about 240 µg), of the chimeric polypeptide per dose. A corresponding dose may be administered once, twice, three times, or more often. In particular, a single dose may be administered for immunization / vaccination of the subject. Further doses can be administered, particularly for revaccination or as booster / refresher, e.g., after a period of about 6 months to about 5 years (for example, after about 1 year). The chimeric polypeptide, the nucleic acid, or the composition can be administered to the subject by any convenient route of administration, including, but not limited to, the parenteral route, e.g., by intravenous, intramuscular, subcutaneous, or intradermal administration (e.g., injection). Alternatively, intranasal administration is also envisaged herein. A preferred route of administration is intramuscular administration (particularly intramuscular injection). The herein provided complex may be administered accordingly. In a sixth aspect, the present invention provides a nucleic acid vector comprising the nucleic acid (according to the second aspect). In a seventh aspect, the present invention provides a host cell comprising the nucleic acid (according to the second aspect) and / or the nucleic acid vector (according to the sixth aspect). In this context, it is preferred that the host cell is an isolated host cell, particularly an immortalized cell line (suitable for the production of biological products, such as polypeptides), more preferably a Chinese Hamster Ovary (CHO) cell line, a Human Embryonic Kidney (HEK) cell line, or an insect cell line (such as, e.g., SF9). A CHOcell line is herein preferred. Exemplary CHO cell lines include: CHO, ExpiCHO, CHO-S, CHO-K1, CHO-DG44 and CHO-DXB11. In an eighth aspect, the present invention provides a method of production of a chimeric polypeptide, wherein the method comprises:(i) culturing the host cell according to the seventh aspect; or(ii) expressing a chimeric polypeptide from the nucleic acid (according to the second aspect), and / orthe nucleic acid vector (according to the sixth aspect), optionally in a cell-free transcription-translation system. Said method may further comprise obtaining and / or purifying the chimeric polypeptide. In a ninth aspect, the present invention provides a method of production of an antigen-binding molecule (particularly an antibody) that specifically binds to an hMPV fusion protein, the method comprising: (i) immunizing an animal (preferably a non-human mammal) with the chimeric polypeptide according the first aspect, or the nucleic acid according to the second aspect, or the composition according to the third aspect; (ii) identifying and / or isolating a B cell from the immunized animal, which specifically binds to the hMPV fusion protein; and (iii) producing the antigen-binding molecule expressed by that B cell. Said method may further comprise obtaining and / or purifying the antigen-binding molecule. The present invention also provides a method of production of an antigen-binding molecule (particularly an antibody) that specifically binds to an hMPV fusion protein, the method comprising: (i) immunizing an animal (preferably a non-human mammal) with the herein provided complex; (ii) identifying and / or isolating a B cell from the immunized animal, which specifically binds to the hMPV fusion protein; and (iii) producing the antigen-binding molecule expressed by that B cell. Said method may further comprise obtaining and / or purifying the antigen-binding molecule.As shown in the appended examples (see, in particular, Example 4), the chimeric polypeptides adopt thedesired pre-fusion conformation. Accordingly, when employing the herein provided chimeric polypeptides, the herein provided nucleic acids, or the herein provided compositions in the method according to the ninth aspect, said method may preferably result in the production of an antigen-binding molecule (particularly an antibody) that specifically binds to an hMPV fusion protein in the pre-fusion conformation and / or an antigen-binding molecule (particularly an antibody) that specifically binds to a complex (particularly a homotrimeric complex) of an hMPV fusion protein in the pre-fusion conformation. In a tenth aspect, the present invention provides an antigen-binding molecule obtainable (or obtained) by the method according to the ninth aspect. Accordingly, said antigen-binding molecule is preferably an antigen-binding molecule that specifically binds to an hMPV fusion protein in the pre-fusion conformation and / or an antigen-binding molecule that specifically binds to a (preferably homotrimeric) complex of an hMPV fusion protein in the pre-fusion conformation. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present specification pertains. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described herein. The following definitions apply throughout the present specification and the claims, unless specifically indicated otherwise. As used herein, unless explicitly indicated otherwise or contradicted by context, the terms “a” and “an” are used interchangeably with “one or more” and “at least one”. In addition, these terms also specifically include the narrower meaning “one”, “exactly one” or “only one”. Accordingly, the articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element. As used herein, the term “and / or” refers to and encompasses any one of the corresponding items as well as any and all possible combinations of two or more of the corresponding items. Unless contradicted by context, this term also includes the specific meaning of “either … or”. Further, the terms “about” and “approximate”, as used herein, when referring to a measurable value such as an amount, dose, time, temperature, activity, level, number, frequency, percentage, dimension,size, amount, weight, position, length and the like, is meant to encompass variations of, e.g., ± 15%, ±10%, ± 5%, ± 1%, ± 0.5%, or even ± 0.1% of the specified amount, dose, time, temperature, activity, level, number, frequency, percentage, dimension, size, amount, weight, position, length and the like. In instances in which the terms “about” and “approximate” are used in connection with the location orposition of regions within a reference polypeptide, these terms encompass variations of, e.g., ± up to 20amino acid residues, ± up to 15 amino acid residues, ± up to 10 amino acid residues, ± up to 5 amino acid residues, ± up to 4 amino acid residues, ± up to 3 amino acid residues, ± up to 2 amino acid residues, or even ± 1 amino acid residue. Moreover, whenever the term “about” or “approximate” is used, the present invention also specifically and preferably relates to the corresponding exact value (without variation). It is to be understood that wherever numerical ranges are provided / disclosed herein, all values and subranges encompassed by the respective numerical range are meant to be encompassed within the scope of the invention. Accordingly, the present invention specifically and individually relates to each value (and particularly each integer) that falls within a numerical range disclosed herein, including the upper and lower endpoints of each numerical range, as well as each subrange encompassed by a numerical range disclosed herein. As used herein, the terms “optional”, “optionally” and “may” denote that the indicated feature may be present but can also be absent. Whenever the term “optional”, “optionally” or “may” is used, the present invention specifically relates to both possibilities, i.e., that the corresponding feature is present or, alternatively, that the corresponding feature is absent. For example, if a component of a composition is indicated to be “optional”, the invention specifically relates to both possibilities, i.e., that the corresponding component is present (contained in the composition) or that the corresponding component is absent from the composition. The term “adjuvant” as used herein refers to a compound that, when used in combination with a specific immunogen (e.g., a (poly)peptide, chimeric polypeptide, chimeric polypeptide complex, polynucleotide and nucleic acid construct of the present disclosure) in a composition, will augment the resultant immune response, including intensification and / or broadening the specificity of either or both antibody and cellular immune responses. In the context of the present disclosure, an adjuvant will preferably enhance the specific immunogenic effect of the active agents of the present disclosure. The term “adjuvant” is typically understood not to comprise agents which confer immunity by themselves. An adjuvant assists the immune system unspecifically to enhance the antigen-specific immune response by, e.g., promoting presentation of an antigen to the immune system or induction of an innate immune response. As used herein, the term “antigen” and its grammatically equivalent expressions (e.g., “antigenic”) refer to a compound, composition, or substance that may be specifically bound by the products of specific humoral or cellular immunity, such as an antibody molecule or T-cell receptor. Antigens can be any type of molecule including, for example, haptens, simple intermediary metabolites, sugars (e.g., oligosaccharides), lipids, and hormones as well as macromolecules such as complex carbohydrates (e.g., polysaccharides), phospholipids, and proteins. Common categories of antigens include, but are not limited to, viral antigens, bacterial antigens, fungal antigens, protozoan and other parasitic antigens, tumor antigens, antigens involved in autoimmune disease, allergy and graft rejection, toxins, and other miscellaneous antigens. As mentioned herein above, the present invention particularly relates to antigens (i.e., chimeric polypeptides) comprising hMPV fusion proteins (or fragments thereof). By “antigen-binding molecule” is meant a molecule that has binding affinity for a target antigen. It will be understood that this term extends to immunoglobulins, immunoglobulin fragments and non- immunoglobulin derived protein or other non-protein frameworks that exhibit antigen-binding activity. Representative antigen-binding molecules that are useful in the practice of the present disclosure include polyclonal and monoclonal antibodies as well as their fragments (such as Fab, Fab’, F(ab’)2, or Fv), single chain (scFv) and single domain antibodies (including, for example, shark and camelid antibodies), and fusion proteins comprising an antibody, and any other modified configuration of the immunoglobulin molecule that comprises an antigen binding / recognition site. An antibody includes an antibody of any class, such as IgG, IgA, or IgM (or sub-class thereof), and the antibody need not be of any particular class. Depending on the antibody amino acid sequence of the constant region of its heavy chains, immunoglobulins can be assigned to different classes. There are five major classes of immunoglobulins: IgA, IgD, IgE, IgG, and IgM, and several of these may be further divided into subclasses (isotypes), e.g., IgG1, IgG2, IgG3, IgG4, IgA1 and IgA2. The heavy-chain constant regions that correspond to the different classes of immunoglobulins are called α, δ, ε, γ, and µ, respectively. The subunit structures and three-dimensional configurations of different classes of immunoglobulins are well known. Antigen-binding molecules also encompass dimeric antibodies, as well as multivalent forms of antibodies. In some embodiments, the antigen-binding molecules are chimeric antibodies in which a portion of the heavy and / or light chain is identical with or homologous to corresponding sequences in antibodies derived from a particular species or belonging to a particular antibody class or subclass, while the remainder of the chain(s) is identical with or homologous to corresponding sequences in antibodies derived from another species or belonging to another antibody class or subclass, as well as fragments of such antibodies, so long as they exhibit the desired biological activity. Also contemplated are humanized antibodies, which are generally produced by transferring complementarity determining regions (CDRs) from heavy and light variable chains of a non-human (e.g., rodent, preferably mouse) immunoglobulin into a human variable domain. Typical residues of human antibodies are then substituted in the framework regions of the non-human counterparts. The use of antibody components derived from humanized antibodies obviates potential problems associated with the immunogenicity of non-human constant regions. General techniques of cloning non-human, particularly murine, immunoglobulin variable domains as well as techniques for producing humanized monoclonal antibodies are well-known in the art and described in the literature. Humanized antibodies include “primatized” antibodies in which the antigen-binding region of the antibody is derived from an antibody produced by immunizing macaque monkeys with the antigen of interest. Also contemplated as antigen-binding molecules are humanized antibodies. Further examples of an “antigen-binding molecule” include any of the above-described agents, which can be obtained, e.g., by using the method according to the ninth aspect of theinvention. The term “anti-parallel”, as used herein, refers to a proteinaceous polymer in which regions or segments of the polymer are in a parallel orientation but have opposite polarities. As used herein, the term “binds specifically” refers to a binding reaction which is determinative of the presence of a chimeric polypeptide or complex of the present disclosure in the presence of a heterogeneous population of molecules including macromolecules such as proteins and other biologics. In specific embodiments, the term “binds specifically” when referring to an antigen-binding molecule is used interchangeably with the term “specifically immuno-interactive” and the like to refer to a binding reaction which is determinative of the presence of a chimeric polypeptide or complex of the present disclosure in the presence of a heterogeneous population of proteins and other biologics. Under designated assay conditions, a molecule binds specifically to a chimeric polypeptide or complex of the disclosure and does not bind in a significant amount to other molecules (e.g., proteins or antigens) present in the sample. In antigen-binding molecule embodiments, a variety of immunoassay formats may be used to select antigen-binding molecules that are specifically immuno-interactive with a chimeric polypeptide or complex of the disclosure. For example, solid-phase ELISA immunoassays are routinely used to select monoclonal antibodies that are specifically immuno-interactive with a protein. The term “chimeric”, when used in reference to a molecule, means that the molecule contains portions that are derived from, obtained or isolated from, or based upon two or more different origins or sources. Thus, a polypeptide is chimeric when it comprises two or more amino acid sequences of different origin and includes (1) polypeptide sequences that are not found together in nature (i.e., at least one of the amino acid sequences is heterologous with respect to at least one of its other amino acid sequences), or (2) amino acid sequences that are not naturally adjoined. It is understood that a “chimeric polypeptide” is a “polypeptide”. Accordingly, in the context of the present invention a “chimeric polypeptide” may also be referred to as a “polypeptide”. By “coding sequence” is meant any nucleic acid sequence that contributes to the code for the polypeptide product of a gene or for the final mRNA product of a gene (e.g., the mRNA product of a gene following splicing). By contrast, the term “non-coding sequence” refers to any nucleic acid sequence that does not contribute to the code for the polypeptide product of a gene or for the final mRNA product of a gene. The term “a nucleic acid encoding a polypeptide” may be used interchangeably with the term “a nucleic acid comprising a coding sequence encoding a polypeptide” or the like. The terms “coiled coil” or “coiled coil structure” are used interchangeably herein to refer to a structural motif in proteins, in which two or more α-helices (most often 2-7 α-helices) are coiled together like the strands of a rope (dimers and trimers are the most common types). Many coiled coil type proteins are involved in important biological functions such as the regulation of gene expression, e.g., transcription factors. Coiled coils often, but not always, contain a repeated pattern, hpphppp or hppphpp, of hydrophobic (h) and polar (p) amino-acid residues, referred to as a heptad repeat (as further explained herein below). This repeating pattern in a (poly)peptide sequence naturally folds into an α-helical secondary structure resulting in the presentation of the hydrophobic residues along one face of the helix and the hydrophilic residues along the opposite face forming an amphipathic structure. The most favorable way for two or three such helices to arrange themselves in a water-filled environment is to wrap or sequester the hydrophobic faces of the helix against each other leaving the hydrophilic amino acids solvent exposed. It is thus the burial of hydrophobic surfaces, which provides the thermodynamic driving force for oligomerization of the α-helices and the stability of the structure. The packing in a coiled-coil interface is exceptionally tight. The α-helices may be parallel or anti-parallel, and usually adopt a left-handed super-coil. Although disfavored, a few right-handed coiled coils have also been observed in nature and in designed proteins. The term “coiled coil” or “coiled coil structure” is well- known in the art. Particular reference in this regard is made to review papers concerning coiled-coil structures, such as for example, Cohen and Parry (Proteins 1990, 7:1-15); Kohn and Hodges (TrendsBiotechnol 1998, 16:379-389); Schneider et al. (Fold Des 1998, 3:R29-R40); Harbury et al. (Science 1998,282:1462-1467); Mason and Arndt (Chem-BioChem 2004, 5:170-176); Lupas and Gruber (Adv ProteinChem 2005, 70:37-78); Woolfson (Adv Protein Chem 2005, 70:79-112); Parry et al. (J Struct Biol 2008,163:258-269); and Mcfarlane et al. (Eur J Pharmacol 2009, 625:101-107).The term “heptad repeat region” (as used, e.g., in the expressions “first heptad repeat region” (FHRR) and “second heptad repeat region” (SHRR)), are well-known in the art and are further explained in WO 2023 / 187743 which is incorporated herein by reference in its entirety. A heptad repeat region is a structural motif that forms the basis of most coiled coils. Alpha-helical coiled coils have been characterized at the level of their amino acid sequences in that each helix is constituted of a series of heptad repeats. A heptad repeat (heptad unit, heptad) is a 7-residue sequence motif which can be encoded as hpphppp, wherein each “h” is a hydrophobic amino acid residue and each “p” is apolar (i.e., hydrophilic) amino acid residue. Occasionally, p-residues are observed at h-positions, and viceversa. A heptad repeat is also often encoded by the patterns a-b-c-d-e-f-g (abcdefg) or d-e-f -g-a-b-c (defgabc), in which case the indices “a” to “g” refer to the conventional heptad positions at which typical amino acid types are observed. By convention, the indices “a” and “d” denote the positions of the core residues (central, buried residues) in a coiled coil. The typical amino acid types that are observed at core a- and d-positions are hydrophobic amino acid residues; at all other positions (non-core positions), predominantly polar (hydrophilic) residues are observed. Thus, conventional heptad patterns “hpphppp” match with the pattern notation “abcdefg” (while “hppphpp” patterns match with the pattern notation “defgabc”, this notation being used for coiled coils starting with a hydrophobic residue at a d- position). The heptad repeat regions (HRRs) as referred to herein include at least 2, and suitably 3 or more(preferably consecutive, i.e. uninterrupted) heptad repeats in individual α-helices of the coiled coilstructure. Each series of consecutive heptad repeats in a helix is denoted a “heptad repeat sequence” (HRS). The start and end of a heptad repeat sequence is preferably determined on the basis of the experimentally determined three-dimensional (3D) structure, if available. If a 3D structure is not available, the start and end of a heptad repeat sequence is preferably determined on the basis of an optimal overlay of a (hpphppp)n or (hppphpp)n pattern with the actual amino acid sequence, where “h” and “p” denote hydrophobic and polar (hydrophilic) residues, respectively, and where “n” is a number equal to or greater than 2. The start and end of each heptad repeat sequence is taken to be the first and last hydrophobic residue at an a- or d-position, respectively. Conventional h-residues are preferably selected from valine, isoleucine, leucine, methionine, phenylalanine, tyrosine, tryptophan, histidine, glutamine, threonine, serine and alanine, more preferably from valine, isoleucine, leucine and methionine, and even more preferably isoleucine. Conventional p-residues are preferably selected from glycine, alanine, cysteine, serine, threonine, histidine, asparagine, aspartic acid, glutamine, glutamic acid, lysine and arginine. In case this method does not permit unambiguous assignment of amino acid residues to a heptad repeat sequence, a more specialized analysis method can be applied, such as, e.g.,the COILS method of Lupas et al. (Science 1991, 252(5009): 1162-1164; DOI:10.1126 / science.252.5009.1162; see also: https: / / bio.tools / coils). As used herein the term “complementary” and grammatically equivalent expressions thereof refer to the characteristic of two or more structural elements (e.g., peptide, polypeptide, nucleic acid, small molecule, or portions thereof etc.) of being able to hybridize, oligomerize (e.g., dimerize), interact or otherwise form a complex with each other. For example, “complementary regions of a polypeptide” are capable of coming together to form a complex. As used herein, the term “complex” refers to an assemblage or aggregate of molecules (e.g., peptides, polypeptides, etc.) in direct and / or indirect contact with one another. In specific embodiments, “contact”, or more particularly, “direct contact” means that two or more molecules are close enough so that attractive noncovalent interactions, such as Van der Waal forces, hydrogen bonding, ionic and hydrophobic interactions, and the like, dominate the interaction of the molecules. In such embodiments, a complex of molecules (e.g., a peptide and polypeptide) is formed under conditions such that the complex is thermodynamically favored (e.g., compared to a non-aggregated, or non-complexed, state of its component molecules). As used herein the term “complex”, unless described otherwise, refers to the assemblage of two or more molecules (e.g., peptides, polypeptides or a combination thereof). Herein, the term “complex” may specifically refer to the assemblage of three polypeptides. Throughout this specification, unless the context requires otherwise, the words “comprise,” “comprises” and “comprising”, as well as “contain”, “contains” and “containing”, will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements. Thus, use of the term “comprising” and the like indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present. By “consisting of” is meant including, and limited to, whatever follows the phrase “consisting of”. Thus, the phrase “consisting of” indicates that the listed elements are required or mandatory, and that no other elements may be present. Throughout the present specification, the term “comprising” (or “containing” or the like) also includes the narrower meanings of “consisting essentially of” and “consisting of”. Accordingly, whenever the term “comprising” (or “containing”, “having” or the like) is used, the invention also specifically relates to the corresponding subject-matter defined by the term “consisting essentially of” as well as the corresponding subject-matter defined by the term “consisting of” (in place of “comprising” or “containing”). As used herein, the terms “conjugated”, “linked”, “fused” or “fusion” and their grammatical equivalents, in the context of joining together of two or more elements or components or domains by whatever means including chemical conjugation or recombinant means (e.g., by genetic fusion) are used interchangeably. Methods of chemical conjugation (e.g., using heterobifunctional crosslinking agents) are known in the art. More specifically, as used herein, a “(poly)peptide” – “structure-stabilizing moiety” fusion or conjugate refers to the genetic or chemical conjugation of the (poly)peptide, which is suitably in a metastable, pre-fusion conformation, to a structure-stabilizing moiety. A “conservative amino acid substitution” is one in which the amino acid residue is replaced with an amino acid residue having a similar side chain. Families of amino acid residues having similar side chains have been defined in the art, which can be generally sub-classified in the context of amino acid substitutionsas shown in Table 1:Table 1 Amino acid sub-classification Sub-classes Amino acidsAcidic Aspartic acid, Glutamic acidBasic Noncyclic: Arginine, Lysine; Cyclic: HistidineCharged Aspartic acid, Glutamic acid, Arginine, Lysine, HistidineSmall Glycine, Serine, Alanine, Threonine, ProlineHydrophilic (polar) / neutral Asparagine, Histidine, Glutamine, Cysteine, Serine, Threonine Hydrophilic (polar) / large Asparagine, GlutamineHydrophobic (non-polar) Tyrosine, Valine, Isoleucine, Leucine, Methionine, Phenylalanine,Tryptophan Aromatic Tryptophan, Tyrosine, PhenylalanineResidues that influence Glycine and Proline chain orientation Conservative amino acid substitution also includes groupings based on side chains. For example, a group of amino acids having aliphatic side chains is glycine, alanine, valine, leucine, and isoleucine; a group of amino acids having aliphatic-hydroxyl side chains is serine and threonine; a group of amino acids having amide-containing side chains is asparagine and glutamine; a group of amino acids having aromatic side chains is phenylalanine, tyrosine, and tryptophan; a group of amino acids having basic side chains is lysine, arginine, and histidine; and a group of amino acids having sulfur-containing side chains is cysteine and methionine. For example, it is reasonable to expect that replacement of a leucine with an isoleucine or valine, an aspartate with a glutamate, a threonine with a serine, or a similar replacement of an amino acid with a structurally related amino acid will not have a major effect on the properties of the resulting variant polypeptide. Whether an amino acid change results in a functional polypeptide can readily bedetermined by assaying its activity. Conservative substitutions are shown in Table 2 under the headingof exemplary and preferred substitutions. Amino acid substitutions falling within the scope of the disclosure, are, in general, accomplished by selecting substitutions that do not differ significantly in their effect on maintaining (a) the structure of the peptide backbone in the area of the substitution, (b) the charge or hydrophobicity of the molecule at the target site, or (c) the bulk of the side chain. After the substitutions are introduced, the variants are screened for biological activity. Table 2 Exemplary and Preferred Amino Acid Substitutions Original Residue Exemplary Substitutions Preferred SubstitutionsAla Val, Leu, Ile ValArg Lys, Gln, Asn LysAsn Gln, His, Lys, Arg GlnAsp Glu GluCys Ser SerGln Asn, His, Lys, AsnGlu Asp, Lys AspGly Pro ProHis Asn, Gln, Lys, Arg ArgIle Leu, Val, Met, Ala, Phe, Norleu LeuLeu Norleu, Ile, Val, Met, Ala, Phe IleLys Arg, Gln, Asn ArgMet Leu, Ile, Phe LeuPhe Leu, Val, Ile, Ala LeuPro Gly GlySer Thr ThrThr Ser SerTrp Tyr TyrTyr Trp, Phe, Thr, Ser PheVal Ile, Leu, Met, Phe, Ala, Norleu LeuThe herein provided nucleic acids encoding chimeric polypeptides may be constructs for example comprising a nucleic acid sequence encoding an hMPV fusion protein and a structure-stabilizing moiety. The term “construct” refers to a recombinant genetic molecule including one or more isolated nucleic acid sequences from different sources. Thus, constructs are chimeric molecules in which two or more nucleic acid sequences of different origin are assembled into a single nucleic acid molecule and include any construct that contains (1) nucleic acid sequences, including regulatory and coding sequences that are not found together in nature (i.e., at least one of the nucleotide sequences is heterologous with respect to at least one of its other nucleotide sequences), or (2) sequences encoding parts of functional RNA molecules or proteins not naturally adjoined, or (3) parts of promoters that are not naturally adjoined. Representative constructs include any recombinant nucleic acid molecule such as a plasmid, cosmid, virus, autonomously replicating polynucleotide molecule, phage, or linear or circular single stranded or double stranded DNA or RNA nucleic acid molecule, derived from any source, capable of genomic integration or autonomous replication, comprising a nucleic acid molecule where one or more nucleic acid molecules have been operably linked. Constructs of the present disclosure will generally include the necessary elements to direct expression of a nucleic acid sequence of interest that is also contained in the construct, such as, for example, a target nucleic acid sequence or a modulator nucleic acid sequence. Such elements may include control elements such as a promoter that is operably linked to (so as to direct transcription of) the nucleic acid sequence of interest, and often includes a polyadenylation sequence as well. Within certain embodiments of the disclosure, the construct may be contained within a vector. In addition to the components of the construct, the vector may include, for example, one or more selectable markers, one or more origins of replication, such as prokaryotic and eukaryotic origins, at least one multiple cloning site, and / or elements to facilitate stable integration of the construct into the genome of a host cell. Two or more constructs can be contained within a single nucleic acid molecule, such as a single vector, or can be containing within two or more separate nucleic acid molecules, such as two or more separate vectors. An “expression construct” generally includes at least a control sequence operably linked to a nucleotide sequence of interest. In this manner, for example, promoters in operable connection with the nucleotide sequences to be expressed are provided in expression constructs for expression in an organism or part thereof including a host cell. For the practice of the present disclosure, conventional compositions and methods for preparing and using constructs and host cells are well known to one skilled in the art, see for example, Molecular Cloning: A Laboratory Manual, 3rdedition Volumes 1, 2, and 3. J. F. Sambrook, D. W. Russell, and N. Irwin, Cold Spring Harbor Laboratory Press, 2000. By “effective amount”, in the context of treating, inhibiting the development of, or preventing a condition is meant the administration of an amount of an agent or composition to an individual in need of such treatment, inhibition or prophylaxis, either in a single dose or as part of a series, that is effective for the prevention of incurring a symptom, holding in check such symptoms, and / or treating existing symptoms, of that condition. The effective amount will vary depending upon the health and physical condition of the individual to be treated, the taxonomic group of the individual to be treated, the formulation of the composition, the assessment of the medical situation, and other relevant factors. It is expected that the amount will fall in a relatively broad range that can be determined through routine trials. By the term “linker”, or “flexible linker”, it is meant a molecule or group of molecules (such as a monomer or polymer) that connects two molecules and often serves to place the two molecules in a desirable configuration. The terms “linker”, and “flexible linker”, may herein be used synonymously. The term “linker” or “flexible linker” as used herein preferably refers to a proteinaceous molecule containing at least one amino acid residue, usually at least two amino acids residues joined by peptide bond(s), which molecule permits two polypeptides linked thereby to move more freely relative to one another, as compared to their movement without the flexible linker. In certain embodiments, the flexible linker provides increased rotational freedom for two polypeptides linked thereby than the two linked polypeptides would have in the absence of the flexible linker. Such freedom of relative movement or rotational freedom allows polypeptides joined by the flexible linker to perform their individual functions or elicit their activities with less structural hindrance. A flexible linker may be characterized by the absence of secondary structures such as helices or β-sheets or a maximal secondary structure content of 10%, 20% 30% or 40%. Non-limiting examples of flexible linkers include the amino acid sequences GGSGG (SEQ ID NO: 34), GSG, GS, GGSG (SEQ ID NO: 35), GSGS (SEQ ID NO: 36), AS, GGGS (SEQ ID NO: 37), G4S (SEQ ID NO: 38), (G4S)2 (SEQ ID NO: 39), (G4S)3 (SEQ ID NO: 40), (G4S)4 (SEQ ID NO: 41), G4SG (SEQ ID NO: 42), GSGG (SEQ ID NO: 43) and GSGGS (SEQ ID NO: 44). Additional flexible linker sequences are well known in the art. In various embodiments, the flexible linker contains or consists of at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 amino acid residues. In some of the same and other embodiments, the flexible linker contains or consists of up to 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 amino acid residues. In some of the same and other embodiments, the flexible linker contains or consists of between about 1 to about 30 amino acid residues, between about 1 to about 25 amino acid residues, between about 1 to about 20 amino acid residues, between about 1 to about 15 amino acid residues, between about 1 to about 12 amino acid residues, between about 1 to about 10 amino acid residues, between about 1 to about 8 amino acid residues, between about 1 to about 6 amino acid residues, between about 1 to about 5 amino acid residues, between about 1 to about 4 amino acid residues, or between about 1 to about 3 amino acid residues. In some of the same and other embodiments, the flexible linker contains or consists of between about 2 to about 30 amino acid residues, between about 2 to about 25 amino acid residues, between about 2 to about 20 amino acid residues, between about 2 to about 15 amino acid residues, between about 2 to about 12 amino acid residues, between about 2 to about 10 amino acid residues, between about 2 to about 8 amino acid residues, between about 2 to about 6 amino acid residues, between about 2 to about 5 amino acid residues, or between about 2 to about 4 amino acid residues. In some of the same and other embodiments, the flexible linker contains or consists of between about 3 to about 30 amino acid residues, between about 3 to about 25 amino acid residues, between about 3 to about 20 amino acid residues, between about 3 to about 15 amino acid residues, between about 3 to about 12 amino acid residues, between about 3 to about 10 amino acid residues, between about 3 to about 8 amino acid residues, between about 3 to about 6 amino acid residues, or between about 3 to about 5 amino acid residues. In certain embodiments, the flexible linker contains or consists of 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 amino acid residues. In particular, a flexible linker may be composed of amino acid residues (e.g., having any of the above-mentioned exemplary numbers of amino acid residues), wherein preferably at least about 70% (more preferably at least about 80%, even more preferably at least about 90%, even more preferably at least about 95%, still more preferably 100%) of said amino acid residues are selected from glycine, serine and alanine; more preferably, at least about 70% (more preferably at least about 80%, even more preferably at least about 90%, even more preferably at least about 95%, still more preferably 100%) of said amino acid residues are selected from glycine and serine. In some embodiments, the said amino acid residues are all glycine. As used herein, the term “moiety” refers to a portion of a molecule, which may be a functional group, a set of functional groups, and / or a specific group of atoms within a molecule, that is responsible for a characteristic chemical, biological, and / or medicinal property of the molecule. The term “oligomer” refers to a molecule that consists of more than one but a limited number of monomer units in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Oligomers include, but are not limited to, dimers, trimers, tetramers, pentamers, hexamers, heptamers, octamers, nonamers, decamers and the like. An oligomer can be a macromolecular complex formed by non-covalent bonding of macromolecules like proteins. In this sense, a homo-oligomer would be formed by identical molecules and by contrast, a hetero-oligomer would be made of at least two different molecules., or a “heterotrimeric polypeptide complex” consisting of three polypeptide subunits in which at least one subunit polypeptide is non-identical. In the context of the herein provided chimeric polypeptides, it is preferred that the hMPV fusion protein and the structure-stabilizing moiety are operably linked (optionally by a linker). The term “operably connected” or “operably linked” as used herein refers to a juxtaposition wherein the components so described are in a relationship permitting them to function in their intended manner. For example, a regulatory sequence (e.g., a promoter) “operably linked” to a nucleotide sequence of interest (e.g., a coding and / or non-coding sequence) refers to positioning and / or orientation of the regulatory sequence relative to the nucleotide sequence of interest to permit expression of that sequence under conditions compatible with the regulatory sequence. The regulatory sequences need not be contiguous with the nucleotide sequence of interest, so long as they function to direct its expression. Thus, for example, intervening non-coding sequences (e.g., untranslated, yet transcribed, sequences) can be present between a promoter and a coding sequence, and the promoter sequence can still be considered “operably linked” to the coding sequence. Likewise, “operably connecting” an hMPV fusion protein to a structure-stabilizing moiety (SSM) encompasses positioning and / or orientation of the structure-stabilizing moiety (SSM) such that it can, under suitable conditions (e.g., in aqueous solution and / or physiological conditions), associate with the structure-stabilizing moieties (SSMs) of two further chimeric polypeptides to form a trimer, wherein preferably, in the trimer, the FHRRs and SHRRs of the three SSMs are associated in the form of a six-helix bundle. The terms “patient”, “subject”, “host” or “individual” used interchangeably herein, refer to any subject, particularly a vertebrate subject, and even more particularly a mammalian subject, for whom therapy or prophylaxis is desired. Suitable vertebrate animals that fall within the scope of the disclosure include, but are not restricted to, any member of the subphylum Chordata including primates (e.g., humans,monkeys and apes, and includes species of monkeys such from the genus Macaca (e.g., cynomolgusmonkeys such as Macaca fascicularis, and / or rhesus monkeys (Macaca mulatta)) and baboon (Papioursinus), as well as marmosets (species from the genus Callithrix), squirrel monkeys (species from thegenus Saimiri) and tamarins (species from the genus Saguinus), as well as species of apes such aschimpanzees (Pan troglodytes)), rodents (e.g., mice, rats, guinea pigs), lagomorphs (e.g., rabbits, hares), bovines (e.g., cattle), ovines (e.g., sheep), caprines (e.g., goats), porcines (e.g., pigs), equines (e.g., horses), canines (e.g., dogs), felines (e.g., cats), avians (e.g., chickens, turkeys, ducks, geese, or otherpoultry, companion birds such as canaries, budgerigars etc.), marine mammals (e.g., dolphins, whales),reptiles (snakes, frogs, lizards etc.), and fish. A preferred subject is a human, particularly a human in needof eliciting an immune response to a fusion protein of an enveloped virus (such as hMPV), or complex thereof. However, it will be understood that the aforementioned terms do not imply that symptoms are present. By “pharmaceutically acceptable carrier” is meant a solid or liquid filler, diluent or encapsulating substance that can be safely used in topical or systemic administration to an animal, preferably a mammal, including humans. Representative pharmaceutically acceptable carriers include any and all solvents, dispersion media, coatings, surfactants, antioxidants, preservatives (e.g., antibacterial agents, antifungal agents), isotonic agents, absorption delaying agents, salts, preservatives, drugs, drug stabilizers, gels, binders, excipients, disintegration agents, lubricants, sweetening agents, flavoring agents, dyes, such like materials and combinations thereof. Except insofar as any conventional carrier is incompatible with the active ingredient(s), its use in the pharmaceutical compositions is contemplated. The terms “peptide”, “polypeptide”, “(poly)peptide” and “protein” are used herein interchangeably and refer to a polymer of two or more amino acids linked via amide bonds (i.e., peptide bonds) that are formed between an amino group of one amino acid and a carboxyl group of another amino acid. The amino acids comprised in the peptide, polypeptide, (poly)peptide, or protein, which are also referred to as amino acid residues, may be selected from the 20 standard proteinogenic α-amino acids (i.e., Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, His, Ile, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr, and Val) but also from non- proteinogenic and / or non-standard α-amino acids (such as, e.g., ornithine, citrulline, homolysine, pyrrolysine, 4-hydroxyproline, α-methylalanine (i.e., 2-aminoisobutyric acid), norvaline, norleucine, terleucine (tert-leucine), labionin, or an alanine or glycine that is substituted at the side chain with a cyclic group such as, e.g., cyclopentylalanine, cyclohexylalanine, phenylalanine, naphthylalanine, pyridylalanine, thienylalanine, cyclohexylglycine, or phenylglycine) as well as β-amino acids (e.g., β- alanine), γ-amino acids (e.g., γ-aminobutyric acid, isoglutamine, or statine) and δ-amino acids. Preferably, the amino acid residues comprised in the peptide, polypeptide or protein are selected from α-amino acids, more preferably from the 20 standard proteinogenic α-amino acids (which can be present as the L-isomer or the D-isomer, and are preferably all present as the L-isomer). The peptide, polypeptideor protein may be unmodified or may be modified, e.g., at its N-terminus, at its C-terminus and / or at a functional group in the side chain of any of its amino acid residues (particularly at the side chain functional group of one or more Lys, His, Ser, Thr, Tyr, Cys, Asp, Glu, and / or Arg residues). Such modifications may include, e.g., the attachment of any of the protecting groups described for the corresponding functional groups in: Wuts PG & Greene TW, Greene’s protective groups in organic synthesis, John Wiley & Sons, 2006. Such modifications may also include the covalent attachment of one or more polyethylene glycol (PEG) chains (forming a PEGylated peptide, polypeptide or protein), the covalent attachment of albumin, the glycosylation and / or the acylation with one or more fatty acids (e.g., one or more C8-30 alkanoic or alkenoic acids; forming a fatty acid acylated peptide, polypeptide or protein). Moreover, such modified peptides, polypeptide or proteins may also include peptidomimetics, provided that they contain at least two amino acids that are linked via an amide bond (formed between an amino group of one amino acid and a carboxyl group of another amino acid). The amino acid residuescomprised in the peptide, polypeptide or protein may, e.g., be present as a linear molecular chain(forming a linear peptide, polypeptide or protein) or may form one or more rings (corresponding to a cyclic peptide, polypeptide or protein). The peptide, polypeptide or protein may also form oligomers consisting of two or more identical or different molecules. Accordingly, peptides, polypeptides and proteins may form dimers, trimers and higher oligomers, wherein the peptide, polypeptide or protein molecules forming such dimers, trimers etc. may be identical or non-identical. The corresponding higher order structures are, consequently, termed homo- or heterodimers, homo- or heterotrimers, and homo- or heterooligomers (etc.). Such dimers, trimers and oligomers are likewise embraced by the terms “peptide”, “polypeptide”, “(poly)peptide” and “protein”. The term “amino acid” refers, in particular, to any one of the 20 standard proteinogenic α-amino acids (i.e., Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, His, Ile, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr, and Val) but also to non-proteinogenic and / or non-standard α-amino acids (such as, e.g., ornithine, citrulline, homolysine, pyrrolysine, 4-hydroxyproline, α-methylalanine (i.e., 2-aminoisobutyric acid), norvaline, norleucine, terleucine (tert-leucine), labionin, or an alanine or glycine that is substituted at the side chain with a cyclic group such as, e.g., cyclopentylalanine, cyclohexylalanine, phenylalanine, naphthylalanine, pyridylalanine, thienylalanine, cyclohexylglycine, or phenylglycine) as well as β-amino acids (e.g., β- alanine), γ-amino acids (e.g., γ-aminobutyric acid, isoglutamine, or statine) and / or δ-amino acids as well as any other compound comprising at least one carboxylic acid group and at least one amino group. Unless defined otherwise, an “amino acid” preferably refers to an α-amino acid, more preferably to any one of the 20 standard proteinogenic α-amino acids (which can be present as the L-isomer or the D- isomer, and are preferably present as the L-isomer). Whenever an amino acid is referred to herein, unless explicitly stated otherwise, it is preferred that the corresponding amino acid is an L-amino acid. Likewise, whenever an amino acid sequence is referred to herein, unless explicitly stated otherwise, it is preferred that all amino acids in the corresponding amino acid sequence are L-amino acids. As used herein, the term “post-fusion conformation” of a viral fusion protein refers to the structure of the virus fusion protein which is in a terminal conformation (i.e., formed at the end of the fusion process) and is the most energetically favorable state. In the post-fusion conformation, the fusion peptides or loops of the fusion protein are brought into close proximity with the fusion protein transmembrane domain. The specific structural elements that facilitate formation of the hairpin structure vary according to the class of enveloped fusion protein. For example, the post-fusion conformation of a Class I fusion protein is characterized by interaction between the endogenous FHRR region and the endogenous SHRR region of individual Class I fusion proteins to form a hairpin structure characterized by a six-helix bundle, comprising three endogenous SHRR and three endogenous FHRR regions. Alternatively, the post-fusion conformation of a Class III fusion protein is characterized by interaction between the internal fusion loops and the C-terminal transmembrane region which facilitates the formation of a hairpin structure. Post-fusion conformations of individual viral fusion proteins have been determined by electron microscopy and / or x-ray crystallography, such structures are readily identifiable when viewed in negatively stained electron micrographs and / or by a lack of pre-fusion epitopes. As used herein, the term “pre-fusion conformation”, “pre-fusion state” or the like of a viral fusion protein refers to the structure of the viral fusion protein which is in a meta-stable confirmation (i.e., in a semi- stable conformation that is not the most energetically favorable terminal conformation) and upon appropriate triggering is able to undergo conformational rearrangement to the terminal post-fusion conformation. Typically, pre-fusion conformations of viral fusion proteins contain a hydrophobic sequence, referred to as the fusion peptide or fusion loop, that is located internally within the pre-fusion conformation and cannot interact with either the viral or host cell membranes. Upon triggering this hydrophobic sequence is inserted into the host cell membrane and the fusion protein collapses into the post-fusion hairpin like conformation. The pre-fusion conformation of viral fusion proteins varies according to the class of enveloped fusion protein. Each class is characterized by non-interacting structural elements that subsequently associate in the energetically favorable post-fusion conformation. For example, the pre-fusion conformation of a Class I fusion protein is dependent on the endogenous FHRR region not interacting with the endogenous SHRR region of individual fusion proteins of the trimer, thereby not permitting formation of a hairpin structure characterized by a six-helix bundle. Alternatively, the pre-fusion conformation of a Class III fusion protein is dependent a central a-helical coiled coil not interacting with fusion loop(s) at the C-terminal region of individual fusion proteins of the trimer, thereby not permitting formation of a hairpin structure. Pre-fusion conformations of individual viral fusion proteins have been determined by electron microscopy and / or X-ray crystallography, such structures are readily identifiable when viewed in negatively stained electron micrographs and / or by pre-fusion epitopes that are not present on post-fusion conformations. The term “sequence identity”, as used herein, refers to the sequence match between two (poly)peptides or nucleic acids. The (poly)peptide or nucleic acid sequences to be compared are aligned to give maximum identity, for example, using bioinformatics tools for pairwise alignment such as EMBOSSNeedle (https: / / www.ebi.ac.uk / Tools / psa / emboss_needle / ; see also Madeira F, et al. The EMBL-EBIsearch and sequence analysis tools APIs in 2019. Nucleic Acids Research.2019 Jul;47(W1):W636-W641. DOI: 10.1093 / nar / gkz268). When the same position in the sequences to be compared is occupied by the same nucleobase or amino acid residue, then the respective molecules are identical at that very position. Accordingly, the “sequence identity”, “percent identity” or “percent sequence identity” is a function of the number of matching positions divided by the number of positions compared and multiplied by 100%. For example, if 6 out of 10 sequence positions are identical, then the identity is 60%. The “identity” or“percent (%) identity” between two amino acid sequences can, e.g., be determined by using theNeedleman-Wunsch algorithm (Needleman, S.B. and Wunsch, CD. A general method applicable to thesearch for similarities in the amino acid sequence of two proteins. J Mol Biol. 1970;48(3):443-53. DOI:10.1016 / 0022-2836(70)90057-4.) which has been incorporated into EMBOSS Needle, using a BLOSUM62 matrix, a "gap open penalty" of 10, a "gap extend penalty" of 0.5, a false "end gap penalty", an "end gap open penalty" of 10 and an "end gap extend penalty" of 0.5. The percent (%) identity is typically determined over the entire length of the query sequence on which the analysis is performed. Two molecules having the same primary amino acid or nucleic acid sequence are identical irrespective of any chemical and / or biological modification. For example, two antibodies having the same primary amino acid sequence, but different glycosylation patterns are identical by this definition. In case of nucleic acids, for example, two molecules having the same sequence but different linkage components such as thiophosphate instead of phosphate are identical by this definition. Accordingly, the skilled person can readily determine the ‘corresponding positions’ in a reference amino acid sequence. For example, a position 'corresponding to' a specific position in a reference sequence (e.g., SEQ ID NO: 28) can be determined by aligning a variant sequence with the reference sequence using a standard algorithm, such as the Needleman-Wunsch algorithm with default parameters as implemented in EMBOSS Needle, and identifying the residue in the variant sequence that aligns with the specified residue in the reference sequence. As used herein, the terms “treatment”, “treating”, and the like, refer to obtaining a desired pharmacologic and / or physiologic effect. The effect may be prophylactic in terms of completely or partially preventing a disease or symptom thereof and / or may be therapeutic in terms of a partial or complete cure for a disease and / or adverse effect attributable to the disease. “Treatment”, as used herein, covers any treatment of a disease in a mammal, particularly in a human, and includes: (a) preventing the disease from occurring in a subject which may be predisposed to the disease but has notyet been diagnosed as having it; (b) inhibiting the disease, i.e., arresting its development; and (c) relievingthe disease, i.e., causing regression of the disease.The terms “wild-type”, “native” and “naturally occurring” are used interchangeably herein to refer to a gene or gene product that has the characteristics of that gene or gene product when isolated from a naturally occurring source. A wild type, native or naturally occurring gene or gene product (e.g., a polypeptide) is that which is most frequently observed in a population and is thus arbitrarily designed the “normal” or “wild-type” form of the gene or gene product. The terms “T3 PD”, “T3-PD”, “T3PD“, or the like may herein be used synonymously. The same applies to the other herein exemplified antigens (e.g., “T4-P” can for example also be termed “T4 P”). The term “adverse event” or “AE”, as used herein, refers to any untoward medical occurrence in a clinical trial participant administered an investigational medicinal product (IMP) or a control (such as a placebo or a comparator drug), which does not necessarily have a causal relationship with the treatment. An AE can therefore be any unfavorable and unintended sign, including an abnormal laboratory or instrumental finding, symptom, or disease that is temporally associated with the use of the IMP, whether or not considered related to the IMP. This includes, for example, a significant or unexpected worsening of the condition under study, the exacerbation of a chronic or intermittent pre-existing condition, the detection or diagnosis of a new condition after IMP administration, or events that occur as a result of protocol- mandated procedures. The term “solicited adverse event”, as used herein, refers to a predefined list of adverse events that are proactively monitored during a clinical study due to their frequent occurrence as reactions to vaccines. In the context of the present invention, solicited AEs specifically include local injection site reactions such as pain, redness, and swelling, and systemic reactions such as fever, headache, shivering, fatigue, muscle pain (myalgia), joint pain (arthralgia), and gastrointestinal symptoms. By definition within the clinical protocol, all such solicited AEs are considered to be related to the IMP. The term “unsolicited adverse event” refers to an adverse event that is not part of the predefined list of solicited AEs and is spontaneously reported by a study participant or detected by an investigator. Unlike solicited AEs, each unsolicited AE is subject to a formal medical judgment by a blinded investigator or delegate to determine its causal relationship to the administration of the IMP. Based on a comprehensive review of all available clinical data, the unsolicited AE is thereby classified as either ‘related’ or ‘unrelated’ to the IMP. As used herein, the term “reactogenicity” refers to the physical manifestation of the inflammatory response to vaccination and is characterized by the occurrence of expected local and systemic adverse reactions. The reactogenicity profile of a vaccine is assessed by monitoring a predefined set of solicited adverse events within a specified time period following administration. The term “serious adverse event” or “SAE”, as used herein, refers to any adverse event that, at any dose, meets one or more of the following criteria: (a) results in death; (b) is life-threatening, meaning the participant was at risk of death at the time of the event; (c) requires inpatient hospitalization or prolongation of existing hospitalization; (d) results in persistent or significant disability or incapacity; (e) is a congenital anomaly or birth defect; or (f) is another important medical event that, based on medical or scientific judgment, may jeopardize the participant or may require medical or surgical intervention to prevent one of the other outcomes listed. Examples of such other important medical events include, but are not limited to, invasive or malignant cancers, allergic bronchospasm requiring intensive treatment, or blood dyscrasias or convulsions that do not result in hospitalization. As used herein, the terms “severity” or “intensity” of an adverse event are categorized into grades to provide an objective measure of the event’s impact. The specific criteria are defined by the context of the adverse event as follows: In the context of solicited systemic adverse events, severity is defined as: ^ Grade 1 (mild): The event is present and easily tolerated by the participant (e.g., for fever, a temperature of ≥ 38.0°C to ≤ 38.5°C). ^ Grade 2 (moderate): The event is present and interferes with normal activity (e.g., for fever, a temperature of > 38.5°C to ≤ 39.0°C). ^ Grade 3 (severe): The event is present and prevents normal activity (e.g., for fever, a temperature of > 39.0°C). In the context of solicited local adverse events, severity is defined as: ^ For pain: Grade 1 (mild) refers to pain that does not interfere with normal activity; Grade 2 (moderate) refers to pain that interferes with normal activity when the limb is moved; and Grade 3 (severe) refers to significant pain at rest that prevents normal activity. ^ For redness or swelling: Grade 1 (mild) refers to a diameter of > 25 mm to ≤ 50 mm; Grade 2 (moderate) refers to a diameter of > 50 mm to ≤ 100 mm; and Grade 3 (severe) refers to a diameter of > 100 mm. In the context of unsolicited adverse events, severity is defined as: ^ Grade 1 (mild): The AE is easily tolerated by the participant, causing minimal discomfort, and does not interfere with normal everyday activities. ^ Grade 2 (moderate): The AE is sufficiently discomforting to interfere with normal everyday activities. ^ Grade 3 (severe): The AE prevents normal everyday activities.Each embodiment described herein is to be applied mutatis mutandis to each other embodimentdescribed herein, unless specifically stated otherwise. Accordingly, the present invention specifically relates to each combination of features and embodiments described herein, including any combination of general and / or preferred features / embodiments. The present invention further relates to the following items: 1. A chimeric polypeptide comprising a human metapneumovirus (hMPV) fusion protein and a structure-stabilizing moiety, wherein said hMPV fusion protein comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 28, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI or NKI, wherein said structure-stabilizing moiety comprises a first heptad repeat region (FHRR) and a second heptad repeat region (SHRR), wherein said FHRR comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32, and wherein said SHRR comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 33. 2. The chimeric polypeptide according to item 1, wherein said hMPV fusion protein comprises any one or more, preferably all, of the following amino acid residues: (i) a cysteine residue at position 122 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position; (ii) a cysteine residue in position 129 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position; and / or (iii) a proline residue in position 167 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position. 3. The chimeric polypeptide according to item 1 or 2, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI. 4. The chimeric polypeptide according to any one of items 1 to 3, wherein said hMPV fusion protein comprises the amino acid sequence set forth in SEQ ID NO: 28. 5. The chimeric polypeptide according to any one of items 1 to 4, wherein said FHRR and said SHRR are linked via a linker, preferably wherein said linker is a peptide linker. 6. The chimeric polypeptide according to item 5, wherein said linker comprises an amino acid sequence selected from GGSGG (SEQ ID NO: 34), GSG, GS, GGSG (SEQ ID NO: 35), GSGS (SEQ ID NO: 36), AS, GGGS (SEQ ID NO: 37), G4S (SEQ ID NO: 38), (G4S)2 (SEQ ID NO: 39), (G4S)3 (SEQ ID NO: 40), (G4S)4 (SEQ ID NO: 41), G4SG (SEQ ID NO: 42), GSGG (SEQ ID NO: 43), and GSGGS (SEQ ID NO: 44). 7. The chimeric polypeptide according to item 5 or 6, wherein said linker comprises the amino acid sequence GGSGG (SEQ ID NO: 34). 8. The chimeric polypeptide according to any one of items 1 to 7, wherein said structure-stabilizing moiety comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 31. 9. The chimeric polypeptide according to item 8, wherein said structure-stabilizing moiety comprises the amino acid sequence set forth in SEQ ID NO: 31. 10. The chimeric polypeptide according to any one of items 1 to 9, wherein said hMPV fusion protein and said structure-stabilizing moiety are linked via a linker, preferably wherein said linker is a peptide linker. 11. The chimeric polypeptide according to item 10, wherein said linker comprises an amino acid sequence selected from GSG, GS, GGSGG (SEQ ID NO: 34), GGSG (SEQ ID NO: 35), GSGS (SEQ ID NO: 36), AS, GGGS (SEQ ID NO: 37), G4S (SEQ ID NO: 38), (G4S)2 (SEQ ID NO: 39), (G4S)3 (SEQ ID NO: 40), (G4S)4 (SEQ ID NO: 41), G4SG (SEQ ID NO: 42), GSGG (SEQ ID NO: 43), and GSGGS (SEQ ID NO: 44). 12. The chimeric polypeptide according to item 11, wherein said linker comprises the amino acid sequence GSG. 13. The chimeric polypeptide according to any one of items 1 to 12, wherein said chimeric polypeptide comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 29. 14. The chimeric polypeptide according to any one of items 1 to 13, wherein said chimeric polypeptide consists of the amino acid sequence set forth in SEQ ID NO: 29. 15. A nucleic acid comprising a nucleic acid sequence encoding the chimeric polypeptide according to any one of items 1 to 14. 16. The nucleic acid according to item 15, wherein said nucleic acid is a ribonucleic acid (RNA), preferably a messenger RNA (mRNA). 17. A composition comprising the chimeric polypeptide according to any one of items 1 to 14 or the nucleic acid according to item 15 or 16, preferably wherein said composition is a pharmaceutical composition. 18. A kit comprising the chimeric polypeptide according to any one of items 1 to 14, the nucleic acid according to item 15 or 16, or the composition according to item 17. 19. The chimeric polypeptide according to any one of items 1 to 14, the nucleic acid according to item 15 or 16, the composition according to item 17, or the kit according to item 18, for use in the immunization and / or the vaccination of a subject. 20. A method for the immunization and / or the vaccination of a subject, wherein the method comprises administering the chimeric polypeptide according to any one of items 1 to 14, the nucleic acid according to item 15 or 16, the composition according to item 17, or the kit according to item 18 to said subject. 21. Use of the chimeric polypeptide according to any one of items 1 to 14, or the nucleic acid according to item 15 or 16, in the manufacture of a medicament for the immunization and / or the vaccination of a subject. 22. The chimeric polypeptide according to any one of items 1 to 14, the nucleic acid according to item 15 or 16, the composition according to item 17, or the kit according to item 18, for use in the prevention of an hMPV infection. 23. A method for the prevention of an hMPV infection, wherein the method comprises administering the chimeric polypeptide according to any one of items 1 to 14, the nucleic acid according to item 15 or 16, the composition according to item 17, or the kit according to item 18, to a subject in need thereof. 24. Use of the chimeric polypeptide according to any one of items 1 to 14, or the nucleic acid according to item 15 or 16, in the manufacture of a medicament for the prevention of an hMPV infection. 25. A nucleic acid vector comprising the nucleic acid according to item 15. 26. A host cell comprising the nucleic acid according to item 15 or 16, and / or the nucleic acid vector according to item 25. 27. A method of production of a chimeric polypeptide, wherein the method comprises: (i) culturing the host cell according to item 26; or (ii) expressing a chimeric polypeptide from the nucleic acid according to item 15 or 16, and / or the nucleic acid vector according to item 25, optionally in a cell-free transcription- translation system. 28. A method of production an antigen-binding molecule that specifically binds to an hMPV fusion protein, the method comprising: (i) immunizing an animal with the chimeric polypeptide according to any one of items 1 to 14, or the nucleic acid according to item 15 or 16, or the composition according to item 17; (ii) identifying and / or isolating a B cell from the immunized animal, which specifically binds to the hMPV fusion protein; and (iii) producing the antigen-binding molecule expressed by that B cell. 29. An antigen-binding molecule obtainable by the method according to item 28. The invention is also described by the following illustrative and non-limiting figures. The appended figures show: Figure 1. Yield of antigen panels as determined post-purification via absorbance at 280nm. Figure 2. Top Panel: Size Exclusion Chromatography (“SEC”; Superdex™ 200 HR) for comparison of antigen hMPV panel of C-terminal truncations T1-T19. Peak at approximately 10-11ml represents the expected size for the soluble trimer (EXP_CA50). Lower Panel: Relative percentage of high molecular weight (HMW) protein (7-9.5ml), trimer (9.5-12ml) and low molecular weight (LMW) protein (12-15ml). Figure 3. Analysis of mAb binding to hMPV panel of C-terminal truncations T1-T19 determined by ELISA. Figure 4. Yield of hMPV_F_CT5s antigens T3, T3P, T3D, T3PD, T4, T4P and T4PD as determined post- purification via absorbance at 280nm. Data presented as mean + / - standard deviation. Figure 5. SDS-PAGE analysis of hMPV_F_CT5s antigens T3, T3P, T3D, T3PD, T4, T4P and T4PD following incubation at 4°C, 25°C or 40°C for 5 days. Molecular weight markers are displayed in each unlabeled lane of the SDS-PAGE, with kDa shown on the right. Figure 6. Analysis by Size Exclusion Chromatography (Superdex™ 200 HR) of hMPV_F_CT5s antigens T3, T3P, T3D, T3PD, T4, T4P and T4PD either soon after purification (Time 0) or following incubation at 25°C or 40°C for 5 days. Figure 7. Comparison of mAb kDvalues calculated by ELISA for hMPV_F_CT5s antigens T3, T3P, T3D, T3PD, T4, T4P and T4PD at 4°C, 25°C or 40°C for 5 days. Figure 8. Analysis by Fab-Shift Size Exclusion Chromatography (Superdex™ 200 HR) of hMPV_F_CT5s antigens T3, T3P, T3D, T3PD, T4, T4P and T4PD with MPE8 FabCtag. Figure 9. T3 PD manufacturing yield from clonal cell line fed-batch production (top) and liquid stability of T3 PD drug product (bottom). Data presented as mean + / - standard deviation. Figure 10. Neutralisation of hMPV strain by mouse sera following administration of two doses of either T3 PD, non-stabilised Fsol or best in class comparator DSCavEs2. Data presented as geometric mean + / - geometric standard deviation. Figure 11. SDS-PAGE analysis of purified T3PD, with native signal peptide (A), and T3PD IgK, with IgK signal peptide (B). M = marker; R = reduced protein; NR = non-reduced protein.Figure 12. SEC analysis of T3PD (A) and comparator hMPV antigen DSCavEs2 (B) using a Superdex 200Increase 10 / 300 GL (Cytiva). Figure 13. Relative kD values determined by ELISA of hMPV specific mAbs binding to non-stabilised Fsol, DSCavEs2, v3B_Δ12 and MPV T3PD.Figure 14. Immunogenicity in BALB / c mice. (A) Serum IgG titre to the hMPV F antigens (black), and theIgG titre specific to either the foldon or MC2S trimer stabilising domain (grey). Titres were determinedby ELISA and expressed as EC50 values. (B) Relative proportion of foldon / MC2S reactivity calculated as apercentage of the total antigen specific titre.Figure 15. Ability of hMPV antigens to deplete neutralizing antibodies form human plasma. hMPV PRNT50with titration of pooled human plasma (black circles) or human plasma depleted of antigen-reactive antibodies via incubation with Sepharose immobilised antigens DsCavEs2 (blue squares) and T3PD (purple triangles).Figure 16. Antigen Stability in liquid formulation. MPV T3PD stability analysis out to 12 months at 2-8°Cas determined by percentage of product as a single MW by reduced CE-SDS, SE-HPLC, or by potency testing capture ELISA with paired neutralising antibodies.Figure 17. Relative proportion of peptide accessibility between v3B-Δ12 and T3PD mapped onto the pre-fusion structure of hMPV F. Arrows ‘A’ and ‘B’ indicate areas of increased hydrogen / deuterium exchange (HDX) for T3PD relative to v3B-Δ12 and arrow ‘C’ indicate an area of lower HDX for T3PD relative to v3B- Δ12.Figure 18. Comparative analysis of Deuterium uptake by T3PD compared to v3BΔ12. Line plotrepresentation of difference in deuterium uptake (B-factor) assessed across three timepoints. Positive values indicate increased Deuterium exchange in T3PD relative to v3BΔ12 (as indicated by shaded barsA-H in panel A), and negative values indicate increased deuterium exchange in v3BΔ12 relative to T3PD(as indicated by shaded bars I-O in panel B).Figure 19. (A) RSV-A and (B) RSV-B Neutralisation Titre prior to vaccination (Baseline) and at 30 daysfollowing vaccination with either Placebo, Arexvy (GSK) or VXB-241 at 4 doses levels (60 µg, 120 µg, 240 µg or 480 µg).Figure 20. (A) hMPV-A and (B) hMPV-B Neutralisation Titre prior to vaccination (“Baseline”) and at 30days following vaccination with either Placebo, Arexvy (GSK) or VXB-241 at 4 doses levels (60 µg, 120 µg, 240 µg or 480 µg; “Day30”). List of sequences The present invention further provides the following amino acid sequences, which are also described in the concurrently filed sequence listing forming part of the present specification. It will be understood that the present invention relates to the sequences disclosed herein, including in the following list of sequences, as well as the sequences disclosed in the concurrently filed sequence listing. In case of any discrepancy between any one of the sequences disclosed in the present specification, including in the list of sequences herein below, and the corresponding sequence disclosed in the sequence listing, the present invention specifically and individually relates to each one of the respective sequences. SEQ ID NO: 1 HMPV fusion protein full lengthAlso referred to as ‘FWT’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRILSSAEKGNTGFIIVIILTAVLGSTMILVSVFIIIKKTKKPTGAPPELSGV wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is the transmembrane domain; and^ Underlined text is the cytoplasmic domain.SEQ ID NO: 2 HMPV F CT5S T1 Also referred to as ‘T1’ MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRILSGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAA NQTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 3HMPV F CT5S T2Also referred to as ‘T2’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRILGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAAN QTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 4HMPV F CT5S T3Also referred to as ‘T3’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRIGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQ THIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 5HMPV F CT5S T4Also referred to as ‘T4’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQ THIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 6HMPV F CT5S T5Also referred to as ‘T5’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQT HIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM. SEQ ID NO: 7HMPV F CT5S T6Also referred to as ‘T6’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHI AQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 8HMPV F CT5S T7Also referred to as ‘T7’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ GSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIA QRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 9HMPV F CT5S T8Also referred to as ‘T8’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDG SGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQ RDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 10HMPV F CT5S T9Also referred to as ‘T9’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVGS GLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQ RDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 11HMPV F CT5S T10Also referred to as ‘T10’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALGSG LANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRD ARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 12HMPV F CT5S T11Also referred to as ‘T11’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQAGSGL ANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRD ARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 13HMPV F CT5S T12Also referred to as ‘T12’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQGSGLA NATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDA RRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 14HMPV F CT5S T13Also referred to as ‘T13’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSGSGLAN ATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDAR RI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 15HMPV F CT5S T14Also referred to as ‘T14’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENGSGLANA TAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 16HMPV F CT5S T15Also referred to as ‘T15’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIEGSGLANAT AAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 17HMPV F CT5S T16Also referred to as ‘T16’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIGSGLANATA AQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 18HMPV F CT5S T17Also referred to as ‘T17’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESGSGLANATAA QQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 19HMPV F CT5S T18Also referred to as ‘T18’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFEGSGLANATAA QQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 20HMPV F CT5S T19Also referred to as ‘T19’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFGSGLANATAAQ QEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 21 HMPV fusion protein DS CavEs2 Foldon (Hsieh et al., Nat Commun 2022, DOI: 10.1038 / s41467- 022-28931-3)Also referred to as ‘DSCavEs2’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLT KSALRELRTVSADQLAREEQIENPRRRRFVLGAIACGVATAAAVTAGVAIAKCIRLESEVTAIKNCLKKTNECV STLGCGVRVLATAVRELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISKD LMTDAELARAISNMPTSAGQIKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCS EKKGNYACLLREDQGWYCQNAGSTVYYPCEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCK VSCGRHPISMVALSPLGALVACYKGVSCSIGSNRVGIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQ HVIKGRPVSSSFDPVKFPQDQFNVALDQCFESIENSQALVDQSNRILSSAEKGNTGGGGSGYIPEAPRDG QAYVRKDGEWVLLSTFLGRSLEVLFQGPGHHHHHHHHSAWSHPQFEK wherein: ^Italicized text corresponds to the signal peptide of HMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to Foldon SSM. SEQ ID NO: 22 HMPV fusion protein solubleAlso referred to as ‘Fsol’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRRRRFVLGAIALGVATAAAVTAGVAIAKCIRLESEVTAIKNALKKTNEAVSTLGCGVRVLATAV RELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISKDLMTDAELARAISNMPTSAGQI KLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSCGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQCFESIENSQALVDQ SNRILSSAEKGNTGSGSEPEA wherein: ^Italicized text corresponds to the signal peptide of HMPV fusion protein; and^ Bold text is a flexible linker.SEQ ID NO: 23HMPV F CT5S T3-PAlso referred to as ‘T3-P’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRIGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQ THIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 24HMPV F CT5S T3-DAlso referred to as ‘T3-D’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIADLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRIGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQ THIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 25HMPV F CT5S T3-PDAlso referred to as ‘T3-PD’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRIGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQ THIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 26HMPV F CT5S T4-PAlso referred to as ‘T4-P’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNALKKTNEAVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQ THIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM. SEQ ID NO: 27HMPV F CT5S T4-PDAlso referred to as ‘T4-PD’MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQ THIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 28 T3-PD hMPV fusion protein (without signal peptide, without MC2S SSM) LKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELRTVSADQLAREEQIENPR QSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATAVRELKDFVSKNLTRAINK NKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQIKLMLENRAMVRRKG FGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTVYYPNEKDCETRGDHV FCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRVGIIKQLNKGCSYITNQ DADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQSNRI SEQ ID NO: 29T3-PD hMPV fusion protein (without signal peptide, with MC2S SSM; herein also referred to as VXB-221)LKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELRTVSADQLAREEQIENPR QSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATAVRELKDFVSKNLTRAINK NKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQIKLMLENRAMVRRKG FGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTVYYPNEKDCETRGDHV FCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRVGIIKQLNKGCSYITNQ DADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQSNRIGSGLANATAAQ QEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM. SEQ ID NO: 30 T3-PD signal peptide (native hMPV signal peptide) MSWKVVIIFSLLITPQHG SEQ ID NO: 31 MC2S SSM – complete sequence LANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRD ARRI SEQ ID NO: 32 MC2S FHRR LANATAAQQEVLEAQYAMVQHIAKGIRILEARVAR SEQ ID NO: 33 MC2S SHRR NHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI SEQ ID NO: 34 Linker GGSGG SEQ ID NO: 35 Linker GGSG SEQ ID NO: 36 Linker GSGS SEQ ID NO: 37 Linker GGGS SEQ ID NO: 38 Linker GGGGS SEQ ID NO: 39 Linker GGGGSGGGGS SEQ ID NO: 40 Linker GGGGSGGGGSGGGGS SEQ ID NO: 41 Linker GGGGSGGGGSGGGGSGGGGS SEQ ID NO: 42 Linker GGGGSG SEQ ID NO: 43 Linker GSGG SEQ ID NO: 44 Linker GSGGS SEQ ID NO: 45 Linker GSGGSG SEQ ID NO: 46 IgK signal peptide MGWSCIILFLVATATGVHS SEQ ID NO: 47T3-PD hMPV fusion protein (with IgK signal peptide, with MC2S SSM)MGWSCIILFLVATATGVHSLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALREL RTVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATA VRELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAG QIKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGST VYYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNR VGIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVD QSNRIGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAAN wherein: ^Italicized text corresponds to the IgK signal peptide;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM.SEQ ID NO: 48 MC2 FHRR LANATAAQQEVLEAQYAMVQHIAKGIRILEARVAR SEQ ID NO: 49 MC2 SHRR NHTWQQWEEEIEQHEGNLSLLLREAALQVHIAQRDARRI SEQ ID NO: 50 MC2 SSM (non-silenced clamp) – complete sequence LANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTWQQWEEEIEQHEGNLSLLLREAALQVHIAQRDA RRI SEQ ID NO: 51T3-PD hMPV fusion protein (with signal peptide, with MC2S SSM)MSWKVVIIFSLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELR TVSADQLAREEQIENPRQSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATAV RELKDFVSKNLTRAINKNKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQ IKLMLENRAMVRRKGFGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTV YYPNEKDCETRGDHVFCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRV GIIKQLNKGCSYITNQDADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQ SNRIGSGLANATAAQQEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTWQQWEEEIEQHEGNLSLLLREAALQV HIAQRDARRI wherein: ^Italicized text corresponds to the native signal peptide of hMPV fusion protein;^ Bold text is a flexible linker; and^ Underlined text corresponds to the MC2 SSM.SEQ ID NO: 52T3-PD hMPV fusion protein (without signal peptide, with MC2S SSM)LKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCADGPSLIKTELDLTKSALRELRTVSADQLAREEQIENPR QSRFVLGAIALGVATAAAVTAGVAIAKTIRLESEVTAIKNCLKKTNECVSTLGNGVRVLATAVRELKDFVSKNLTRAINK NKCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSNMPTSAGQIKLMLENRAMVRRKG FGILIGVYGSSVIYMVQLPIFGVIDTPCWIVKAAPSCSEKKGNYACLLREDQGWYCQNAGSTVYYPNEKDCETRGDHV FCDTAAGINVAEQSKECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRVGIIKQLNKGCSYITNQ DADTVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPVKFPEDQFNVALDQVFESIENSQALVDQSNRIGSGLANATAAQ QEVLEAQYAMVQHIAKGIRILEARVARGGSGGNHTWQQWEEEIEQHEGNLSLLLREAALQVHIAQRDARRI wherein: ^Bold text is a flexible linker; and^ Underlined text corresponds to the MC2 SSM. SEQ ID NO: 76 HMPV fusion protein v3B-Δ12_D454C-V458C (Ou, Chen et al.2023) Referred to as ‘v3B-Δ12’ MSWKVMIIISLLITPQHGLKESYLEESCSTITEGYLSVLRTGWYTNVFTLEVGDVENLTCTDGPSLIKTELDLTKSALRELK TCSADQGSGGSGATAAAVTAGIAIAKTIRLESEVNAIKGCLKTTNECVSTLGNGVRVLATAVRELKEFVSKNLTSAINKN KCDIPDLKMAVSFSQFNRRFLNVVRQFSDNAGITPAISLDLMTDAELARAVSYMPTSAGQIKLMLENRCMVRRKGFG ILIGVYGSSVIYMVQLPIFGVIDTPCWIIKAAPSCSEKDGNYACLLREDQGWYCKNAGSTVYYPNDKDCETRGDHVFCD TAAGINVAEQSRECNINISTTNYPCKVSTGRHPISMVALSPLGALVACYKGVSCSIGSNRVGIIKQLPKGCSYITNQDAD TVTIDNTVYQLSKVEGEQHVIKGRPVSSSFDPIKFPECQFNCALDQVFESIENSQALVDQSNKILNSAESAIGGYIPEAPR wherein: ^Italicized text corresponds to the native signal peptide;^ Bold text are flexible linkers; and^ Underlined text corresponds to the Foldon SSM, Histidine and Streptavidin purificationtags. SEQ ID NO: 77 VXB-213 - RSV fusion protein stabilized by the MC2S SSM SGQNITEEFYQSTCSAVSKGYLSALRTGWYTSVITIELSNIKENKCNGTDAKVKLIKQELDKYKNAVTELQLLMQSTPAT GSGSAICSGVAVCKVLHLEGEVNKIKSALLSTNKAVVSLSNGVSVLTFKVLDLKNYIDKQLLPILNKQSCSISNIETVIEFQ QKNNRLLEITREFSVNAGVTTPVSTYMLTNSELLSLINDMPITNDQKKLMSNNVQIVRQQSYSIMCIIKEEVLAYVVQL PLYGVIDTPCWKLHTSPLCTTNTKEGSNICLTRTDRGWYCDNAGSVSFFPQAETCKVQSNRVFCDTMNSRTLPSEVNL CNVDIFNPKYDCKIMTSKTDVSSSVITSLGAIVSCYGKTKCTASNKNRGIIKTFSNGCDYVSNKGVDTVSVGNTLYCVNK QEGKSLYVKGEPIINFYDPLVFPSDEFDASISQVNEKINQSLAFIRKGSGLANATAAQQEVLEAQYAMVQHIAKGIRILE ARVARGGSGGNHTTWQQWEEEIENHTGNLTLLLREAANQTHIAQRDARRI wherein: ^Bold text is a flexible linker; and^ Underlined text corresponds to the MC2S SSM. EXAMPLES Certain embodiments of the present invention are described with reference to the following examples, which are provided for the purpose of illustration only and are not intended to limit the scope of the invention. Example 1: Antigen panel 1 design and screening To determine the optimal linkage between the C-terminus of hMPV F and the structure-stabilising domain “molecular clamp 2 silenced” (MC2S, alternatively referred to as CT5S) a panel of 19 antigens(outlined in Table 3) were cloned, expressed in ExpiCHO-S and purified by affinity chromatography withMC2S specific resin AVI-8740 (Avitide / Repligen; see WO 2024 / 054656 which is incorporated herein by reference in its entirety). The panel of antigens were then compared by yield (Figure 1), SEC (Figure 2) and ELISA (Figure 3). From the yield comparison it was evident that multiple truncations had improved yield, in particular truncations T3 to T6 and T15 gave the highest yield. From SEC analysis (Figure 2) truncations showed variability in the relative percentage of trimer present with T2, T3, T5 and T10 showing >60% trimer.ELISA with a panel of five hMPV specific mAbs MPE8 (Corti, Bianchi et al.2013), DS7 (Williams, Chen etal.2007), MPV458, MPV467 and MPV487 (Banerjee, Huang et al.2022) (Figure 3) revealed a substantial difference between truncated antigens. The antigens with the most favourable pattern were T3, T4 and T6, which showed strongest binding for antibodies MPE8 and MPV-487. Based on the aggregate of yield, SEC and ELISA analysis, two lead truncation lengths T3 and T4 were selected to progress into further analysis.

[0002] Table 3. List of hMPV antigen panel 1 constructs. Signal C- SEQ Abbreviated Clamp Stabilization Full name Backbone peptide terminal ID name linker domain length NO: hMPV-F-CT5S-T1 T1 Wild Type1 Native 482 GSG CT5S 02hMPV-F-CT5S-T2 T2 Wild Type1 Native 481 GSG CT5S 03hMPV-F-CT5S-T3 T3 Wild Type1 Native 480 GSG CT5S 04hMPV-F-CT5S-T4 T4 Wild Type1 Native 479 GSG CT5S 05hMPV-F-CT5S-T5 T5 Wild Type1 Native 478 GSG CT5S 06hMPV-F-CT5S-T6 T6 Wild Type1 Native 477 GSG CT5S 07hMPV-F-CT5S-T7 T7 Wild Type1 Native 476 GSG CT5S 08hMPV-F-CT5S-T8 T8 Wild Type1 Native 475 GSG CT5S 09hMPV-F-CT5S-T9 T9 Wild Type1 Native 474 GSG CT5S 10hMPV-F-CT5S-T10 T10 Wild Type1 Native 473 GSG CT5S 11hMPV-F-CT5S-T11 T11 Wild Type1 Native 472 GSG CT5S 12hMPV-F-CT5S-T12 T12 Wild Type1 Native 471 GSG CT5S 13hMPV-F-CT5S-T13 T13 Wild Type1 Native 470 GSG CT5S 14hMPV-F-CT5S-T14 T14 Wild Type1 Native 469 GSG CT5S 15hMPV-F-CT5S-T15 T15 Wild Type1 Native 468 GSG CT5S 16hMPV-F-CT5S-T16 T16 Wild Type1 Native 467 GSG CT5S 17hMPV-F-CT5S-T17 T17 Wild Type1 Native 466 GSG CT5S 18hMPV-F-CT5S-T18 T18 Wild Type1 Native 465 GSG CT5S 19hMPV-F-CT5S-T19 T19 Wild Type1 Native 464 GSG CT5S 20DSCavEs2-Foldon DSCavEs2 DSCavEs22 Native 490 N / A Foldon 21hMPV-Fsol Fsol Wild Type1 Native 490 N / A N / A 221Wild type hMPV subtype sequenced2Pre-fusion stabilised fusion protein containing structure stabilising mutations; Q100R, S101R, T365C,V463C, T127C, N153C, A185P, L219K, V231C (Hsieh, Rush et al.2022)Example 2: Antigen panel 2 including additional structure stabilising mutagenesis To attempt to further improve stability, two structure-based stabilisation mutations were selected for incorporation into the lead antigens hMPV_F_CT5s_T3 and hMPV_F_CT5s_T4: a proline mutation,A185P (Battles, Mas et al. 2017) and / or a disulfide bridge insertion A140C / A147C (Stewart-Jones,Gorman et al.2021). A panel of antigens (outlined in Table 4) were cloned, expressed in ExpiCHO-S andpurified by affinity chromatography with MC2S specific resin AVI-8740 (see WO 2024 / 054656). Following purification, the new panel of antigens was compared based on yield (Figure 4). A panel of different screens was conducted both on freshly purified material stored at 4°C or following 5 days incubation at either 25 or 40°C. This panel of screens included SDS-PAGE (Figure 5), SEC (Figure 6) and ELISA (Figure 7). A Fab Shift SEC experiment was also performed where antigen, MPE8 Fab, and preincubated antigen + MPE8 Fab were separated by SEC to assess in-solution binding of MPE8 (Figure 8). In the SDS-PAGE analysis (Figure 5), a band at 75 kDa was visible representing the noncleaved antigen. A high MW band at the top of the gel is likely to be due to aggregated protein due to incomplete denaturing during heating and is unlikely to be relevant. Lower molecular weight bands are likely host cell contaminants. All antigens stored at 40°C showed a high level of stability, however at 25°C proteolytic cleavage resulted in degradation of antigens T3, T3P, T4 and T4P. It was notable that antigens including the disulfide bridge insertion (A140C / A147C) were resistant to proteolytic degradation at 25°C. The SEC analysis (Figure 6) indicates that the full panel of antigens (T3, T3P, T3D, T3PD, T4, T4P and T4PD) are all primarily soluble trimer with little to distinguish between antigens, except for an indication that T3 and T4 appear to contain a slightly higher level of aggregate, and monomer compared to antigens containing A185P (P) and / or disulfide bridge insertion A140C / A147C (D). In SEC analysis following incubation for 5 days at 25 or 40°C, 3 antigens (T3D, T3PD and T4PD) all showed remarkable stability with the amount of soluble trimer more or less unchanged. ELISA analysis completed with a panel of three hMPV specific mAbs (Figure 7), revealed differences in binding of mAbs particularly MPE8, with T3PD and T4PD showing low nanomolar affinity for all three mAbs. Furthermore, mAb binding to T3PD and T4PD was unaffected by incubation at 25 or 40°C. In the Fab Shift SEC (Figure 8) all antigens showed a decrease, to varying levels, of the size of the peak located at 15-17 ml, corresponding to unbound MPE8 Fab. This result provides evidence that MPE8 can bind to the corresponding antigens to varying degrees. Furthermore, the peak corresponding to trimeric hMPV antigen, located at 9.5-11ml, shifted to the left (i.e., lower retention volumes / higher MW) indicative of Fab attachment to the hMPV antigen. Of note, T3PD showed the biggest reduction in the Fab peak potentially indicating the highest amount of antigen with the MPE8 epitope in the desired conformation. Based on the combined findings of yield, SDS-PAGE, SEC, ELISA, Fab shift SEC and stability at elevated temperatures (Table 5) it was evident that both proline mutation A185P and disulfide bridge insertion A140C / A147C provided improved stability and homogeneity to MC2S-stabilised hMPV F. The antigen incorporating both changes, hMPV-F-CT5S-T3-PD (SEQ ID NO: 25), obtained without signal peptide (SEQ ID NO: 29), was selected as the lead construct to take forward into large scale manufacturing. Table 4. List of hMPV antigen panel 2 constructs. Structure Abbre stabilisin C- Stabilizati Backbo Signal Clamp SEQ ID Full name viated g termina on ne peptide linker NO: name mutation l length domain s hMPV-F- Wild T3P1Native A185P 480 GSG CT5S 23CT5S-T3-P Type hMPV-F- Wild A140C, T3D1Native480 GSG CT5S 24CT5S-T3-D Type A147C hMPV-F- A185P, Wild CT5S-T3- T3PD1Native A140C,480 GSG CT5S 25Type PD A147C hMPV-F- Wild T4PNative A185P 479 GSG CT5S 26CT5S-T4-P Type1hMPV-F- A185P, Wild CT5S-T4- T4PD Native A140C,479 GSG CT5S 27Type1PD A147C

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Through selection of a stable clonal cell line and process optimisation, the production yield was increased to approximately 4 g / L (Figure 9). Liquid stability of the purified drug product was also assessed with a panel of release tests including Capillary Electrophoresis(CE-SDS), SEC and potency ELISA using pre-fusion specific monoclonal antibody MPE8 (Corti, Bianchi etal. 2013) (Figure 9). Stability analysis revealed no significant change within the antigen following 6months incubation at 2-8°C. Similarly, incubation of T3PD at 2-8°C, for 9 or 12 months did not or not substantially affect the stability of the antigen assessed by CE-SDS, SEC, and AM14 potency ELISA (Figure 16). The neutralising immune response elicited in mice following vaccination with dose matched purified T3 PD (SEQ ID NO: 25), non-stabilized hMPV-Fsol (SEQ ID NO: 22) and best-in-class comparator hMPV- DSCavEs2-Foldon (SEQ ID NO: 21) was determined. Groups of 8 BALB / c mice were immunised twice, three weeks apart, by intramuscular (IM) administration of 2 µg antigen mixed 1:1 with AddaVaxTMadjuvant (Invivogen). Blood serum collected three weeks following the second dose was heat inactivated and the neutralization of hMPV strain CAN97-83 was measured by Plaque Reduction Neutralization Titration (Figure 10). T3 PD stimulated a strong neutralising immune response to hMPV (geomean = 1,384; 95%CI = 1,094 – 1,762), which was approximately 2.5-fold higher relative to the dose-matched hMPV-DSCavEs2-Foldon (geomean = 558; 95%CI = 336 - 971) and approximately 5.5-fold higher relative to non-stabilized hMPV-Fsol (geomean = 252; 95%CI = 99 - 778). In summary the results demonstrate that T3 PD can be manufactured in high amounts, is compatible with long term liquid stability at 2-8°C, and was shown to elicit a strong neutralising immune response in a naïve mouse immunisation schedule. Example 4: Use of alternate signal peptides for expression of VXB-221 Two 50ml transient expression cultures were performed to compare MPV T3PD antigen produced with the native signal peptide (i.e., SEQ ID NO: 30) and the IgK signal peptide (i.e., SEQ ID NO: 46). The recombinant antigens were expressed in mammalian suspension culture (ExpiCHO-S, Thermofisher) and purified from clarified supernatants by affinity chromatography using ligand-resin with affinity for MC2S (AVI-8740) and then compared for yield and percentage trimer by SEC (Table 6) and SDS-PAGE (Figure 11). With the IgK signal peptide 8.59 mg of antigen was purified but with the native signal peptide 37.32 mg of antigen was purified. The purity of the antigen was roughly equivalent by SDS-PAGE and SEC. Collectively, this demonstrates that the herein provided antigens, and in particular T3-PD, can be produced using different signal peptides. Table 6: Yield Homogeneity SECT3PD 37.32 93.53%T3-PD IgK 8.59 96.42%Example 5: Comparison of purified T3PD to comparator antigens hMPV DsCavEs2 and v3B The recombinant antigens were expressed in mammalian suspension culture (ExpiCHO-S, Thermofisher) and purified from clarified supernatants by immunoaffinity chromatography using either a ligand-resin with affinity for MC2S (AVI-8740) or monoclonal antibodies 101F (McLellan, Chen et al.2010) and then analysed by SEC (Figure 12). T3PD showed high percentage of soluble trimeric homogeneity (90%), while comparator antigen DSCavEs2 was in both trimeric (54%) and monomeric (43%) conformations. hMPV DsCavEs2 produced two overlapping peaks within the trimeric fractions, suggesting that the trimer may adopt distinct conformational or oligomeric states. To determine whether the proteins were correctly folded, epitope presentation was tested by ELISA using panels of mAbs. For the hMPV antigens, a broader panel of mAbs targeting diverse epitopes across the topology of preF was tested against the antigen series in an indirect ELISA format (Figure 13). MPV T3PD was shown to be recognised with sub-nanomolar binding to five of the six mAbs tested. Only DS7 (Williams, Chen et al. 2007), which binds to the post-fusion conformation or a pre / post intermediate, showed low affinity for MPV T3PD. Neutralising antibodies with moderate or strong pre-fusion preference (MPV467, MPV-487 and MPE8) (Wen, Mousa et al. 2017, Banerjee, Huang et al. 2022), showed similar sub-nanomolar affinities for VXB-221 and comparator antigens DSCavEs2 and v3B- Δ12_D454C-V458C, but poor reactivity for hMPV Fsol. Neutralising antibodies known to recognise the internal trimer interface (MPV-458 and M8C10) (Banerjee, Huang et al.2022, Xiao, Fridman et al.2022), were able to bind with sub-nanomolar affinity to MPV-Fsol, DSCavEs2 and MPV T3PD but showed poor affinity for v3B-Δ12_D454C-V458C, which contains an interprotomer disulphide bridge and has been optimised to adopt a closed, pre-fusion conformation (Ou, Chen et al.2023). Example 6 Immunogenicity to MC2S compared to Foldon BALB / c mice were immunised with 2 µg of the recombinant antigens by intramuscular injection (IM) in 2 doses, 3 weeks apart, with a squalene-based oil-in-water adjuvant, AddaVax (InvivoGen). To ensure RSV and hMPV Fsol antigens were fully in the post-fusion state they were first heated to 70°C for 2 hrs. At completion, analysis of mouse sera by ELISA indicated that the immunisations had been effective in inducing IgG against each hMPV F (Figure 14). Each antigen also elicited a response to the trimer stabilising domain that was detectable via ELISA with a non-homologous antigen (Nipah F) stabilised with either foldon or MC2S. By comparing EC50 values for total antigen-specific IgG and tag-specific IgG for each mouse we were able to estimate the proportion of tag-specific response. This response was considered undesirable as it will not contribute to protection. For MPV T3PD the geometric mean of the relative response to MC2S was found to be 6.3%. For the hMPV F DSCavEs2 comparator, which also contains foldon, the geometric mean (GM) of the relative response to foldon was 3.1%. This response was somewhat variable with very low titres for 5 individual mice and relatively high titres for the remaining 3 mice. Example 7. Comparison between T3PD and DSCavEs2 for the potential to deplete neutralising antibodies from a human plasma The ability of each antigen to deplete neutralising antibodies from a pooled human plasma sample was compared either by directly pre-incubating plasma with antigens in solution or with immobilised antigens coupled to CNBR Activated Sepharose (Invitrogen). Both DSCavEs2 and T3PD were able to efficiently deplete hMPV neutralising antibodies, although hMPV Fsol was not tested (Figure 15). Example 8. Solvent accessibility of MPV T3PD compared to v3B_Δ12 determined by hydrogen / deuterium exchange mass spectrometry (HDX-MS) Purified antigens for MPV T3PD compared to v3B_Δ12 were provided to a CRO specialising in HDX-MS. Fractional uptake of deuterium was compared between the two proteins with notable differences observed throughout their sequences. Overall, T3PD had more sequences that displayed increased uptake of deuterium. Eight specific amino acid stretches were identified in which T3PD was more accessible for deuterium exchange; A: aa113-141, B: aa196-218, C: aa233-241, D aa256-266, E: aa284- 294, F: aa310-327, G: aa370-388, and H: aa416-433 (Figure 18A). Conversely, seven specific sequences were identified in which T3PD was less accessible; I: aa 148-154, J: aa221-226, K: aa245:250, L: aa294- 309, M: aa350-360, N: aa397-406, and O: aa438-445 (Figure 18B). Overall, it appears that VXB-221 is more solvent exposed, as the changes in the positive direction tend to be larger. The differences in uptake of deuterium between the two proteins was overlaid onto the prefusion structure of hMPV F (Figure 17). It was observed that two large patches (indicated by arrows A and Bwithin Figure 17) of increased deuterium uptake for T3PD were mapped to the internal trimer interface.Conversely a region of lower deuterium uptake was observed on the protein surface near the base of the trimer. This region is surface exposed in both proteins, however the higher stability provided by MC2S in this region could decrease deuterium uptake.Collectively, and in view of Example 5 and Figure 13, this suggests that MPV T3PD adopts an openconformation allowing for the binding of antibodies to internal, yet surface-exposed epitopes. This could contribute the surprisingly strong induction of neutralizing antibodies described herein above. Example 9. Phase 1 clinical trial of a vaccine comprising MPV T3PD A phase 1 randomized, placebo- and active-controlled (limited to an RSV-antigen as positive control), observer-blind, dose-finding study was conducted to evaluate the safety, reactogenicity, and immunogenicity of four dose levels of VXB-241, a bivalent vaccine comprising the herein provided T3PD hMPV antigen (SEQ ID NO: 29; VXB-221) and antigen VXB-213 (SEQ ID NO: 77), an RSV fusion protein ectodomain antigen also stabilized by the silenced molecular clamp in a prefusion conformation. VXB- 241 consists of the above-mentioned antigens (i.e., VXB-213 and VXB-221) formulated in phosphate buffered saline, 0.02% polysorbate 80 without any adjuvant. The active control is the marketed RSV vaccine Arexvy (GSK), comprising 120 µg of RSVPreF3-Antigen and AS01E adjuvant. Phosphate buffered saline, 0.02% polysorbate 80 was employed as placebo. In the run-in study, a total of 16 younger adults 18 to 40 years of age (yoa), were randomised in a 1:3 ratio and received either placebo or one of four dose levels of VXB-241; 60 µg (containing 30 µg of VXB- 213 and 30 µg of VXB-221), 120 µg (containing 60 µg of VXB-213 and 60 µg of VXB-221), 240 µg (containing 120 µg of VXB-213 and 120 µg of VXB-221) or 480 µg (containing 240 µg of VXB-213 and 240 µg of VXB-221). No safety issues were reported in the run-in study at any dose level, thereby supporting progression into the main study conducted in older adults. In the main study, a total of 128 older adults 60 to 83 yoa were randomised in a 1:1:3 ratio and received either placebo, active comparator, Arexvy, or one of four dose levels of VXB-241; 60 µg (containing 30 µg of VXB-213 and 30 µg of VXB-221), 120 µg (containing 60 µg of VXB-213 and 60 µg of VXB-221), 240 µg (containing 120 µg of VXB-213 and 120 µg of VXB-221) or 480 µg (containing 240 µg of VXB-213 and 240 µg of VXB-221). On Day 1 of the run-in and the main studies, all participants had a blood sample collected and subsequently received the 1st injection of the Investigational Medicinal Product (IMP; i.e., VXB-241), Arexvy (with Arexvy only being administered in the main study), or placebo. Each younger adult in the run-in study, only received a single vaccination and were to remain in the study for ~6-7 months. Each older adult participant has received 1 injection of IMP, with a second dose to be administer at ~12 month after the first dose. These participants will remain in the study for ~24-25 months. Following IMP administration, participants of the run-in and the main studies were instructed to record any post vaccination local and systemic reactions in an electronic diary through Day 8. Reactogenicity was assessed through 10 predetermined adverse events / AEs (referred to as solicited AEs), which are known early reactions to vaccination common to many vaccines, 3 of which are local (pain, redness, swelling at the injection site, also referred to as injection site reactions), and 7 systemic (fever, headache, shivering, fatigue, myalgia, arthralgia, and gastrointestinal symptoms). Participants then returned on Day8 for post-dose safety assessment by assessing unsolicited AEs (see Table 8).Throughout both studies all participants had further blood draws on day 30 (1 month post vaccination) and day 182 (6 months post vaccination). Older adults in the main study, will also return to the clinic at day 364 (1 year post vaccination), although the study is ongoing, and this is yet to occur. At this time, participants will be randomised to receive either placebo or vaccination. These participants will then return for further blood draws on day 394 (1 month post booster vaccination), day 546 (6 months post booster vaccination), and day 728 (1 year post booster vaccination). An interim readout was conducted at Day 30 post vaccination at which time the primary safety and immunogenicity endpoints were assessed as set out in Table 7. Table 7. Primary objectives and endpoints. Objective EndpointSafety. Exclude high incidence of Proportion of older adults with 1 or more unsolicited AEs in early onset unsolicited AEs in older the month after 1st IMP injection (Table 8). adults. Reactogenicity. Exclude highProportion of older adults with 1 or more solicited AEs in incidence of solicited AEs in olderthe 7 days after 1st IMP injection (Tables 9 to 11).adults. Immunogenicity. Confirm that atGeometric mean fold increase (GMFI) of RSV-A, RSV-B, least 1 dose level of VXB-241 is highly hMPV-A, and hMPV-B serum neutralizing antibody titers immunogenic 1 month after 1st IMP from pre-injection baseline (Visit 2) to ~1 month after 1st injection in older adultsIMP injection (Visit 4) in older adults (Figures 19 and 20).Immunogenicity. Assess theEstimates of dose-response curves for GMFIs of RSV-A, RSV- relationship between VXB-241 dose B, hMPV-A, and hMPV-B serum neutralizing antibody titers level and neutralizing antibody from pre-injection baseline to ~1 month after 1st IMP response in older adults.injection in older adults (Figures 19 and 20).Overall vaccination with VXB-241 was safe and well tolerated. Unsolicited adverse events, also referred to as Treatment Emergent Adverse Events (TEAEs), were recorded in 58 of 128 (45.3%) older adults, with similar rates observed in participants who received placebo, Arexvy or VXB-241 (Table 8). The vast majority of TEAEs were mild or moderate in nature (Grade 1 or 2) and the majority were defined as unrelated to IMP administration upon assessment by a blinded investigator or delegate. Any solicited AEs (either local or systemic) were recorded in 59 of 128 (46.1%) older adults, with local solicited AEs recorded in 31 of 128 (24.2%) older adults and systemic solicited AEs recorded in 38 of 128 (29.7%) older adults (Tables 9-11). Local AEs were most common amongst Arexvy vaccinated participants (16 of 21; 71.4%) compared to participants who received VXB-241 (15 of 86; 17.4%) or placebo (1 of 21; 4.8%), with injection site pain the most frequent local solicitated AE. Systemic AEs were most common amongst Arexvy vaccinated participants (12 of 21; 57.1%) compared to participants who received VXB- 241 (31 of 86; 36.0%) or placebo (5 of 21; 23.8%). Fatigue, headache and muscle pain (myalgia) were the most common systemic AEs. It is likely that the increased rate of solicited AEs for Arexvy (GSK) is due to the inclusion of adjuvant AS01E compared to VXB-241, which does not include any adjuvant, as a similar result reported during the phase 1 trial of Arexvy (GSK) testing when both unadjuvanted and AS01E adjuvanted formulations were tested (Leroux-Roels et al.2022). The number of participants experiencing unsolicited AEs and solicited AEs did not increase with an increase in VXB-213 and VXB-221 antigen doses. At day 30 following IMP administration, there was no change in the RSV-A or RSV-B neutralising titre following vaccination with placebo, however following vaccination with the active control, Arexvy (GSK) the RSV-A titre and RSV-B neutralising titres increased by a geometric mean fold increase (GMFI) of 10.81 and 4.91-fold respectively (Figure 19). Vaccination with VXB-241 resulted in an increase in RSV-A and RSV-B neutralising titres at each dose level tested. At the lowest dose levels tested (60 µg) this increase was below that of Arexvy; RSV-A GMFI = 3.99 and RSV-B GMFI = 3.96. However, at higher dose (120µg, 240 µg or 480µg) the GMFI in RSV-A and RSV-B neutralising titres was similar or higher compared to Arexvy; RSV-A GMFI = 9.72, 15.06 and 11.2, and RSV-B GMFI = 5.59, 8.07 and 8.26, respectively. Of note, the VXB-241 dose level of 240 µg contains the 120 µg of RSV preF (VXB-213), which is equivalent to the dose of RSV preF with Arexvy. At this dose level the GMFI in RSV-A and RSV-B neutralising titres was roughly 50% higher compared to Arexvy (RSV-A: 15.06 vs 10.81, and RSV-B 8.07 vs 4.91). There was no change in the hMPV-A or hMPV-B neutralising titre following vaccination with either placebo or Arexvy (GSK), whereas vaccination with VXB-241 resulted in an increase in hMPV-A and hMPV-B neutralising titres at each dose level tested (Figure 20). No substantial difference in the GMFI of hMPV-A or hMPV-B, was observed between dose levels; hMPV-A GMFI = 4.49 - 7.50, and hMPV-B GMFI = 3.88 – 6.56. Of note, the VXB-241 dose level of 240 µg, which contains the 120 µg of hMPV preF (VXB-221), gave the highest GMFI in hMPV-A and hMPV-B neutralising titres. At this dose level the GMFI against RSVA, RSV-B, hMPV-A and hMPV-B were all substantially higher than that reported in a clinical trial of alternate RSV / hMPV bivalent vaccine (IVX-A12) even in comparison to adjuvanted IVX-A12, where the maximum GMFI were reported as 4-fold and 3-fold for RSV-A and RSV-B, and 5-fold and 4-fold for hMPV-A and hMPV-B (Shapiro, Sanchez-Crespo et al.2025).

[0004] ReferencesBanerjee et al., Proc Natl Acad Sci U S A 2022, DOI: 10.1073 / pnas.2203326119Battles et al., Nat Commun 2017, DOI: 10.1038 / s41467-017-01708-9Corti et al., Nature 2013, DOI: 10.1038 / nature12442Galiano et al., J Med Virol 2006, DOI: 10.1002 / jmv.20586Hsieh et al., Nat Commun 2022, DOI: 10.1038 / s41467-022-28931-3Leroux-Roels, et al., J Infect Dis.2022, Safety and immunogenicity of a respiratory syncytial virusprefusion F (RSVPreF3) candidate vaccine in older adults: phase I / II randomized clinical trial McLellan, et al. (2010). "Structure of a major antigenic site on the respiratory syncytial virus fusion glycoprotein in complex with neutralizing antibody 101F." J Virol 84(23): 12236-12244. Ou, et al. (2023). "Structure-based design of a single-chain triple-disulfide-stabilized fusion- glycoprotein trimer that elicits high-titer neutralizing responses against human metapneumovirus." PLoS Pathog 19(9): e1011584.Shapiro, et al., Open Forum Infect Dis 2025, Randomized Phase 1 Clinical Trial of a Respiratory SyncytialVirus and Human Metapneumovirus Combination Protein-Based Virus-like Particle Vaccine in Adults 60-75 Years of Age.Stewart-Jones et al., Proc Natl Acad Sci U S A 2021, DOI: 10.1073 / pnas.2106196118Wen, et al. (2017). "Structural basis for antibody cross-neutralization of respiratory syncytial virus and human metapneumovirus." Nat Microbiol 2: 16272.Williams et al., J Virol 2007, DOI: 10.1128 / JVI.00106-07Xiao, et al. (2022). "Profiling of hMPV F-specific antibodies isolated from human memory B cells." Nat Commun 13(1): 2546. Each of the above-listed publications is incorporated herein by reference in its entirety.

Claims

CLAIMS 1. A chimeric polypeptide comprising a human metapneumovirus (hMPV) fusion protein and a structure-stabilizing moiety, wherein said hMPV fusion protein comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 28, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI or NKI, wherein said hMPV fusion protein comprises any one or more of the following amino acid residues: (i) a cysteine residue at position 122 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position, (ii) a cysteine residue in position 129 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position, and / or (iii) a proline residue in position 167 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position, wherein said structure-stabilizing moiety comprises a first heptad repeat region (FHRR) and a second heptad repeat region (SHRR), wherein said FHRR comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 32, and wherein said SHRR comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO:

33.

2. The chimeric polypeptide according to claim 1, wherein said hMPV fusion protein comprises all of the following amino acid residues: (i) a cysteine residue at position 122 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position; (ii) a cysteine residue in position 129 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position; and (iii) a proline residue in position 167 in the amino acid sequence set forth in SEQ ID NO: 28 or at a position corresponding to said position.

3. The chimeric polypeptide according to claim 1 or 2, wherein said hMPV fusion protein comprises a C-terminal amino acid sequence consisting of the amino acid sequence NRI.

4. The chimeric polypeptide according to any one of claims 1 to 3, wherein said hMPV fusion protein comprises the amino acid sequence set forth in SEQ ID NO: 28.

5. The chimeric polypeptide according to any one of claims 1 to 4, wherein said FHRR and said SHRR are linked via a linker, preferably wherein said linker is a peptide linker.

6. The chimeric polypeptide according to claim 5, wherein said linker comprises an amino acid sequence selected from GGSGG (SEQ ID NO: 34), GSG, GS, GGSG (SEQ ID NO: 35), GSGS (SEQ ID NO: 36), AS, GGGS (SEQ ID NO: 37), G4S (SEQ ID NO: 38), (G4S)2 (SEQ ID NO: 39), (G4S)3 (SEQ ID NO: 40), (G4S)4 (SEQ ID NO: 41), G4SG (SEQ ID NO: 42), GSGG (SEQ ID NO: 43), and GSGGS (SEQ ID NO: 44).

7. The chimeric polypeptide according to claim 5 or 6, wherein said linker comprises the amino acid sequence GGSGG (SEQ ID NO: 34).

8. The chimeric polypeptide according to any one of claims 1 to 7, wherein said structure-stabilizing moiety comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO:

31.

9. The chimeric polypeptide according to claim 8, wherein said structure-stabilizing moiety comprises the amino acid sequence set forth in SEQ ID NO:

31.

10. The chimeric polypeptide according to any one of claims 1 to 9, wherein said hMPV fusion protein and said structure-stabilizing moiety are linked via a linker, preferably wherein said linker is a peptide linker.

11. The chimeric polypeptide according to claim 10, wherein said linker comprises an amino acid sequence selected from GSG, GS, GGSGG (SEQ ID NO: 34), GGSG (SEQ ID NO: 35), GSGS (SEQ ID NO: 36), AS, GGGS (SEQ ID NO: 37), G4S (SEQ ID NO: 38), (G4S)2 (SEQ ID NO: 39), (G4S)3 (SEQ ID NO: 40), (G4S)4 (SEQ ID NO: 41), G4SG (SEQ ID NO: 42), GSGG (SEQ ID NO: 43), and GSGGS (SEQ ID NO: 44).

12. The chimeric polypeptide according to claim 11, wherein said linker comprises the amino acid sequence GSG.

13. The chimeric polypeptide according to any one of claims 1 to 12, wherein said chimeric polypeptide comprises an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO:

29.

14. The chimeric polypeptide according to any one of claims 1 to 13, wherein said chimeric polypeptide consists of the amino acid sequence set forth in SEQ ID NO: 29.

15. The chimeric polypeptide according to any one of claims 1 to 14, wherein said chimeric polypeptide further comprises an N-terminal signal peptide.

16. The chimeric polypeptide according to claim 15, wherein said signal peptide comprises (or preferably consists of) an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 30 or 46, preferably SEQ ID NO:

30.

17. The chimeric polypeptide according to claim 15 or 16, wherein said signal peptide comprises (or preferably consists of) the amino acid sequence set forth in SEQ ID NO: 30 or 46, preferably SEQ ID NO:

30.

18. The chimeric polypeptide according to any one of claims 15 to 17, wherein said chimeric polypeptide comprises (or preferably consists of) an amino acid sequence having at least about 90% sequence identity to the amino acid sequence set forth in SEQ ID NO: 25 or 47, preferably SEQ ID NO:

25.

19. The chimeric polypeptide according to any one of claims 15 to 18, wherein said chimeric polypeptide consists of the amino acid sequence set forth in SEQ ID NO: 25 or 47, preferably SEQ ID NO:

25.

20. A nucleic acid comprising a nucleic acid sequence encoding the chimeric polypeptide according to any one of claims 1 to 19.

21. The nucleic acid according to claim 20, wherein said nucleic acid is a ribonucleic acid (RNA), preferably a messenger RNA (mRNA), preferably wherein said nucleic acid encodes the chimeric polypeptide according to any one of claims 15 to 19.

22. A complex comprising three subunits, wherein each subunit is a chimeric polypeptide according to any one of claims 1 to 14.

23. A composition comprising the chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 15 or 16, or the complex according to claim 22, preferably wherein said composition is a pharmaceutical composition.

24. A kit comprising the chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 20 or 21, the complex according to claim 22, or the composition according to claim 23.

25. The chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 20 or 21, the complex according to claim 22, the composition according to claim 23, or the kit according to claim 24, for use in the immunization and / or the vaccination of a subject.

26. A method for the immunization and / or the vaccination of a subject, wherein the method comprises administering the chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 20 or 21, the complex according to claim 22, the composition according to claim 23, or the kit according to claim 24 to said subject.

27. Use of the chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 20 or 21, or the complex according to claim 22, in the manufacture of a medicament for the immunization and / or the vaccination of a subject.

28. The chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 20 or 21, the complex according to claim 22, the composition according to claim 23, or the kit according to claim 24, for use in the prevention of an hMPV infection.

29. A method for the prevention of an hMPV infection, wherein the method comprises administering the chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 20 or 21, the complex according to claim 22, the composition according to claim 23, or the kit according to claim 24, to a subject in need thereof.

30. Use of the chimeric polypeptide according to any one of claims 1 to 14, the nucleic acid according to claim 20 or 21, or the complex according to claim 22, in the manufacture of a medicament for the prevention of an hMPV infection.

31. A nucleic acid vector comprising the nucleic acid according to claim 20.

32. A host cell comprising the nucleic acid according to claim 20 or 21, and / or the nucleic acid vector according to claim 31.

33. A method of production of a chimeric polypeptide, wherein the method comprises: (i) culturing the host cell according to claim 32; or(ii) expressing a chimeric polypeptide from the nucleic acid according to claim 20 or 21, and / or the nucleic acid vector according to claim 31, optionally in a cell-free transcription- translation system.

34. A method of production an antigen-binding molecule that specifically binds to an hMPV fusion protein, the method comprising: (i) immunizing an animal with the chimeric polypeptide according to any one of claims 1 to 14, or the nucleic acid according to claim 20 or 21, or the composition according to claim 23; (ii) identifying and / or isolating a B cell from the immunized animal, which specifically binds to the hMPV fusion protein; and (iii) producing the antigen-binding molecule expressed by that B cell.

35. An antigen-binding molecule obtainable by the method according to claim 34.

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