The present invention relates to vaccines having an increased level of safety comprising recombinant
vaccinia viruses. The invention also relates to methods for stimulating a protective immune response in an immunized host using the vaccines of the invention. The vaccines and recombinant
vaccinia viruses of the invention comprise a first
nucleic acid comprising an expression control sequence and a second
nucleic acid comprising an exogenous
nucleic acid encoding a conditional replication
gene product, wherein the expression control sequence is operably linked to the exogenous nucleic acid. The exogenous nucleic acid may, by its expression or non-expression, confer upon the
recombinant vaccinia virus either sensitivity or dependence upon an exogenous molecule (e.g. a
drug) or a condition. Importantly, to allow the recombinant
vaccinia viruses of the invention to replicate normally under permissive conditions, the exogenous nucleic acid is inserted into a non-essential locus, e.g., the E2L / E3L inter-genic locus, K1L / K2L inter-genic locus, the
superoxide dismutate locus, and the 7.5 K locus.