An effective prophylactic mucosal gene expression vaccine (GXV), made up of a cocktail of at least 4 different plasmid DNAs encoding corresponding RSV antigens, coacervated with chitosan to formulate nanospheres. In a murine model of RSV infection, intranasal administration with GXV results in significant induction of RSV-specific antibodies, nasal IgA antibodies, cytotoxic T lymphocytes, and IFN-.gamma. production in the lung and splenocytes. A single dose of GXV induces a drastic reduction of viral titers.