Intramuscular inoculation of live respiratory syncytial virus induces immunity in cotton rats.

Intramuscular inoculation of infant or weanling cotton rats with 10(2.2) to 10(4) plaque-forming units of respiratory syncytial virus induced significant or complete resistance to infection in both the upper and lower portions of the respiratory tract. This resistance did not appear to be the result of in vitro neutralization of virus during homogenization of tissue. Virus was not recovered from the local site of inoculation after 5 min and was never detected in the respiratory tract of intramuscularly inoculated rats. Attempts to detect viral antigens at the site of inoculation by using indirect immunofluorescence were unsuccessful. However, inactivation of infectivity of three different strains of respiratory syncytial virus markedly reduced or completely ablated antigenicity and protective efficacy by the intramuscular route. This suggests that these viruses underwent limited replication, perhaps restricted to an abortive cycle, at the local site of inoculation. An immunosuppressive effect of passive maternally derived immunity was observed. Only 50% of weanling rats possessing passive maternal serum antibody were successfully immunized by intramuscular vaccination with live virus.