Efficacy of live attenuated influenza A/Scotland/74 (H3N2) virus vaccine against challenge with influenza A/Victoria/3/75 (H3N2) virus.

For evaluation of heterologous protection by live attenuated influenza virus vaccine, 42 healthy volunteers with low titers of or no antibody to A/Scotland/74 (H3N2) and A/Victoria/3/75 (H3N2) influenza viruses were given live attenuated A/Scotland/74 (H3N2) virus vaccine or placebo by the intranasal route with no resultant vaccine-related illness. Seventeen of 21 vaccine recipients and none of 21 placebo recipients developed antibody conversion. Thirty-seven days after administration of vaccine or placebo, all subjects were challenged intranasally with wild-type A/Victoria/3/75 influenza virus. Five placebo recipients and no vaccine recipients developed moderately severe illness, whereas 10 vaccine recipients and three placebo recipients developed no illness (p less than 0.025). Although 16 of 21 vaccine recipients fulfilled criteria of infection with the challenge virus, shedding of virus was significantly less frequent, less prolonged, and of significantly lower magnitude than that in the placebo recipients. Thus, live attenuated influenza virus vaccine showed a significant protective effect against illness following challenge with heterologous wild-type virus. The protective effect and the negligible side effects of this vaccine merit consideration of its use in a large-scale field trial.