Natural challenge of subjects vaccinated with WRL 105 strain live influenza vaccine in a residential community.

In the spring of 1975 an outbreak of influenza associated with influenza virus A/Scotland/840/74 virus occurred in a residential college where the previous autumn some students had been immunised with a single intranasal dose of Recombinant WRL 105 (H3N2) (A/Okuda/57xA/Finland/4/74) strain live attenuated infleunza virus vaccine. During the outbreak none of seven students who had been vaccinated suffered from influenza but an influenzal illness did occur in four of eleven who had received placebo nose drops only. Fourfold or greater increases in hemagglutinating inhibiting antibodies between sera collected three weeks after vaccination and those collected three months later following the influenza epidemic were found in one of seven vaccinated and in nine of eleven placebo treated subjects and also in one student who had received neither vaccine nor placebo; a difference in serological response which is statistically significant (p less than 0.01). Thus recombinant WRL 105 live attenuated vaccine was found to have conferred substantial protection against a current wild strain of influenza A virus without causing any appreciable untoward reactions.