New live attenuated influenza A/England/42/72 (H3N2) vaccine (Alice): reactogenicity,, immunogenicity, and protection efficacy.

The Alice strain of influenza A/England/42/72 (H3N2) live attenuated vaccine, when given by the intranasal route to 133 volunteers, was relatively nonreactogenic; only 12% of the vaccinees had upper respiratory tract symptoms after immunization. Seroconversion in 87.2% of subjects whose titers of humoral hemagglutination-inhibiting antibody before immunization were less than 1:8 demonstrated the immunogenicity of the vaccine. The overall seroconversion rate was 66.1% (geometric mean titer of hemagglutination-inhibiting antibody, 1:40.9). Antibody levels were unchanged at six months in 95.5% and at 12 months in 91.7% of the vaccinees. Because of the lack of natural influenza A infection during the season monitored, an experimental challenge study with homotypic virus (influenza A/Udorn/307/72 [H3N2]) was conducted eight months after immunization of a sample population of 22 subjects. Twelve of these subjects were vaccinees whose titers of hemagglutination-inhibiting antibody were greater than or equal to 1:64, and 10 were unimmunized controls whose titers of hemagglutination-inhibiting antibody were less than or equal to 1:8. The group with titers of greater than or equal to 1:64 represented 40.2% of the immunized population. The vaccine was highly effective in this selected group, with a 91% protection efficacy against illness and a 100% rate of protection against illness associated with influenza A infection.