New Live Attenuated Influenza A/England/42/72 (H3N2) Vaccine (Alice): Reactogenicity, Immunogenicity, and Protection Efficacy

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Abstract
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 <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/307172 [H3 N2]) 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 ⩾1:64, and 10 were unimmunized controls whose titers of hemagglutination-inhibiting antibody were ⩽1:8. The group with titers of ⩾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.

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