Objective: To evaluate the efficacy of currently used inactivated influenza vaccine during a severe epidemic caused by antigenically drifted influenza type A(H3N2) and well-matched type B viruses during the 1992-1993 season.
Design: Prospective nonrandomized controlled trial.
Setting: An urban general hospital pediatric asthma clinic in Japan.
Participants: A total of 137 children with moderate to severe asthma (mean age, 7.0 years; range, 2 to 14 years).
Intervention: Eighty-five children received trivalent split-antigen vaccine containing A/Beijing/352/89 (H3N2) and B/Bangkok/163/90 (B/Panama/45/90-like strain). Fifty-two were unvaccinated.
Main outcome measures: Protection against infection was determined using hemagglutination inhibition test and virus isolation. Clinical efficacy was estimated based on febrile episodes with antibody rise or virus isolation.
Results: Although marked antigenic drift in hemagglutinin was demonstrated in the epidemic virus (A/Beijing/32/92-like strain), the protection against influenza type A(H3N2) infection was 67.5% (P < .01). The protection against influenza type B infection was 43.7% (P < .01), although the epidemic influenza type B viruses were antigenically almost identical to the vaccine strain. Inactivated vaccine was not effective for protection against influenza type B infection in children younger than 7 years. High clinical effectiveness was demonstrated in children at least 7 years of age during the epidemic.
Conclusions: Our data suggest that current inactivated vaccine is highly effective for protection against influenza type A(H3N2) virus infection regardless of antigenic drift. In contrast, the protective efficacy obtained by vaccination may not be sufficient against influenza type B virus infection, and especially in young children, it does not offer protection.