Evaluation of seasonal influenza vaccination effectiveness based on antibody efficacy among the institutionalized elderly in Japan.

Influenza vaccination efficacy was evaluated in 114 institutionalized elderly people in 2002/03. Strain A/H3N2 was isolated; 44 and 8 subjects had sudden-onset fever (> or = 37.8 degrees C) and kit-diagnosed influenza, respectively. Odds ratios adjusted for age, sex, comorbidity, and vaccine strain (OR(adj)) were determined using multiple logistic regression. Seroprotected patients (haemagglutination-inhibition antibody titre > or = 1:40) had lower incidence of fever (OR(adj), 0.35; 95% confidence interval [CI], 0.09-1.28) and kit-diagnosed influenza (OR(adj), 0.35; 95% CI, 0.03-4.64) than patients without seroprotection (antibody efficacy, approximately 65%). Seroprotective levels of vaccination-induced antibodies probably prevent influenza among the institutionalized elderly, although statistical significance could not be confirmed owing to the sample size.