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 (≥37.8 °C) and kit-diagnosed influenza, respectively. Odds ratios adjusted for age, sex, comorbidity, and vaccine strain (ORadj) were determined using multiple logistic regression. Seroprotected patients (haemagglutination-inhibition antibody titre ≥1:40) had lower incidence of fever (ORadj, 0.35; 95% confidence interval [CI], 0.09-1.28) and kit-diagnosed influenza (ORadj, 0.35; 95% CI, 0.03-4.64) than patients without seroprotection (antibody efficacy, ∼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.