Retrospective assessment of influenza vaccine effectiveness among the non-institutionalized elderly population in Japan

To construct a framework for monitoring the effectiveness of influenza vaccine, we retrospectively surveyed elderly (≥65 years old) people living in rural areas of Kyoto prefecture, Japan, using a self-administered questionnaire, for their influenza-related symptoms during the peak influenza season of 2003–2004. A total of 2301 elderly individuals responded, of whom 1533 (66.6%) had been vaccinated. The odds ratio (OR) of vaccination for fever of ≥38 °C, adjusted for sex, age and underlying conditions, was 0.77 (95% confidence interval (CI), 0.40–1.47), and the OR for a diagnosis of influenza in clinics was 0.81 (95% CI, 0.41–1.57). These findings suggested that the vaccine was 20% effective, although this effectiveness was not statistically significant.