Influenza vaccine effectiveness among elderly persons living in the community during the 2003--2004 season.

To examine the effectiveness of influenza vaccine among community-dwelling elderly (65-79 years old), we conducted a population-based cohort study during the 2003--2004 influenza season. A total of 4787 elderly individuals were interviewed regarding acute febrile illness, hospital visits, hospitalization and death by telephone every month. The vaccination status and physician-diagnosed clinical influenza (hereinafter referred as clinical influenza) were determined based on data obtained from the city office and hospitals, respectively. After adjusting for confounders, the odds ratio (OR) of vaccination for influenza-like illness (ILI) with high-fever, which was defined as an acute febrile illness (> or =38.5 degrees C) during the epidemic period, was 0.38 (95% confidence interval [CI], 0.17-0.85) and the OR for clinical influenza was 0.76 (95%CI, 0.28-2.06). Due to the inadequate sample size, ORs for preventing hospitalization for influenza or pneumonia (OR, 0.37; 95%CI, 0.09-1.47) and death (OR, 3.68; 95%CI, 0.75-18.12) were not conclusive. These results suggested that vaccination was therefore effective for elderly persons living in the community.