Effectiveness of Influenza vaccines in reducing risk of acute febrile illness among community-dwelling elderly, 2006-07 seasons : Population-based cohort study in Japan
Background: Annual vaccinations are recommended for groups with high-risk medical conditions, as well as the residents of nursing homes. In general, little is known about the vaccine effectiveness of vaccines for Influenza-like illness (ILI) in community-dwelling elderly. Methods: A population based cohort study was conducted during the 2006-2007 (06/07) influenza season to examine the effectiveness of an influenza vaccine among community-dwelling elderly. We selected 1,000 elderly citizens ranging from 65 to 74 years old randomly from a population registry of Sapporo in September 2006. Baseline survey for them was conducted in October or November 2006, and 542 (54.2%) subjects responded with an informed consent. We excluded one person because he passed away before the follow-up survey. Thus, we analyzed 541 subjects. We followed-up the participants concerning acute febrile illnesses, hospitalizations, and so on every prior month from December 2006 to April 2007 though telephone interviews. The Chi-square test and Mann-Whitney U-test were used to compare vaccinated group to non-vaccinated group, and Cox’s hazard model was conducted to control for potential confounding factors. Results: After adjusting for confounders, the vaccination decreased acute fevers higher than or equal to 37.5°C (Hazard ratio (HR) =0.42, 95% confidence interval (CI)=(0.20, 0.90)) from December 2006 to March 2007, but was not associated with the risk of ILI (HR=1.25, 95%CI=(0.29, 5.37)). Conclusion: An influenza vaccination may decrease an acute fever during an influenza epidemic season in community-dwelling elderly
