Efficacy of influenza vaccine in elderly persons in welfare nursing homes: Reduction in risks of mortality and morbidity during an influenza A (H3N2) epidemic

Background. The effect of influenza vaccination on the occurrence and severity of influenza virus infection in a population residing in nursing homes was studied through a program by the Osaka Prefectural Government, which is the first and official support for influenza vaccination of the elderly population during an influenza A (H3N2) epidemic in Japan. Methods. A cohort study located in the Osaka Prefecture, Japan, followed the outcomes of elderly nursing home residents who received influenza vaccinations (n = 10,739) in comparison with control subjects who did not receive influenza vaccinations (n = 11,723) and monitored clinically the onset of serious morbidity and mortality of influenza illness. Subjects were 22,462 persons older than 65 years who resided in 301 welfare nursing homes in the Osaka Prefecture, Japan during an influenza A (H3N2) epidemic in 1998 to 1999. Results. Of 22,462 individuals living in 301 nursing homes, 10,739 received either one dose (2027 subjects) or two doses (8712 subjects) of inactivated, subunit trivalent influenza vaccine. Through the period from November 1998 to March 1999, there were 950 cases of influenza infection diagnosed clinically with cases by virus isolation and/or serology. There were statistically significantly fewer clinical cases of influenza, hospital admissions due to severe infection, and deaths due to influenza in the vaccinated cohort (256 cases, 32 hospital admissions, and one death) compared with the unvaccinated controls (694 cases, 150 hospital admissions, and five deaths). Vaccination was equally effective in those who received one dose of vaccine as in those who received two doses. No serious adverse reactions to vaccination were recorded. Thus, influenza vaccination is safe and effective in this population and should be an integral part of the routine care of persons aged 65 years and older residing in nursing homes. Conclusions. This study provides an analysis of the clinical efficacy of influenza vaccination in a large cohort of nursing home residents in Japan. Annual influenza vaccine administration requires the attention of all nursing home attendants, physicians, and public health organizations.