Effectiveness of inactivated influenza vaccine among nursing home residents during an influenza type A (H3N2) epidemic.

OBJECTIVES To evaluate the use of influenza vaccine in nursing homes and its effectiveness in reducing the likelihood of influenza-like illness. DESIGN A retrospective case-control study with active identification of influenza infection. SETTING All nursing homes in a seven-county study area in southern lower Michigan were eligible for participation. Analyses were based on data collected from 23 homes with documented influenza transmission. PARTICIPANTS Persons aged 65 years or older who were residents of the nursing homes under study during the influenza type A(H3N2) outbreak in 1989-1990. MEASUREMENTS Residents were identified as cases or controls based on occurrence of febrile respiratory illness meeting a case definition. Demographic and underlying illness information were gathered as were data on the use of influenza vaccine, antibiotics, and antivirals. Characteristics of the nursing homes were also recorded. Logistic regression analyses were carried out to determine vaccine effectiveness. MAIN RESULTS Determinants of vaccine use were different from those observed in a parallel community-based study. In a multivariate model that considered the effects of resident and nursing home characteristics, vaccinated residents were significantly less likely than those who were not vaccinated to have an influenza-like illness (OR = .58 (95% CI, .43-.78), P < .001, imputed vaccine effectiveness estimate of 42%). Vaccination was more effective in younger residents (those aged 65 to 84) then in older residents (those older than 84 years). CONCLUSIONS Influenza vaccination was effective in reducing the likelihood of influenza-like illness in nursing home residents. Effectiveness appeared to be related to age, which may function as a surrogate for related immunologic factors. Older nursing home residents should be targeted for newer vaccines and/or potential prophylactic use of antivirals.