Influenza vaccine effectiveness among elderly nursing home residents: a cohort study.

Outbreaks of influenza in nursing homes still occur, even when a large portion of residents have been inoculated with inactivated vaccine. Data were collected in 1991--1992 from 83 eligible skilled nursing homes located in southern Lower Michigan to determine the effectiveness of inactivated influenza vaccine in preventing influenza-like illness and influenza-associated pneumonia. Surveillance was conducted to identify the occurrence of influenza in the homes and, at the end of the season, specific data were gathered on all residents of homes with influenza activity. Age- and sex-adjusted estimates of vaccine effectiveness were calculated using Cox proportional hazards models for each nursing home. Estimates were pooled using precision-based weights calculated from data for each home. Vaccine was found to be 33% effective in preventing total respiratory illness (influenza-like illness and clinically diagnosed pneumonia). In prevention of pneumonia alone, vaccine was 43% effective. The estimate for prevention of pneumonia rose to 55% if the period under consideration was limited to the time of peak influenza activity. Given the number of eligible homes and the cohort methodology used, the results support continuation of current policy, encouraging use of vaccine in all nursing home residents.