Risk factors for outbreaks of influenza in nursing homes. A case-control study.

To determine risk factors for outbreaks of influenza virus infections in chronic-care facilities for the elderly, the authors compared the characteristics of two groups of nursing homes in Genesee County, Michigan, in 1982-1983, following a community-wide epidemic caused by A/Bangkok/1/79-like (H3N2) viruses: seven homes in which an outbreak occurred (case homes) and six homes with sporadic illnesses only (control homes). The two groups were similar in many respects, including the physical characteristics of the facilities, visitation and staffing patterns, infection control practices, and demographic and clinical profiles of residents. Case homes, however, had larger resident populations than control homes (median 160 vs. 92, respectively; p = 0.028) and a lower proportion of residents who had been vaccinated with A/Bangkok antigen the previous autumn (median 51% vs. 81%, respectively; p = 0.047). Based on a retrospectively determined estimate of the efficacy of influenza vaccine in preventing illness (43%), the number of residents presumed to be susceptible to the epidemic strain was higher in case homes than in control homes (median 102 vs. 61, respectively; p = 0.0047) and was the most important factor selected by a stepwise, multivariate logistic regression model (p less than 0.0005). These findings are consistent with mathematical models of vaccine-induced herd immunity in closed populations and suggest that high rates of vaccination can reduce the risk of influenza outbreaks in this setting.