An Outbreak of Influenza A (H3N2) in a Well Immunized Nursing Home Population

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Abstract
To describe the epidemiologic features of an outbreak of influenza A that occurred in a skilled nursing home although over 90 percent of the resident population had previously received influenza vaccine. Retrospective cohort study. Skilled nursing home facility in western New York State. Nursing home residents and patient-care staff. Incidence of influenza-like illness among vaccinated versus unvaccinated nursing home residents and staff. Thirty-seven of 124 residents (attack rate = 30%) and 18 of 146 staff (attack rate = 12%) had an influenza-like illness. Staff illness began 16 days prior to onset among residents. Six cases of pneumonia and three influenza-related deaths occurred, all among the vaccinated residents. Ninety percent of the nursing home residents and 10% of the staff received the influenza vaccine prior to the outbreak. The calculated vaccine efficacies were minus 21% and plus 45% for residents and staff, respectively. While antigenic drift of the circulating influenza virus was the major factor in the apparent vaccine failure, the observed poor staff immunization rate (10%) and absence of surveillance which precluded the use of amantadine chemoprophylaxis suggest that the use of these strategies may be of importance in controlling influenza outbreaks in nursing homes.