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

OBJECTIVE: 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. DESIGN: Retrospective cohort study. SETTING: Skilled nursing home facility in western New York State. PATIENTS: Nursing home residents and patient-care staff. MAIN OUTCOME MEASURE: Incidence of influenza-like illness among vaccinated versus unvaccinated nursing home residents and staff. RESULTS: 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. CONCLUSION: 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.