An influenza outbreak in an immunized nursing home population: Inadequate host response or vaccine failure?

Objective:To describe an outbreak of influenza A in an immunized nursing home population. Design: Observational cohort study. Setting:Subjects include residents and staff members of a skilled nursing facility in Rochester, Minnesota, at the time of an influenza outbreak in November 1996. Methods: Review of medical records, incident reports, and Minnesota State Health Department data. Results: During a 5-day period, 27 of 62 residents (attack rate = 44%), and 16 of 67 staff members (attack rate = 24%) developed symptoms consistent with acute influenza infection. Influenza A was confirmed by throat culture in the index case and in three other initial cases. Ninety-five percent of residents and 72% of staff members with direct patient contact had been immunized 4ú8 weeks prior to the outbreak. Two symptomatic residents (5%) were hospitalized, and one nonhospitalized resident died (2.3% mortality). The viral strain isolated from the outbreak (A/Wuhan/H3N2) was antigenically identical to the one (A/Nanchang/H3N2) contained in the vaccine. Conclusion: Annual influenza vaccination of nursing home residents and staff members, though advisable, does not afford complete protection from infection of nursing home residents or staff members. The higher attack rates among nursing home residents compared to staff members suggest that older nursing home residents may mount an inadequate host response to influenza vaccination or exposure.