Outbreaks of influenza A among elderly hospital inpatients

To describe two outbreaks of influenza A that occurred in wards catering for assessment, treatment and rehabilitation of elderly patients at the Princess Margaret Hospital in Christchurch.

Infected patients and staff were identified clinically, and influenza subsequently confirmed or supported in some patients by viral antigen detection, culture of throat and nasopharyngeal swabs or serology.

Overall, the attack rate of clinical influenza in the two wards affected was 13 of 27 patients (48%) and 15 of 26 patients (58%) respectively. Of all those with influenza, 13 patients (46%) developed lower respiratory tract infection and there were two deaths (7%) at least in part attributed to influenza. Patients in single rooms were not protected from infection. The vaccination rate among all patients was 18%. Immunised patients had a lower attack rate (21%) than those who had not received influenza vaccine that season (40%). The attack rate amongst staff in each ward was 69% and 36% respectively, and the overall staff vaccination rate was 12%.

These outbreaks illustrate the severe consequences of influenza in an institutional setting.