Comparison of adult patients hospitalised with pandemic (H1N1) 2009 influenza and seasonal influenza during the "PROTECT" phase of the pandemic response.

OBJECTIVE: To compare the patient characteristics, clinical features and outcomes of adult patients hospitalised with pandemic (H1N1) 2009 influenza and seasonal influenza.

DESIGN AND SETTING: Retrospective medical record review of all patients admitted to Liverpool Hospital, Sydney, with laboratory-confirmed influenza from the initiation of the "PROTECT" phase of the pandemic response on 17 June until the end of our study period on 31 July 2009.

MAIN OUTCOME MEASURES: Severity of illness; requirement for admission to the intensive care unit (ICU) and/or invasive ventilation; mortality.

RESULTS: Sixty-four adults were admitted to Liverpool Hospital with influenza, 48 with pandemic (H1N1) 2009 influenza and 16 with seasonal influenza. Thirteen patients were admitted to the ICU. Seven required invasive ventilation, with 2 patients requiring ongoing extracorporeal membrane oxygenation (ECMO). Five patients died (mortality rate, 8%) with two deaths occurring after the study period. Patients with pandemic (H1N1) 2009 influenza were younger and less likely to be immunocompromised than patients with seasonal influenza. However, the clinical features of pandemic (H1N1) 2009 influenza and seasonal influenza were similar.

CONCLUSIONS: Our findings show that the clinical course and outcomes of pandemic (H1N1) 2009 influenza virus are comparable to those of the current circulating seasonal influenza in Sydney. The high number of hospital admissions reflects a high incidence of disease in the community rather than an enhanced virulence of the novel pandemic influenza virus.