Background: Comparative data on severity and treatment of seasonal, pandemic and post-pandemic influenza virus infections are scarce. Objectives: To systematically analyze characteristics of hospitalized patients with influenza in the post-pandemic period compared to seasonal and pandemic influenza. Study design: Clinical and virological data of patients hospitalized in a tertiary referral hospital with post-pandemic influenza (2010-2011) were compared with those during seasonal influenza epidemics (2007-2009) and the influenza A(H1N1)pdm09 pandemic (2009-2010). Results: 82 patients were admitted during the post-pandemic period, compared to 85 during the pandemic and 60 during seasonal influenza epidemics. No differences were observed in the occurrence of complicated illness and the need for intensive care. However, radiographic pneumonia was significantly more often diagnosed in patients with influenza A(H1N1)pdm09 compared to patients with seasonal influenza A (25% versus 71% in pandemic, p= 0.004, and 55% in post-pandemic, p= 0.047). Oseltamivir was more frequently prescribed in post-pandemic and pandemic patients compared to previous influenza seasons (48.9% resp. 76.5% versus 6.5%, p< 0.0001). During the post-pandemic period, patients with influenza B were significantly less often treated with oseltamivir compared to patients with influenza A (27.0% versus 48.9%, p= 0.043), although the course of illness in patients with influenza B was comparable with influenza A. No upsurge of oseltamivir resistance was observed. Conclusions: In our center, severity of illness was comparable for all influenza seasons, although more radiographic pneumonia was diagnosed in patients with influenza A(H1N1)pdm09. Despite the increased use of oseltamivir, no increase in oseltamivir resistance was detected.

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Journal of Clinical Virology
Erasmus MC: University Medical Center Rotterdam