Overrepresentation of influenza A(H1N1)pdm09 virus among severe influenza cases in the 2011/12 season in four European countries.

In France, Ireland, Spain and the United Kingdom, the influenza season 2011/12 started in the final weeks of 2011 and has been dominated by influenza A(H3) viruses with minimal circulation of influenza A(H1N1) pdm09 and B viruses. A relatively greater proportion, however, of influenza A(H1N1)pdm09 viruses were reported in hospitalised laboratory-confirmed influenza cases in four countries. Compared to the season 2010/11, the proportion of subtype A(H3) among hospitalised cases has increased, associated with a larger proportion of cases in the youngest and oldest age groups.