The study a prospective cohort of adults hospitalized for at least 24 h with laboratory-confirmed pandemic influenza A (H1N1) 2009 virus pneumonia at 14 Spanish teaching hospitals during the pandemic period before the introduction of the vaccination campaign against the influenza A (H1N1) 2009 virus (12 June to 10 November 2009) and the first post-pandemic influenza season (1 December 2010 to 31 March 2011)