2009 H1N1: risk factors for hospitalization in a matched case-control study

In order to compare sociodemographical data and preexisting risk medical conditions in patients requiring hospital admission for 2009 pandemic influenza A (H1N1) virus infection and those managed on an outpatient basis, a prospective observational, matched case-control study in 36 hospitals of the Spanish National Health Service was conducted from July 2009 to February 2010. Cases were patients aged 6 months to 18 years hospitalized for influenza syndrome, in whom 2009 influenza A (H1N1) virus infection was confirmed using real-time reverse-transcription polymerase chain reaction. Controls were patients aged 6 months to 18 years with confirmed 2009 influenza A (H1N1) infection managed on an outpatient basis. There were 195 cases and 184 controls. In a multivariate model, hospitalization was more frequent in children aged