All children and adolescents <18 years of age with laboratory-confirmed PIA admitted to one of the three hospitals covering pediatric in-patients in the area of Wuerzburg (Northern Bavaria / Germany) from July 2009 to March 2010, covering the influenza season 2009/2010, were included. The study population size in our defined catchment area corresponded to approximately 60,300 children younger than 18 years of age (Bavarian State Office for Statistics and Data Processing 2009, https://www.statistik.bayern.de).