The study period extended from July 1, 2009, to January 31, 2010. Children were included if they were aged from 0 to 18 years, presented with clinical symptoms compatible with influenza (fever and/or respiratory signs/symptoms) and had either positive PCR results for influenza A/H1N1v (proven cases), or an antigen and/or a positive culture for influenza A (probable cases). The latter cases were included because virtually no other seasonal influenza A viruses were circulating in Belgium during the epidemic period (less than 0.4%, data from the Belgian National Institute of Public Health). Moreover, specific H1N1v PCR confirmation was no longer carried out routinely at the end of the epidemic, due to the high cost of this testing and the limited number of cases after December 2009.