Molecular evolution of HA gene of the influenza A H1N1 pdm09 strain during the consecutive seasons 2009-2011 in Hangzhou, China: Several immune-escape variants without positively selected sites

The results of genetic surveillance in this study might account for frequent outbreaks of the influenza A H1N1 pdm09 strains since December 2010 and the disappearance after three months circulation. It facilitates early detection of antigenic sites for the virus to escape immunological restraint in 2010/2011 season. Continuous monitoring of antigenic changes is recommended.