WHO recommendations for the viruses used in the 2013-2014 Northern Hemisphere influenza vaccine: Epidemiology, antigenic and genetic characteristics of influenza A(H1N1)pdm09, A(H3N2) and B influenza viruses collected from October 2012 to January 2013

In February the World Health Organisation (WHO) recommends influenza viruses to be included in influenza vaccines for the forthcoming winter in the Northern Hemisphere. These recommendations are based on data collected by National Influenza Centres (NICs) through the WHO Global Influenza Surveillance and Response System (GISRS) and a more detailed analysis of representative and potential antigenically variant influenza viruses from the WHO Collaborating Centres for Influenza (WHO CCs) and Essential Regulatory Laboratories (ERLs). This article provides a detailed summary of the antigenic and genetic properties of viruses and additional background data used by WHO experts during development of the recommendations of the 2013-2014 Northern Hemisphere influenza vaccine composition.