Epidemiological, antigenic and genetic characteristics of seasonal influenza A(H1N1), A(H3N2) and B influenza viruses: Basis for the WHO recommendation on the composition of influenza vaccines for use in the 2009–2010 Northern Hemisphere season

Abstract Influenza vaccines form an important component of the global response against infections and subsequent illness caused in man by influenza viruses. Twice a year, in February and September, the World Health Organisation through its Global Influenza Surveillance Network (GISN), recommends appropriate influenza viruses to be included in the seasonal influenza vaccine for the upcoming Northern and Southern Hemisphere winters. This recommendation is based on the latest data generated from many sources and the availability of viruses that are suitable for vaccine manufacture. This article gives a summary of the data and background to the recommendations for the 2009–2010 Northern Hemisphere influenza vaccine formulation.