Influenza Virus Characterisation, June 2013

​Over the course of the 2012–13 season, A(H1N1)pdm09, A(H3N2) and B/Victoria- and B/Yamagata-lineage influenza viruses have co-circulated in ECDC-affiliated countries. The relative prevalences of each has varied between countries.• Type A and type B viruses have been detected in similar proportions but with type A peaking and declining slightly before type B.• A(H1N1)pdm09 viruses have been detected at approximately twice the level of A(H3N2) viruses.• The vast majority of A(H1N1)pdm09 viruses have remained antigenically similar to the vaccine virus, A/California/07/2009, but continued to show genetic drift with an increasing prevalence of genetic group 6 viruses.• The vast majority of A(H3N2) viruses have been antigenically and genetically similar to cell-propagated A/Victoria/361/2011, the prototype vaccine virus for the 2012–13 influenza season.• Viruses of the B/Yamagata-lineage have predominated over those of the B/Victoria-lineage.• B/Victoria-lineage viruses have remained antigenically similar to cell-propagated reference viruses of the B/Brisbane/60/2008 genetic clade.• B/Yamagata-lineage viruses formed two antigenically distinguishable genetic clades: clade 3 represented by B/Wisconsin/1/2010 (the recommended vaccine component for the 2012–13 influenza season) and, in increasing numbers, clade 2 represented by B/Massachusetts/2/2012 (the recommended vaccine component for the 2013–14 influenza season).