Influenza virus characterisation, May 2014

The relative prevalence has varied between countries. Viruses with collection dates after 1 January 2014, from 20 countries, have been received by the WHO Collaborating Centre in London.

• Type A and type B viruses have been received at a ratio of over 20:1.
• A(H3N2) and A(H1N1)pdm09 viruses have been received in similar numbers.
• Recently circulating A(H1N1)pdm09 viruses belonged to genetic subgroups 6B and 6C, with viruses in genetic subgroup 6B predominating greatly. Viruses in subgroups 6B and 6C are antigenically similar to the vaccine virus, A/California/07/2009.
• Recently circulating A(H3N2) viruses have fallen within genetic group 3C represented by the recommended vaccine virus for the 2013–14 and 2014–15 seasons, A/Texas/50/2012, with viruses of genetic subgroup 3C.3 predominating. Antigenic analysis using antisera raised against cell-propagated H3N2 viruses indicates that the majority of circulating viruses are antigenically similar to those in circulation in the 2012–13 and 2013–14 influenza seasons.
• A small set of viruses in genetic subgroup 3C.3 were not recognised well by the panel of antisera and their HA gene sequences encode several amino acid substitutions compared to other viruses in genetic group 3C.3.
• Two genetic clades of B/Yamagata-lineage viruses continue to circulate: clade 3 represented by B/Wisconsin/1/2010 and clade 2 represented by B/Massachusetts/02/2012 (the recommended vaccine component for the 2013–14 and 2014–15 influenza seasons). Viruses in each clade have been received in similar numbers but with viruses in clade 3 predominating in those samples collected in 2014. 
• Few B/Victoria-lineage viruses have been received, and none were analysed in May. Over the season phylogenetic analysis revealed that all B/Victoria-lineage viruses received were in genetic clade 1A and antigenically similar to the prototype virus B/Brisbane/60/2008 and viruses genetically similar to this prototype virus. B/Brisbane/60/2008 has been recommended by WHO as a component in quadrivalent influenza vaccines for 2013–14 and 2014–15 influenza seasons.