Influenza virus characterisation, June 2014

The relative prevalence has varied between countries. Viruses with collection dates after 31 December 2013, from 22 countries, have been received by the WHO Collaborating Centre in London.

• Type A and type B viruses have been received at a ratio of nearly 20:1.
• A(H3N2) and A(H1N1)pdm09 viruses have been received in similar numbers.
• Recently circulating A(H1N1)pdm09 viruses belonged to genetic subgroup 6B. Viruses in subgroup 6B 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. Antisera raised against two reference viruses representative of viruses in genetic subgroup 3C.3 – with HA gene sequences encoding several amino acid substitutions compared to other viruses in genetic group 3C.3 – have been prepared. These antisera recognised the majority of test viruses well.
• 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.
• Antigenic characterisation of two viruses of the B/Victoria lineage was performed in June. Neither virus was recognised well by the antiserum raised against the egg-propagated reference virus, A/Brisbane/60/2008, a virus previously recommended as a component of the trivalent influenza vaccine and recommended as a component of quadrivalent influenza vaccines for 2013–14 and 2014–15 influenza seasons. The test viruses were not recognised well by antisera raised against other reference viruses propagated in eggs. The test viruses were better recognised by some, but not all, antisera raised against reference viruses exclusively propagated in cells. Phylogenetic analysis revealed that all B/Victoria-lineage viruses received in 2014 were in genetic clade 1A, the B/Brisbane/60/2008 genetic clade.