Influenza virus characterisation, July 2014

Surveillance report
Publication series: Influenza Virus Characterisation
Time period covered: 2013 - 2014

​During the 2013–14 season, A(H1N1)pdm09, A(H3N2), B/Victoria- and B/Yamagata-lineage influenza viruses have continued to co-circulate in EU/EEA Member States. The relative prevalence has varied between countries. Viruses with collection dates after 31 January 2014 from 21 countries have been received by the WHO Collaborating Centre in London.

Executive summary

Type A and type B viruses have been received at a ratio of approximately 10:1. A(H3N2) outnumbered A(H1N1)pdm09 viruses at a ratio of 1.4:1.

Recently circulating A(H1N1)pdm09 viruses in Europe, and worldwide, belonged to genetic subgroup 6B. Viruses in this subgroup 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. Viruses of genetic subgroup 3C.3 predominated. In recent months a cluster of viruses have been detected within subgroup 3C.3, designated 3C.3a, with HA gene sequences encoding several amino acid substitutions compared to other viruses in the 3C.3 subgroup. 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 but those in the newly emerging cluster, 3C.3a, are poorly recognised by some antisera.

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 clade 3 predominate in those samples collected since 31 January 2014.

Antigenic characterisation of a small number of viruses of the B/Victoria lineage was performed in July. A single virus was recognised well by the antiserum raised against the egg-propagated reference virus, A/Brisbane/60/2008, 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 majority of the test viruses were better recognised by antisera raised against reference viruses propagated exclusively 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.


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