Influenza virus characterisation, December 2013

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

​The 2013–14 influenza season has been slow to start in EU/EEA countries resulting in a low number (46) of influenza-positive specimens, with collection dates since 1 September 2013, being received by WHO CC, London.

Executive summary

  • Type A and type B viruses have been received in the ratio 9:1.
  • A(H3N2) and A(H1N1)pdm09 viruses have been received in the ratio 2:1.
  • Compared to the 2012–13 influenza season where genetic subgroup 6C dominated among A(H1N1)pdm09 viruses, and based on the current global situation, genetic subgroup 6B viruses have been detected in increasing frequency. Subgroup 6B viruses have been antigenically similar to the vaccine virus, A/California/07/2009.
  • Recently circulating A(H3N2) viruses have fallen within genetic subgroup 3C represented by the recommended vaccine virus for the 2013–14 season, A/Texas/50/2012; some new genetic clusters defined by specific HA amino acid substitutions have been observed for which antigenic characterisation is pending.
  • No B/Victoria-lineage viruses have been received to date, but phylogenetic analysis of recently circulating viruses reveals that all are in genetic clade 1A with no significant HA amino acid substitutions, suggestive of antigenic similarity to cell-propagated reference viruses of the B/Brisbane/60/2008 genetic clade.
  • 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/2/2012 (the recommended vaccine component for the 2013–14 influenza season). The great majority of recently circulating viruses fall within clade 2.