Week 15/2017 (10 – 16 April 2017)

  • Influenza activity across the region decreased further with 41 of 43 countries reporting low influenza activity.
  • The proportion of sentinel specimens testing positive for influenza virus was 14%, lower compared to the previous week (17%).
  • The proportion of type B viruses exceeded the proportion of type A viruses in sentinel detections, similar to recent previous weeks. However, the overall number of type B virus detections remained low.

Season overview

  • After an earlier than usual start to the influenza season (week 46/2016), the influenza season is considered to be over in the majority of countries in the Region, with influenza activity at inter-seasonal levels in 41 countries.
  • From week 40/2016 through week 10/2017, influenza A viruses have predominated, accounting for 90% of all sentinel detections; the great majority (99%) of subtyped influenza A viruses from sentinel sites was A(H3N2).
  • Since week 11/2017 influenza B viruses have predominated, although absolute numbers of type B detections have remained low.
  • Confirmed cases of influenza virus type A infection reported from hospitals have predominantly been in adults aged 65 years or older.
  • Significant excess all-cause mortality has been observed in people aged 15–64 years, and markedly so in people aged 65 years or older, in the majority of the 19 reporting countries or regions. This is commonly seen when the predominant viruses circulating are A(H3N2).
  • Two-thirds of the A(H3N2) viruses genetically characterized belong to subclade (3C.2a1), but remain antigenically similar to the clade 3C.2a vaccine virus, as described in the WHO recommendations for vaccine composition for the northern hemisphere 2017–18. See also the WHO CC London February 2017 report.
  • Vaccine effectiveness estimates for all age groups against A(H3N2) illness suggest moderate effectiveness in Canada (42%), the US (43%) and in Europe (38%).
  • Of the viruses tested so far, only one A(H3N2) virus (<1%) has shown reduced susceptibility to oseltamivir this season.
  • The developments during the season have followed the conclusions of the ECDC risk assessment on seasonal influenza, updated on 25 January 2017, suggesting increased severe outcomes in the elderly due to the prevalence of A(H3N2) viruses, which has put some health care systems under pressure.

Influenza intensity, spread and dominant virus type/subtype