Week 16/2017 (17 – 23 April 2017).

  • Influenza activity across the region decreased further with 38 of 39 countries reporting low influenza activity. 
  • The proportion of sentinel specimens testing positive for influenza viruses was 13%, slightly lower compared to the previous week (15%). 
  • The proportion of type B viruses exceeded the proportion of type A viruses in sentinel detections, similar to recent weeks. However, the overall number of type B virus detections remained low.

Season overview

  • After an earlier than usual start (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 38 of 39 countries that reported for week 16/2017.
  • From week 40/2016 through week 10/2017, influenza A viruses predominated, accounting for 90% of all sentinel detections; the great majority (99%) of influenza A viruses from sentinel sites that were subtyped were 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 type A virus 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 this season, one A(H3N2) virus has shown reduced susceptibility to oseltamivir and a second A(H3N2) virus has shown reduced susceptibility to zanamivir.
  • 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 high prevalence of A(H3N2) viruses, which put some health care systems under pressure.

Influenza intensity, spread and dominant virus type/subtype