Week 20/2017 (15 – 21 May 2017)

  • Influenza activity remained at out-of-season levels in most countries. Low intensity of influenza was reported by all of the 38 reporting countries.
  • The proportion of sentinel specimens testing positive for influenza viruses was 7%, lower than in the previous week (8%). Influenza viruses were detected in 6 countries only and numbers were low.
  • All sentinel detections were type B viruses but numbers were low and the decline since week 14/2017 continued.

Season overview

  • After an earlier start than usual (week 46/2016), influenza activity peaked between weeks 52/2016 and 4/2017. Since week 12/2017, most countries have reported decreased influenza activity with the proportion of sentinel detections returning to the epidemic threshold value (10%) in week 17/2017.
  • From week 40/2016 through week 10/2017, influenza A viruses predominated, accounting for 90% of all sentinel detections. Of those subtyped, 99% were A(H3N2). Since week 11/2017, influenza B viruses have predominated, although absolute numbers of type B detections have remained low and overall have been decreasing since week 14/2017.
  • 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 20 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 influenza viruses tested so far this season, 7 A(H3N2), 1 A(H1N1)pdm09 and 3 B/Victoria lineage viruses show reduced susceptibility to oseltamivir and/or zanamivir.
  • The developments during the season have been consistent with the conclusions of the ECDC risk assessment on seasonal influenza, updated on 25 January 2017, which suggested increased severe outcomes in the elderly due to the high prevalence of A(H3N2) viruses, resulting in pressure on some health care systems.


Influenza intensity, spread and dominant virus type/subtype