Week 11/2017 (13 - 20 March 2017)

  • Influenza activity across the region continued to decrease with the great majority of countries reporting low intensity.
  • The proportion of influenza virus detections among sentinel surveillance specimens continued to decrease, to 17% from 22% in the previous week.
  • This was the first week during the season that the proportion of type B viruses exceeded the proportion of type A viruses in sentinel detections, as is commonly seen in the second half of an influenza season. However, the overall number of type B detections remained low

Season overview

  • Influenza activity started early this season, in week 46/2016, which is the earliest week that the overall influenza virus-positivity rate in sentinel specimens reached 10% since the emergence of A(H1N1)pdm09 viruses in 2009/10.
  • Since week 40/2016, influenza A viruses have predominated, accounting for 94% of all sentinel detections; the great majority (99%) of subtyped influenza A viruses from sentinel sites being A(H3N2).
  • Confirmed cases of influenza virus type A infection reported from hospitals have predominantly been in adults aged 65 years or older. Excess all-cause mortality has been observed substantially in people aged 15–64 years and markedly in people aged 65 years or older in the majority of the 19 reporting countries. This is commonly seen when the predominant viruses circulating are A(H3N2).
  • Two-thirds of the A(H3N2) viruses genetically characterized belong to genetic subclade (3C.2a1), which are in the main antigenically similar to the clade 3C.2a vaccine virus, as mentioned in the WHO recommendations for vaccine composition for the northern hemisphere 2017–18.
  • Recent vaccine effectiveness estimates for all age groups against A(H3N2) illness from Canada (42%), the US (43%) and Europe (38%) are consistent for persons aged 65 years or older.
  • Given typically suboptimal vaccination coverage and the partial effectiveness of influenza vaccines, rapid use of neuraminidase inhibitors (NAIs) for laboratory-confirmed or probable cases of influenza virus-infection should be considered for vaccinated and non-vaccinated patients at risk of developing complications.
  • Of the viruses tested so far, only one A(H3N2) virus (<1%) has shown reduced susceptibility to oseltamivir this season.
  • The progression of the season has confirmed the conclusions of the ECDC risk assessment on seasonal influenza updated on 25 January 2017, namely that expected severe outcomes in the elderly related to the prevalence of A(H3N2) viruses, putting some health care systems under pressure.

Influenza intensity, spread and dominant virus type/subtype