Weekly influenza update, week 16, April 2018

Summary

• Influenza activity was at inter-season levels in all but one reporting country.
• While low in number, 12% of the individuals sampled from primary healthcare settings tested positive for influenza viruses (compared to 23% in the previous week).
• Both influenza virus types A and B were co-circulating with the majority being type A.

2017/18 season overview 

• Influenza viruses have been circulating widely in the Region between weeks 52/2017 and 16/2018 (based on increased proportions - 10% and above - of sentinel specimens testing positive for influenza viruses). This is longer than in recent seasons and may contribute to the severity of this season.
• For the Region overall, the majority of influenza viruses detected were type B, representing a high level of circulation of influenza B viruses compared to recent seasons. B/Yamagata lineage viruses have greatly outnumbered those of the B/Victoria lineage. 
• Different patterns of dominant type and A subtypes were observed between the countries of the Region.
• Of the type A virus detections from sentinel sources, the majority of which were subtyped, A(H1N1)pdm09 viruses have outnumbered A(H3N2) viruses. In non-sentinel sources, similar numbers of A(H3N2) viruses and A(H1N1)pdm09 viruses were reported. 
• While low in numbers, characterized A(H3N2) viruses fell mainly in clade 3C.2a (57%) and subclade 3C.2a1 (42%), while 42% of B/Victoria lineage viruses fell in a subclade of clade 1A viruses that are antigenically distinct from the current trivalent vaccine component.
• The majority of severe cases reported this season were due to influenza virus type B infection and have mostly occurred in persons older than 15 years. 
• Mortality from all causes now appears be have returned to normal expected levels in all 21 participating countries and regions that report to EuroMOMO. 
• Interim results from 5 European studies indicate 25 to 52% vaccine effectiveness against any influenza.