Weekly influenza update, week 18, April/May 2018

Summary

• Influenza activity was at inter-season levels in all but one reporting country.
• 6% of the individuals sampled from primary healthcare settings tested positive for influenza viruses (compared to 11% in the previous week).
• The number of influenza virus detections has further decreased compared to previous weeks.

2017/18 season overview 

• Influenza viruses circulated at high levels in the Region between weeks 52/2017 and 12/2018 (based on increased proportions - 40% and above - of sentinel specimens testing positive for influenza viruses). This is longer than in recent seasons and may have contributed to the severity of this season.
• 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.
• 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 20 participating countries and regions that report to EuroMOMO.
• Interim results from 5 European studies indicate 25 to 52% vaccine effectiveness against any influenza.