Weekly influenza update, week 4, January 2018
Influenza activity was widespread in the majority of reporting countries and while activity was increasing, intensity in most countries was reported as low to medium. Both influenza virus types A and B were co-circulating with a higher proportion of type B viruses. Different proportions of circulating influenza virus types and A subtypes were observed between countries in the Region. Of the individuals sampled, on presenting with ILI or ARI to sentinel primary healthcare sites, 51.9% tested positive for influenza viruses, within the range observed in the previous 3 weeks (42–52%).
2017/18 season overview
- For the Region overall, a higher proportion of type B compared to type A viruses has been detected in sentinel sources, whereas in non-sentinel sources the proportions are roughly similar. Of the type A detections from sentinel sources, A(H1N1)pdm09 viruses have outnumbered A(H3N2) viruses, while in non-sentinel sources more A(H3N2) viruses were reported than A(H1N1)pdm09 viruses.
- For type B viruses from both sentinel and non-sentinel sources, B/Yamagata lineage viruses have greatly outnumbered those of the B/Victoria lineage. No B/Yamagata lineage virus is included in the current trivalent seasonal influenza vaccine.
- Different patterns of dominant type and A subtype were observed across the countries in the Region, which may be due to the relative weights of information being derived from sentinel, non-sentinel and severe influenza case sources of information.
- The majority of severe cases are in adults infected by influenza A(H1N1)pdm09 or type B virus.
- While low in number, 58% of the genetically characterized A(H3N2) viruses belonged to clade 3C.2a, the clade of the vaccine virus described in the WHO recommendations for vaccine composition for the northern hemisphere 2017–18, and 37% to subclade 3C.2a1, with mammalian cell-cultured viruses in both clades being antigenically similar.
- A situation analysis that describes the early season evolving epidemiological pattern was published by WHO Regional Office for Europe in January. A high level of influenza B virus circulation is observed during the first half of the season, compared to previous seasons.
- An early risk assessment based on data from EU/EEA countries was published by ECDC on 20 December 2017.
- Interim or real-time vaccine effectiveness estimates from Canada, Stockholm County and Finland suggest overall vaccine effectiveness of 17-31%, depending on proportion of circulating (sub)types. Effectiveness against influenza B is in the range of 37-55%, despite the circulating lineage not being included in the more commonly used trivalent vaccine.
- Based on data submitted to the EuroMOMO project there has been over the past weeks increased all-cause mortality among the elderly, notably in Southern Europe and the United Kingdom (England and Scotland).
- Additional information on global influenza activity is available from WHO’s biweekly global updates.
See all weekly influenza updates
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