Weekly influenza update, week 52, December 2017

Summary

• Influenza activity was increasing in countries in northern, southern and western Europe.
• Both influenza type A and B viruses were co-circulating and mixed patterns of circulation were observed across the Region.
• Of the individuals sampled, on presenting with ILI or ARI to sentinel primary healthcare sites, 44% tested positive for influenza viruses, an increase from 38% in the previous week.

2017/18 season overview 

• An early risk assessment based on data from EU/EEA countries was published by ECDC on 20 December 2017. First detections indicated circulation of A(H3N2) and B/Yamagata viruses in the highest proportions. As the A(H3N2) subtype dominated last season, a high proportion of the population should be protected.

• From sentinel sources, a higher proportion of type B viruses compared to type A viruses has been detected. Of the type A detections, A(H1N1)pdm09 viruses have outnumbered A(H3N2) 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.

• While low in number, 59% of the genetically characterized A(H3N2) viruses belonged to clade 3C.2a, the vaccine virus clade as described in the WHO recommendations for vaccine composition for the northern hemisphere 2017–18, and 40% to clade 3C.2a1, with viruses in both clades being antigenically similar.

Other news

• The US CDC published a Health Alert Network (HAN) notice, regarding increased A(H3N2) activity that affects mostly people aged over 65 and younger children, leading to more hospitalizations and deaths. Based on the moderate vaccine effectiveness, detailed information on recommended antiviral treatment is provided. See full report here.
• Additional information on global influenza activity is available from WHO’s biweekly global updates.