Influenza virus characterisation, Summary Europe, June 2018

Surveillance report
Publication series: Influenza Virus Characterisation
Time period covered: 2017-2018 influenza season
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European Centre for Disease Prevention and Control. Influenza virus characterisation, summary Europe, June 2018. Stockholm: ECDC; 2018.  

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This is the sixth report of the 2017–18 influenza season. As of week 25/2018, nearly 239 000 influenza detections across the WHO European Region have been reported. Forty-four percent of the detected viruses were type A, with A(H1N1)pdm09 and A(H3N2) viruses being detected in equal numbers. Type B viruses accounted for 56%; B/Yamagata viruses prevailed over B/Victoria viruses at a ratio of over 50:1.

Executive summary

Twenty-nine EU/EEA countries have shared influenza-positive specimens with the London WHO CC, Crick Worldwide Influenza Centre (WIC), since week 40/2017, with 1 455 specimens having collection dates after August 2017.  

Of the 28 A(H1N1)pdm09 test viruses characterised antigenically, all but one showed good reactivity with antiserum raised against the 2017–18 vaccine virus, A/Michigan/45/2015. The 231 test viruses with collection dates from week 40/2017 genetically characterised at the WIC, as others from the European Region recently deposited in the GISAID EpiFlu database, have all fallen in subclade 6B.1, defined by HA1 amino acid substitutions S162N and I216T, the great majority with additional substitutions of S74R, S164T and I295V.

Of 311 A(H3N2) viruses successfully recovered to date, only 86 (28%) had sufficient HA titre to allow antigenic characterisation by HI assay in the presence of oseltamivir, of which 34 were tested since the last report. The majority of these 86 viruses were poorly recognised by antisera raised against the currently used vaccine virus, egg-propagated A/Hong Kong/4801/2014, in HI assays. Of the 298 viruses with collection dates from week 40/2017 genetically characterised at the WIC, 289 were clade 3C.2a (with 161 3C.2a2, 102 3C.2a1, 22 3C.2a3 and four 3C.2a4 subclade viruses) and nine were clade 3C.3a. Of the 102 subclade 3C.2a1 viruses, 96 fell in subgroup 3C.2a1b and three belonged to subgroup 3C.2a1a.

A single B/Victoria-lineage viruses was tested by HI, and it reacted well only with post-infection ferret antisera raised against tissue culture-propagated cultivars of B/Norway/2409/2017 and B/Colorado/06/2017, viruses with a deletion of two amino acids in HA1 (Δ162-163). Of the 43 viruses characterised genetically at the WIC with a collection date after week 40/2017, 11 fell within clade 1A and 32 fell within the subgroup (1A(Δ2)) carrying the HA1 double amino acid deletion.  

A total of 52 B/Yamagata viruses were characterised antigenically, and all reacted well (within fourfold of the homologous titre) with post-infection ferret antiserum raised against egg-propagated B/Phuket/3073/2013, the recommended vaccine virus for use in quadrivalent vaccines for the northern hemisphere 2017–18 and 2018–19 seasons and for trivalent vaccines in the southern hemisphere 2018 season. The 352 viruses with collection dates from week 40/2017 genetically characterised at the WIC – as others recently circulating in the European Region and reported to the GISAID EpiFlu database – fall within genetic clade 3.

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