Influenza virus characterisation, Summary Europe, May 2018

This is the fifth report of the 2017–18 influenza season. As of week 20/2018, nearly 240 000 influenza detections across the WHO Europe region have been reported. Types A and B viruses have been detected in the proportions 44% and 56%, respectively, with A(H1N1)pdm09 viruses being slightly more prevalent than A(H3N2) (1:0.98) and B/Yamagata being significantly more prevalent than B/Victoria viruses (52.5:1).

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 1 281 specimens having collection dates after August 2017.  

The 49 A(H1N1)pdm09 test viruses characterised antigenically showed good reactivity with antiserum raised against the 2017–18 vaccine virus, A/Michigan/45/2015. The 210 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 215 A(H3N2) viruses successfully recovered to date, only 44 (20%) had sufficient HA titre to allow antigenic characterisation by HI assay in the presence of oseltamivir, of which seven were tested since the last report. The majority of these 44 viruses were poorly recognised by antisera raised against the currently used vaccine virus, eggpropagated A/Hong Kong/4801/2014, in HI assays. Of the 251 viruses with collection dates from week 40/2017 genetically characterised at the WIC, 247 were clade 3C.2a (with 144 3C.2a2, 78 3C.2a1, 21 3C.2a3 and four 3C.2a4 subclade viruses) and four were clade 3C.3a. Of the 78 subclade 3C.2a1 viruses 73 and 3, respectively, fell in subgroups 3C.2a1b and 3C.2a1a.

Nine B/Victoria-lineage viruses were tested by HI, and eight 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 41 viruses characterised genetically at the WIC with a collection date after week 40/2017, 11 fell within clade 1A and 30 fell within the subgroup (1A(Δ2)) carrying the HA1 double amino acid deletion.  

A total of 58 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 2018–2018–19 seasons and for trivalent vaccines in the southern hemisphere 2018 season. The 298 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.