Influenza virus characterisation - Summary Europe, March 2022

Of these 90 644 detections, 98% were type A viruses, with A(H3N2)  (93%) dominating over A(H1 N1)pdm09 (7%), and 2 % type B of which only 32 were ascribed to a lineage  with all but one being B/ Victoria This represents a large increase 89 859 , 115 fold in detections compared to the 2020-2021 season, on the back of a large increase 1 548 266, 347%) in the number of samples tested. However, while there have been clear indications of an influenza epidemic in 2021-2022 with the epidemic threshold of 10% positivity within sentinel specimens having been crossed for 10 weeks as of week 13/2022 (unlike in 2020-2021), numbers of detections are significantly reduced compared to earlier seasons (e.g. 44 % reduced compared to 2019-2020) The increased testing but reduced number of influenza detections is undoubtedly related to the emergence of SARS CoV2 and measures introduced to combat it.

Since the February 20 22 characterisation report report, one shipment from an EU/EEA country (Slovenia) was received at the London WHO Collaborating Centre, the Francis Crick Worldwide Influenza Centre (This report focuses on viruses with collection dates after 31 August 2021 for which HA gene sequences were made available in GISAID after 8 February 2022 to 4 April 2022, together with sequences generated and antigenic data determined at the WIC.

Globally relatively few A(H1N1)pdm09 viruses have been detected in the course of the 2021-2022 season with 6B.1A.5a.1 and 6B.1A.5a.2 subgroups being most commonly detected, but with dominance of a particular subgroup varying between countries. The subgroups are antigenically different and 6B.1A.5a.1 viruses have been most numerous in Europe. An emergent genetic group with in this subgroup showing antigenic drift, defined by HA1 P137S and G155E amino acid substitutions has been detected At the February 2022 WHO influenza vaccine composition meeting (the recommendation was to retain A/Victoria/2570/2019-like viruses 6B.1A.5a.2) as the vaccine component for the northern hemisphere 2022-2023 influenza season.

In Europe and across the world A(H3N2) viruses have been dominant with t he vast majority of recently detected viruses falling in subgroup 3C.2a1b.2 a being Bangladesh like (3C.2a1b.2a. 2). While small clusters of viruses showing antigenic drift have emerged among the ‘Bangladesh-like’ viruses, the great majority of these viruses retained good recognition by post-infection ferret antisera raised against A/Darwin/9/2021-like and A/Darwin/6/2021-like (3C.2a1b.2a.2) viruses which were recommended for egg- and cell-based vaccines to be used in the 2022 southern hemisphere season. At the February 2022 WHO VCM, the recommendation was to change the A(H3N2) vaccine components for the northern hemisphere 2022-2023 influenza season to match those to be used in the 2022 southern hemisphere season.

In Europe and across the world, few B/Victoria-lineage viruses have been detected during the 2021-2022 influenza season. All have lost encoding of a three amino acid triplet (HA1 residues 162-164) placing them in subclade V.1A.3 represented by B/Washington/02/2019 the vaccine virus recommended for inclusion in influenza vaccines for the 2021-2022 northern hemisphere season. The great majority of HA sequences from recently detected viruses, in geographically dispersed countries, have fallen in the V1A.3a group defined by a series of HA1 amino acid substitutions including N150K, with most falling in the V1A.3a.2 subgroup with defining HA1 A127T, P144L and K203R amino acid substitutions. Viruses in subgroup V1A.3a.2 are not recognised well by post-infection ferret antisera raised against B/Washington/02/2019-like viruses and B/Austria/1359417/2021-like (V.1A.3a.2) viruses were recommended for use in the southern hemisphere 2022 and the northern hemisphere 2022-2023 influenza seasons.

No cases of infection with circulating B/Yamagata-lineage viruses have been confirmed since March of 2020. All HA gene sequences from the 77 viruses detected in 2020, inclusive of 12 from EU/EEA countries, belong to genetic clade Y3 and carry three HA1 amino acid substitutions (L172Q, D229N and M251V) compared to B/Phuket/3073/2013-like viruses which are still recommended for use in quadrivalent influenza vaccines. There is a need to share all B/Yamagata-lineage viruses detected recently for detailed characterisation to determine if there are any in circulation that are not related to Live Attenuated Influenza  Vaccines.