Influenza virus characterisation, March 2019
This is the fifth report for the 2018–19 influenza season. As of week 14 in 2019, 197 027 influenza detections across the WHO European Region had been reported. Detections were 99.1% type A viruses, with A(H1N1)pdm09 prevailing over A(H3N2), and 0.9% type B viruses, with 72 (67%) of 108 ascribed to a B/Yamagata-lineage.
Executive Summary
Since the February 2019 characterisation report, a further five shipments of influenza-positive specimens from EU/EEA countries have been received at the London WHO CC, the Francis Crick Worldwide Influenza Centre (WIC). A total of 1 037 virus specimens, with collection dates after 31 August 2018, have been received.
A total of 103/105 (98.1%) A(H1N1)pdm09 test viruses characterised antigenically since the February 2019 characterisation report showed good reactivity with antiserum raised against the 2018–19 vaccine virus, A/Michigan/45/2015 (clade 6B.1). The 304 test viruses with collection dates from week 40 of 2018 genetically characterised at the WIC, including an H1N2 reassortant, have all fallen in a 6B.1 subclade, designated 6B.1A, defined by HA1 amino acid substitutions of S74R, S164T and I295V. Of these recently circulating viruses, 273 also have HA1 S183P substitution, often with additional substitutions in HA1 and/or HA2.
Since the last report, only 46 A(H3N2) viruses successfully recovered had sufficient HA titre to allow antigenic characterisation by HI assay in the presence of oseltamivir. These viruses were poorly recognised by antisera raised against the currently used vaccine virus, egg-propagated A/Singapore/INFIMH-16-0019/2016, in HI assays. Of the 247 viruses with collection dates from week 40 of 2018 genetically characterised at the WIC, 224 were clade 3C.2a (with 29 3C.2a2, nine 3C.2a3, five 3C.2a4 and 181 3C.2a1b) and 23 were clade 3C.3a.
Recent clade 1A B/Victoria-lineage viruses carry HA genes that encode HA1 amino acid substitutions of I117V, N129D and V146I were compared to a previous vaccine virus, B/Brisbane/60/2008. Groups of viruses defined by deletions of two [Δ162–163, 1A(Δ2)] or three [Δ162–164, 1A(Δ3)] amino acids in HA1 have emerged, with the triple deletion group having subgroups of Asian and African origin. HI analyses with panels of post-infection ferret antisera have shown these virus groups to be antigenically distinguishable. One virus that was characterised since the last report is of the Asian [Δ162–164, 1A(Δ3)] subgroup. Of the five viruses characterised from EU/EEA countries, one was Δ162–163 and four Δ162–164 (three African and one Asian subgroup).
Including the three B/Yamagata-lineage viruses reported here, nine from the 2018–19 season have been characterised. All have HA genes that fall in clade 3 and encode HA1 amino acid substitutions of L172Q and M251V compared to the vaccine virus B/Phuket/3073/2013, but remain antigenically similar to the vaccine virus that is recommended for use in quadrivalent vaccines for current and subsequent northern hemisphere influenza seasons.
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