Broberg, Eeva KVukovikj, MajaSvartström, OlovHasibra, IrisRiess, MaximilianMelidou, AngelikiERLI-Net networkPozo Sanchez, FranciscoCasas Flecha, InmaculadaPérez-Gimeno, GloriaMazagatos, ClaraLarrauri, Amparo2025-03-112025-03-112024-12Broberg EK, Vukovikj M, Svartström O, Hasibra I, Riess M, Melidou A; Members of the ERLI-Net network; Members of the ERLI-Net network. Antigenic changes in influenza A(H3N2) driven by genetic evolution: Insights from virological surveillance, EU/EEA, week 40/2023 to week 9/2024. Euro Surveill. 2024 Dec;29(50):2400395.1025-496Xhttps://hdl.handle.net/20.500.12105/26419Background: During the 2023/24 influenza season in the European Union/European Economic Area (EU/EEA), influenza viruses A(H1N1)pdm09, A(H3N2) and B/Victoria viruses were co-circulating. Aim: We aimed to describe the circulating influenza viruses by (sub)type, genetic clade, antigenic group and antiviral susceptibility in that season in the EU/EEA. Methods: We collected surveillance data from EU/EEA countries through weekly submissions to The European Surveillance System (TESSy). Data were submitted in strain-based format for weeks 40/2023 to 9/2024. Results: Twenty-nine EU/EEA countries reported 154,718 influenza virus detections (primary care sentinel and non-sentinel combined), of which 97% (150,692) were type A and 3% (4,026) were type B. Of the subtyped influenza A viruses, 30,463 (75%) were influenza A(H1)pdm09 and 10,174 (25%) were influenza A(H3). For 809 (20%) of the type B viruses, the lineage was determined; all were B/Victoria/2/87 lineage, and none were B/Yamagata/16/88 lineage. Genetic diversification of seasonal influenza viruses continued, and clade 5a.2a of A(H1N1)pdm09, 2a.3a.1 of A(H3N2) and V1A.3a.2 of B/Victoria-lineage viruses dominated. Of the A(H3N2) 2a.3a.1 viruses, 23% were antigenically distinct from the 2023/24 vaccine virus. Conclusion: The 2023/24 influenza season was characterised by co-circulation of different influenza (sub)types, antigenically similar to the components recommended for the 2023/24 northern hemisphere vaccine, A/Victoria/4897/2022 (egg-based) and A/Wisconsin/67/2022 (cell culture- or recombinant-based). However, genetic diversification of the viruses continued. The World Health Organization's vaccine recommendations for the northern hemisphere 2024/25 season were updated to include a new A(H3N2) component, while maintaining the current A(H1N1)pdm09 and B/Victoria components.engVoRhttp://creativecommons.org/licenses/by/4.0/EuropeGeneticInfluenzaSurveillanceAntigens, ViralAntiviral AgentsEuropeEuropean UnionEvolution, MolecularHumansInfluenza A Virus, H1N1 SubtypeInfluenza A Virus, H3N2 SubtypeInfluenza B virusInfluenza, HumanPhylogenyPopulation SurveillanceSeasonsSentinel SurveillanceAttribution 4.0 International396687662950240039510.2807/1560-7917.ES.2024.29.50.24003951560-7917Euro surveillance : bulletin Européen sur les maladies transmissibles = European communicable disease bulletinopen access