{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,18]],"date-time":"2026-08-18T04:44:47Z","timestamp":1787028287645,"version":"build-2736575974"},"reference-count":30,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2026,4,14]],"date-time":"2026-04-14T00:00:00Z","timestamp":1776124800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Infectious Disease Reports"],"abstract":"<jats:p>Background: Influenza A(H3N2) continues to evolve rapidly, frequently eroding population immunity and challenging seasonal vaccine strain selection. During the 2025\/26 season, the A(H3N2) subclade K (J.2.4.1) expanded quickly across multiple regions and showed evidence of antigenic divergence in standard assays. Methods: In this study, we combined phylogenetic analyses of hemagglutinin (HA) and neuraminidase (NA) sequences with a systematic synthesis of recent peer-reviewed studies and official surveillance reports to comprehensively define the molecular profile and early epidemiological dynamics of subclade K. Results: Our phylogenetic reconstructions of HA and NA genes confirmed the emergence of a coherent and recently diversified lineage characterized by coordinated evolution of surface glycoproteins and broad geographic representation during 2025. Integration of molecular, temporal, and surveillance evidence further supported rapid expansion with limited early regional structuring. Antigenic analyses reported in peer-reviewed studies described reduced haemagglutination inhibition reactivity to vaccine reference antisera for many subclade K viruses, whereas vaccine effectiveness (VE) estimates from multiple settings remained moderate. Conclusions: Overall, the available genetic, antigenic, and epidemiological evidence indicates that subclade K represents a recently diversified A(H3N2) lineage associated with rapid international spread during the 2025\/26 season, highlighting the importance of integrated HA\/NA genomic surveillance and timely antigenic characterization to support evidence-based vaccine strain selection.<\/jats:p>","DOI":"10.3390\/idr18020037","type":"journal-article","created":{"date-parts":[[2026,4,14]],"date-time":"2026-04-14T08:14:17Z","timestamp":1776154457000},"page":"37","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":4,"title":["Influenza A(H3N2) Subclade K (J.2.4.1): Molecular Characterization, Antigenic Divergence, and Global Spread During the 2025\/26 Season"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9485-3877","authenticated-orcid":false,"given":"Francesco","family":"Branda","sequence":"first","affiliation":[{"name":"Unit of Medical Statistics and Molecular Epidemiology, Universit\u00e0 Campus Bio-Medico di Roma, 00128 Rome, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2585-7567","authenticated-orcid":false,"given":"Nicola","family":"Petrosillo","sequence":"additional","affiliation":[{"name":"Infection Prevention & Control\/Infectious Disease Service, Fondazione Policlinico Universitario Campus Bio-Medico, 00127 Rome, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5921-3180","authenticated-orcid":false,"given":"Giancarlo","family":"Ceccarelli","sequence":"additional","affiliation":[{"name":"Department of Public Health and Infectious Diseases, University Hospital Policlinico Umberto I, Sapienza University of Rome, 00185 Rome, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3501-714X","authenticated-orcid":false,"given":"Fabio","family":"Scarpa","sequence":"additional","affiliation":[{"name":"Department of Biomedical Sciences, University of Sassari, 07100 Sassari, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5849-7326","authenticated-orcid":false,"given":"Marta","family":"Giovanetti","sequence":"additional","affiliation":[{"name":"Department of Science and Bio-Technology, Universit\u00e0 Campus Bio-Medico di Roma, 00128 Rome, Italy"},{"name":"Instituto Ren\u00e9 Rachou, Funda\u00e7\u00e3o Oswaldo Cruz, Belo Horizonte 30190-002, Brazil"},{"name":"Climate Amplified Diseases and Epidemics (CLIMADE) Americas, Belo Horizonte 30190-002, Brazil"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3866-9239","authenticated-orcid":false,"given":"Massimo","family":"Ciccozzi","sequence":"additional","affiliation":[{"name":"Unit of Medical Statistics and Molecular Epidemiology, Universit\u00e0 Campus Bio-Medico di Roma, 00128 Rome, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,4,14]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1840","DOI":"10.1080\/21645515.2018.1462639","article-title":"H3N2 influenza viruses in humans: Viral mechanisms, evolution, and evaluation","volume":"14","author":"Allen","year":"2018","journal-title":"Hum. 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