{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"institution":[{"name":"medRxiv"}],"indexed":{"date-parts":[[2026,9,20]],"date-time":"2026-09-20T06:13:00Z","timestamp":1789884780678,"version":"4.0.1"},"posted":{"date-parts":[[2026,2,22]]},"group-title":"Infectious Diseases (except HIV\/AIDS)","reference-count":20,"publisher":"openRxiv","license":[{"start":{"date-parts":[[2026,2,22]],"date-time":"2026-02-22T00:00:00Z","timestamp":1771718400000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by-nc-nd\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"accepted":{"date-parts":[[2026,2,22]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                <jats:p>An H3N2 variant, named subclade K, continues to circulate widely during the 2025-2026 influenza season. This virus possesses a hemagglutinin (HA) protein that has eleven substitutions relative to the HA of the Northern Hemisphere 2025-2026 H3N2 vaccine strain. Many of these substitutions are in epitopes in well-characterized HA antigenic sites. Despite this, interim vaccine effectiveness studies indicate that the 2025-2026 influenza vaccine provides moderate protection against H3N2 subclade K infection. We previously reported that many individuals who received the 2025-2026 influenza vaccine produced antibodies that inhibit H3N2 subclade K virus cellular attachment. Here, we show these individuals also produced antibodies that neutralize H3N2 subclade K virus infection, and we observed a strong correlation between hemagglutination-inhibition titers and neutralizing antibody titers. We completed additional specificity studies using samples from individuals who did or did not have antibodies that cross-reacted to H3N2 subclade K viruses. Using high-throughput neutralization assays, we determined that antibodies that bound to the vaccine strain but not H3N2 subclade K viruses typically targeted antigenic site B of HA. Conversely, we found that cross-reactive neutralizing antibodies elicited by vaccination commonly targeted antigenic site A, D, and E of HA that are conserved between the vaccine strain and H3N2 subclade K viruses. Additional electron microscopy-based polyclonal epitope mapping studies confirmed that cross-reactive antibodies elicited by vaccination typically target epitopes on the side of HA. Together, our studies provide an immunological explanation of why the 2025-2026 influenza vaccine was partially effective against antigenically advance H3N2 subclade K viruses. Our data suggest that vaccine strains for subsequent seasons need to be carefully considered, since subclade K viruses have already started to acquire additional substitutions in HA antigenic sites targeted by cross-reactive antibodies.<\/jats:p>","DOI":"10.64898\/2026.02.20.26346746","type":"posted-content","created":{"date-parts":[[2026,2,22]],"date-time":"2026-02-22T16:30:14Z","timestamp":1771777814000},"source":"Crossref","is-referenced-by-count":5,"title":["Mapping the specificity of H3N2 strain-specific and cross-reactive human neutralizing antibodies elicited by the 2025-2026 influenza vaccine"],"prefix":"10.64898","author":[{"given":"Jiaojiao","family":"Liu","sequence":"first","affiliation":[{"name":"Department of Microbiology and Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sydney","family":"Gang","sequence":"additional","affiliation":[{"name":"Department 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USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Naiqing","family":"Ye","sequence":"additional","affiliation":[{"name":"Department of Microbiology and Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tachianna","family":"Griffiths","sequence":"additional","affiliation":[{"name":"Department of Microbiology and Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Elizabeth M","family":"Drapeau","sequence":"additional","affiliation":[{"name":"Department of Microbiology and Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Reilly K.","family":"Atkinson","sequence":"additional","affiliation":[{"name":"Department of 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