Eurosurv. Population immunity, post-vaccine and post-infection antibody responses to influenza A (H3N2) subclade J.2 and K (J.2.4.1) viruses, Hong Kon

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Samuel MS Cheng[SUP]1,*[/SUP] , Nancy HL Leung[SUP]1,*[/SUP] , Eunice Y Shiu[SUP]1[/SUP] , Ricky WK Wong[SUP]1[/SUP] , Niki YM Au[SUP]1[/SUP] , Xiaotong Huang[SUP]1[/SUP] , Natalie KM Kwok[SUP]1[/SUP] , Coco HC Chan[SUP]1[/SUP] , John KC Li[SUP]1[/SUP] , Leo CH Tsang[SUP]1[/SUP] , Tsz Chun Kwan[SUP]1[/SUP] , Tim K Tsang[SUP]1[/SUP] , Leo LM Poon[SUP]1,2[/SUP] , Benjamin J Cowling[SUP]1,**[/SUP] , Malik Peiris[SUP]1,2,*[/SUP]​


We assessed haemagglutination inhibition (HAI) antibody titres against 2025 influenza A(H3N2) J.2 vaccine reference viruses and the novel subclade K (J.2.4.1) in age-stratified Hong Kong sera (collected May–July 2025), vaccine recipients and sera from virologically confirmed subclade J.2.2 or K infections. In contrast to mono-specific ferret sera, human sera with high J.2 HAI titres cross-reacted strongly with subclade K. Recombinant haemagglutinin- or egg-based standard dose J.2 A(H3N2) vaccines boosted geometric mean titres to K virus 5.6-fold and 2.6-fold, respectively.

https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2026.31.10.2600185#html_fulltext