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March 15, 2026Eurosurveillance2 citationsOpen Access

Population immunity, post-vaccine and post-infection antibody responses to influenza A (H3N2) subclade J.2 and K (J.2.4.1) viruses, Hong Kong, 2025

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SCSamuel MS ChengNLNancy HL LeungESEunice Y Shiu

Key Points

  • To assess antibody responses in relation to specific influenza A(H3N2) subclades post-vaccination and infection.
  • Analyzed sera collected from Hong Kong participants
  • Measured HAI antibody titres against J.2 and K viruses
  • Stratified samples by age and vaccination status
  • Human sera exhibited significant cross-reactivity with subclade K
  • J.2 vaccine boosted geometric mean titres to K virus by 5.6-fold
  • Egg-based vaccines increased titres to K virus by 2.6-fold

Abstract

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.

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Cite This Study

Cheng et al. (2026) studied this question.

synapsesocial.com/papers/69b6069b83145bc643d1cc19https://doi.org/10.2807/1560-7917.es.2026.31.10.2600185
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