Meeting Report: Fourth Correlates of Protection for Next Generation Influenza Vaccines

BACKGROUND: This report summarizes the discussions and conclusions from the fourth "Correlates of Protection for Next Generation Influenza Vaccines" meeting, which took place in Vienna, Austria, from October 15, 2025, to October 17, 2025. CONCLUSION: Discussions on influenza virus correlates of protection built on prior meetings, with particular emphasis on challenges introduced by novel vaccine platforms and evolving regulatory expectations. Although established immune markers such as hemagglutination inhibiting (HI) antibodies remain central to influenza vaccine evaluation, this year's discussions highlighted the continued uncertainty and challenges around their generalizability across platforms, populations, and age groups. Regulatory perspectives emphasized the need for strong biological and clinical justification linking immune markers to protection. Overall, the meeting highlighted the importance of continued collaboration among academia, industry, and regulatory agencies to advance the identification and validation of correlates of protection, in alignment with priorities outlined in the 2025 influenza research and development road map for influenza vaccines.