Seasonal Influenza Exposure Elicits Functional Antibody and T-cell Responses to A(H5) Influenza Viruses in Humans

Background. Highly pathogenic avian influenza A(H5) viruses pose a pandemic threat, with a history of zoonotic spillovers into humans that are presumed immunologically naive. Whether the general population is currently immunologically naive to circulating A(H5) influenza viruses is unknown. Methods. To evaluate the presence of cross-reactive immune responses to emerging A(H5) clade 2.3.4.4b influenza viruses in the general population, we conducted comprehensive immune profiling on cross-sectional samples from healthcare workers (n=107). Samples were collected in August and September 2024 in the scope of an ongoing prospective follow-up study: Surveillance of rEspiratory viruses iN healThcare and anImal workers in the NethErLands (SENTINEL). Findings. Low-level antibody responses directed against the A(H5) hemagglutinin (HA) head were detected in a limited number of individuals, but without hemagglutination inhibition activity. Nevertheless, we detected in most participants A(H5)-reactive antibodies with Fc-effector functions, likely directed at the conserved HA stalk. Additionally, we observed abundant neuraminidase (NA) inhibiting antibodies against avian N1s and T-cell responses against HAs and NAs from A(H5) influenza viruses. These responses correlated strongly with immune responses targeting an A(H1N1) seasonal influenza virus, indicating they were likely induced by prior exposures. Interpretation. Together, our findings suggest that partial cross-reactive immunity to A(H5) influenza viruses exists in humans, which may play an important role during future outbreaks, potentially by blunting disease severity. Characterizing such baseline immunity is crucial for accurate pandemic risk assessment and preparedness planning. Funding. Netherlands Organization for Health Research and Development (ZonMw), European Union EU4Health program DURABLE, Dutch Ministries of Agriculture, Fisheries, Food Security and Nature and Health, Welfare and Sport, National Institute of Health - National Institute of Allergies and Infectious Diseases (NIH-NIAID). The funding sources had no role in study design, data collection, analysis, interpretation of the data, or the decision to submit the paper for publication.