Characterisation of immune responses targeting highly pathogenic avian influenza A(H5) viruses in health-care workers in the Netherlands: an observational, cross-sectional analysis

BACKGROUND: Highly pathogenic avian influenza A(H5) viruses pose a pandemic threat, with a history of mammalian adaptation and zoonotic spillovers into humans. We aimed to determine whether pre-existing cross-reactive immune responses to A(H5) clade 2.3.4.4b influenza viruses detected between 2020 and 2024 are present in the general population. METHODS: We conducted an observational, cross-sectional study within the prospective Surveillance of Respiratory Viruses in Healthcare and Animal Workers in the Netherlands (SENTINEL) cohort, in which we analysed a subset of health-care workers aged 18 years or older who provided blood samples at a periodic study visit in August or September, 2024. Blood samples were analysed for influenza A(H5)-specific antibody binding, haemagglutination inhibition, Fc-effector functions, neuraminidase (NA) inhibition, and T-cell responses. FINDINGS: We included 107 health-care workers. Participants' median age was 50·0 years (IQR 40·0-58·0); 77 (72%) health-care workers were female, 29 (27%) were male, and one (1%) did not report their biological sex. Virus-specific antibodies were measured in 106 serum samples. Low-level binding antibodies directed against the A(H5) haemagglutinin (HA) head were detected in up to 28 individuals (depending on the antigen), but without haemagglutination inhibition activity. Nevertheless, we detected A(H5)-reactive antibodies with Fc-effector functions in all participants. Additionally, we observed high levels of antibodies with NA inhibition activity (geometric mean titre 208 [95% CI 153-284]) in up to 97% of the health-care workers against avian N1, and T-cell responses against HA and NA from A(H5) influenza viruses in 43-69% (46-74 of 107) health-care workers. A(H5)-specific responses correlated with immune responses targeting A(H1N1). INTERPRETATION: Together, our findings suggest that partial cross-reactive immunity to A(H5) influenza viruses exists in humans, likely induced by previous exposures to seasonal influenza viruses. This partial cross-reactive immunity might play an important role during future outbreaks, potentially by blunting disease severity. Characterising pre-existing baseline immunity is crucial for accurate pandemic risk assessment and preparedness planning. FUNDING: The Netherlands Organization for Health Research and Development, the EU's EU4Health programme DURABLE, the Dutch Ministry of Agriculture, Fisheries, Food Security and Nature, the Dutch Ministry of Health, Welfare and Sport, and the National Institute of Health-National Institute of Allergies and Infectious Diseases.