Inactivated Zoonotic Influenza A(H5N8) Vaccine Induces Robust Antibody Responses Against Recent Highly Pathogenic Avian Influenza Clade 2.3.4.4b A(H5N1) Viruses

In 2023, Finland faced an outbreak of highly pathogenic avian influenza caused by clade 2.3.4.4b A(H5N1) viruses, which spread from wild birds to fur farms. Vaccinations of individuals at-risk, such as fur and poultry farm workers, veterinarians, and laboratory workers, began in June 2024 using the MF59-adjuvanted inactivated (H5N8) vaccine manufactured by Seqirus (based on clade 2.3.4.4b A/Astrakhan/3212/2020). We investigated antibody responses following a two-dose vaccination regimen in 39 subjects. Vaccination induced comparable levels of functional antibodies both against the vaccine virus and two clade 2.3.4.4b viruses, either associated with outbreaks in fur animals in Finland or cattle in the United States. Upon two doses of the vaccine for previously unvaccinated people, the seroprotection rate against the vaccine virus was 83 % (95 % CI 70-97 %, titer [≥]20) and 97 % (95 % CI 90-100 %, titer [≥]40) using microneutralization or hemagglutinin inhibition assays, respectively. In a subset of previously H5-vaccinated individuals, the first dose already led to seroprotective titers, indicative of immunological recall. These data show that the vaccine is expected to confer cross-protection against currently circulating H5 clade 2.3.4.4b viruses.