RNA replicon vaccination confers long-lasting protection against H5N1 avian influenza in 23 zoo bird species.

Highly pathogenic avian influenza (HPAI) H5N1 viruses (clade 2.3.4.4b) cause mass mortality in birds and have infected over 50 mammalian species, including humans. Approved avian vaccines remain limited. We report the use of a propagation-defective vesicular stomatitis virus (VSV) replicon vaccine, VSV∆G(H5(mb)), encoding a modified H5 hemagglutinin from a 2022 Swiss HPAI isolate. Specific pathogen-free chickens immunized twice intramuscularly develop strong neutralizing antibody responses and are fully protected from lethal H5N1 challenge, without viral shedding. The vaccine is also administered to 317 captive birds across 23 species in Bern Animal Park and Basel Zoo. No side effects are observed. Birds without prior exposure to H5Nx develop strong antibody responses after two doses; previously exposed birds respond after one vaccination. After one year, 98.7% retained significant neutralizing antibody levels, indicating durable protection. These findings suggest that VSV∆G(H5(mb)) is a safe, effective vaccine candidate for broad protection against H5N1 across diverse avian species.