A newly published study in Emerging Infectious Diseases offers encouraging findings for conservationists battling the spread of highly pathogenic avian influenza (HPAI) in endangered birds. The study, led by Dr. Todd Katzner of the U.S. Geological Survey and conducted in collaboration with the U.S. Fish and Wildlife Service (USFWS), the U.S. Department of Agriculture (USDA), and other partners, evaluates the safety and immunogenicity of a poultry H5N1 vaccine for use in black vultures and California condors.
The research was initiated in response to a 2023 outbreak of HPAI (clade 2.3.4.4b) that killed over 21 California condors—about 18% of one wild subpopulation—posing a grave threat to a species with fewer than 600 individuals remaining worldwide. With condors teetering on the edge of extinction, the urgent need to explore protective strategies, including vaccination, became clear.
Vaccine Was Safe in Both Vultures and Condors
The trial used a conditionally licensed, inactivated H5N1 avian influenza vaccine developed by Zoetis, Inc. No adverse reactions were observed in either species. While two vultures developed minor, temporary injection-site nodules, condors exhibited no negative behavioral or physical responses.
Strong Antibody Responses in Vultures and Condors
In black vultures, all 20 vaccinated birds developed detectable hemagglutination inhibition (HI) titers, with 95% reaching levels considered protective (>32). In condors, 80% of vaccinated birds developed HI titers, and 45% surpassed the protective threshold. No immune response was detected in unvaccinated control groups of either species.
Prime-Boost Regimen Marginally More Effective
Both a one-dose (1.0 mL) and a prime-boost (two 0.5 mL doses) strategy were tested. While no statistically significant differences in antibody titers were found between regimens, the prime-boost group trended toward stronger and longer-lasting responses, especially in condors. These findings support using a two-dose approach for maximum efficacy when logistically feasible.
No Impact from Sex or Lead Exposure on Response
The study evaluated whether factors such as sex or prior lead exposure—an immunosuppressant and major cause of condor mortality—impacted vaccine response. No statistically significant correlations were found, though birds with lower lead levels tended to show stronger antibody responses, suggesting an area for future investigation.
Implications for Conservation and Public Health
This study provides the first documented evidence supporting the use of HPAI vaccination in critically endangered condors and demonstrates the value of surrogate species (black vultures) in preclinical wildlife vaccine trials. Importantly, it paves the way for vaccination programs as a conservation tool, especially amid rising threats from zoonotic and panzootic pathogens.
Given the success of this pilot program, the USFWS initiated a broader vaccination campaign in 2024. By October, 207 condors had received at least one dose. This proactive intervention may become a model for safeguarding other at-risk avian species, particularly those whose population sizes are too fragile to withstand sudden disease outbreaks.
Ultimately, the study suggests that wildlife vaccination—long discussed but rarely implemented at scale—may offer real promise for preserving biodiversity in the face of emerging infectious disease threats.
For more information, view the full paper at the CDC’s Emerging Infectious Diseases journal.
Katzner TE, Blackford AV, Donahue M, et al. Safety and Immunogenicity of Poultry Vaccine for Protecting Critically Endangered Avian Species against Highly Pathogenic Avian Influenza Virus, United States. Emerging Infectious Diseases. 2025;31(6):1131–1139. doi:10.3201/eid3106.241558