A new peer-reviewed study from the Francis Crick Institute and University College London Hospitals has found that quadrivalent seasonal flu vaccines (QIVs)—the standard annual flu shots—do not generate protective antibodies against two dangerous strains of avian influenza A(H5N1), including the version recently detected in U.S. dairy cattle and linked to human infections.
Published in Emerging Infectious Diseases, the study analyzed blood samples from 61 adults who received flu vaccines across the 2021–2024 seasons. While the vaccines effectively boosted immunity against seasonal flu viruses, they failed to trigger any meaningful immune response against the two H5N1 strains tested.
No Protection Against H5N1
Despite strong responses to the seasonal H1N1 flu virus, none of the participants showed antibody responses that would help neutralize the tested H5N1 viruses—one from a 2023 outbreak in Cambodia, and one from a 2024 U.S. dairy cow linked to recent human infections. This indicates that current flu shots are unlikely to protect people from serious illness if H5N1 spreads more widely among humans.
Rising Risk of Human Transmission
The findings come amid growing concern that H5N1 is evolving in animals, particularly on dairy farms where the virus has been detected in cows, cats, rodents—and in some human cases. U.S. officials have also found traces of the virus in wastewater and in raw milk. While pasteurized milk remains safe, the presence of live virus in unpasteurized products adds another route of possible exposure.
Some human infections so far have been mild, but the virus’s continued spread in mammals raises the risk it could adapt to spread more easily between people. If that happens, current flu vaccines won’t be enough to protect vulnerable populations.
Vulnerable Groups May Be at Greater Risk
The study focused on healthy adults, the group most likely to mount a strong immune response to vaccines. The lack of H5N1 protection even in this group is especially concerning, as older adults, people with weakened immune systems, and others at higher risk for severe illness would likely fare even worse if exposed to this virus.
Why It Matters
This research is a warning signal. Public health efforts cannot rely on seasonal flu vaccines to prevent serious illness from H5N1 if it starts spreading more widely among humans. With limited global supplies of vaccines designed specifically for H5N1, the study reinforces the need for faster development and broader access to targeted vaccines.
The authors call for:
- Accelerated development of H5N1-specific vaccines
- Improved surveillance in animals and humans
- Focused protection for high-risk groups
As H5N1 continues to spread in animals and cause sporadic human infections, this study underscores the urgent need to prepare for the possibility of broader transmission. Seasonal flu vaccines remain an important tool—but they won’t protect against this virus. Moving forward, pandemic preparedness will depend on the rapid development of H5-specific vaccines, improved detection systems, and targeted strategies to protect those most at risk.
Stevenson-Leggett P, Adams L, Greenwood D, et al. Investigation of Influenza A(H5N1) Virus Neutralization by Quadrivalent Seasonal Vaccines, United Kingdom, 2021–2024. Emerging Infectious Diseases. 2025;31(6):1202–1206. doi:10.3201/eid3106.241796