Study: Flu Vaccines Ineffective Against H5N1 Strains in Humans

London, United Kingdom — A recent study from the Francis Crick Institute and University College London Hospitals revealed that quadrivalent seasonal flu vaccines (QIVs) do not produce protective antibodies against dangerous strains of avian influenza A(H5N1). These findings are particularly alarming given the recent H5N1 cases linked to U.S. dairy cattle.

Published in the peer-reviewed journal Emerging Infectious Diseases, the study examined blood samples from 61 adults who received the flu vaccine during the 2021–2024 seasons. While these vaccines prompted a strong immune response against seasonal flu strains, they were ineffective against the tested H5N1 strains.

The researchers highlighted that none of the participants showed sufficient antibody responses to neutralize H5N1 viruses, raising concerns about potential human infections if the virus spreads more widely. The strains in question include one from a 2023 outbreak in Cambodia and another associated with a 2024 U.S. dairy cow.

With H5N1 evolving in various animal populations, notably dairy cattle, public health officials are increasingly worried. Traces of the virus have been identified in not only cows but also in cats, rodents, and even wastewater in certain areas. Though pasteurized milk remains safe for consumption, the detection of live H5N1 in unpasteurized products heightens concerns about human exposure.

While some human infections reported have been mild, the ongoing spread of H5N1 in mammals could enhance the virus’s ability to infect humans. The urgency for effective responses has been reinforced. Current flu vaccines are inadequate to prevent severe illness should H5N1 establish itself in human populations.

The study emphasizes the heightened risk faced by vulnerable groups, including older adults and those with weakened immune systems, as they may experience worse outcomes if infected. This situation is critical enough to call for the development of targeted H5N1 vaccines.

In conclusion, public health experts join the authors in urging for better preparedness against H5N1, advocating that seasonal flu vaccines alone will not suffice. Strategies must involve rapid development and distribution of H5N1-specific vaccines and improved monitoring systems to protect those at greatest risk.