Reports of bird flu turning up in U.S. milk supplies have raised questions about safety at the grocery store. A new study from St. Jude Children’s Research Hospital offers reassuring news: pasteurized milk, even if it contains inactive H5N1 viral fragments, does not appear to affect flu immunity or increase infection risk.
Researchers tested the idea that drinking milk with viral remnants might train the immune system to ignore flu proteins, a process called oral tolerance. If that happened, people could become more vulnerable to influenza later. In experiments with mice, however, scientists found no such effect. Animals given pasteurized milk with inactive H5N1 were no more likely to get sick from flu than those given uncontaminated milk.
“We found that consuming pasteurized milk multiple times, even if it has inactivated H5N1 virus, poses minimal health risks,” said Stacey Schultz-Cherry, PhD, a lead researcher on the study.
The study also highlighted a critical distinction: unpasteurized milk is a different story. When researchers exposed mice to raw milk containing live H5N1 virus, the animals quickly became sick and died. This finding underscores why pasteurization, heating milk to kill pathogens, remains an essential safeguard.
Another experiment mimicked what happens in real life, where most people already have some immunity from flu shots or prior infections. Mice with pre-existing flu immunity were fully protected against a later H5N1 challenge, regardless of what kind of pasteurized milk they drank. Those without prior immunity did not survive, again showing the importance of vaccination in protecting against severe flu.
“It’s reassuring to find that these inactivated H5N1 viral components in pasteurized milk present minimal health risks and don’t alter flu immunity,” Schultz-Cherry said. “However, we also reaffirmed that consuming unpasteurized milk can expose people to this potentially dangerous infectious agent.”
The takeaway: store-bought milk remains safe to drink thanks to pasteurization, but unpasteurized milk continues to carry significant risks.
This study, published in Science Advances, was funded by the National Institute of Allergy and Infectious Diseases, the National Institutes of Health and ALSAC, the fundraising and awareness organization of St. Jude.