Newswise — (MEMPHIS, Tenn. – September 26, 2025) Proteins and genetic material from H5N1 influenza viruses have been found in pasteurized milk in the United States, but a study from St. Jude Children’s Research Hospital shows those inactive viral pieces represent little to no health risk. As the H5N1 outbreak in dairy cows continues in the U.S., commercial milk remains contaminated by viral pieces. However, the pasteurization process kills the virus. The researchers tested whether the resulting noninfectious viral materials could teach the immune system that these viral components were harmless, thus increasing susceptibility to influenza infections. They found that H5N1-contaminated milk that was pasteurized did not significantly affect the immune system in laboratory models. Unpasteurized milk remains a health concern. The findings were published today in Science Advances.

“We found that consuming pasteurized milk multiple times, even if it has inactivated H5N1 virus, poses minimal health risks,” said corresponding author Stacey Schultz-CherryPhD, St. Jude Department of Host-Microbe Interactions. “We observed no benefit or detriment to subsequent influenza infection.”

The scientists were originally concerned that regularly drinking inactivated viral components in milk may teach the immune system that these molecules were safe. This loss of immunity against materials in food is called oral tolerance, which prevents humans from developing unwanted immune reactions against what they consume. However, if people gained oral tolerance to flu’s viral proteins, it could lead to greater susceptibility to later influenza infections if the body no longer recognizes the viral components as part of an invader.

To learn if drinking contaminated pasteurized milk created oral tolerance of influenza viruses, the researchers took uncontaminated milk or milk contaminated with H5N1, which were both then pasteurized, and gave them to mice. The mice were given the milk over five days, mimicking how a person would consume it over time. Weeks later, the scientists challenged the mice with an H5N1 infection. There was no difference in how the infection proceeded between the two groups.

“We found an influenza infection after repeated exposure to H5N1 virus in pasteurized milk was normal, with no adverse events,” said first author Pamela BriglebPhD, St. Jude Department of Host-Microbe Interactions. “We saw no evidence of it worsening the disease.”

Pasteurization and pre-existing immunity prevent pathogenic infections

While pasteurized milk appeared to play no role in altering influenza immunity, unpasteurized milk represented a health threat. Mice exposed to unpasteurized H5N1-infected milk succumbed to their disease in a matter of days, which other groups have also documented.

“We did see that if infected milk wasn’t fully pasteurized, that was still very pathogenic in our model,” Brigleb said. “That highlights the importance of pasteurization, especially in potentially contaminated milk.”

Until this point, the researchers had only used mice that never had the virus. To better account for pre-existing flu immunity from previous infections and vaccinations, as occurs in the human population, the scientists infected mice with a nonlethal dose of H1N1 virus, leaving another group uninfected. They then took regular milk or milk that was H5N1-infected, then pasteurized both, and gave it to the mice for several days. Weeks later, they challenged those mice with the H5N1 virus, finding that prior H1N1 infection was 100% protective against mortality regardless of what type of milk the mice drank. All mice that did not have the pre-existing H1N1 immunity succumbed to their disease.

“Most of the population has had an infection or a vaccination at some point in their life, so we wanted to reflect that in our experiments,” Brigleb said. “We found that whether or not mice received viral particles in pasteurized milk or not, prior immunity still fully protected them from the H5N1 challenge.”

The study confirms that current food safety methods and vaccination practices are likely protecting human health from the H5N1 virus in milk, though the need for continued vigilance remains.

“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. We must continue to watch this virus and mitigate its risk of spilling over into the human population.”

Authors and funding

The study’s other authors are Ericka Kirkpatrick Roubidoux, Lauren Lazure, Brandi Livingston, Victoria Meliopoulos, Bridgett Sharp, Tyler Ripperger, Shelby Patrick, Dorothea Morris and Shaoyuan Tan, all of St. Jude.

The study was supported by grants from the National Institute of Allergy and Infectious Diseases (Contract No. 75N93021C00016, the National Institutes of Health (T32AI106700-08 and F32AI183804) and ALSAC, the fundraising and awareness organization of St. Jude.

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St. Jude Children’s Research Hospital

St. Jude Children’s Research Hospital is leading the way the world understands, treats, and cures childhood catastrophic diseases. From cancer to life-threatening blood disorders, neurological conditions, and infectious diseases, St. Jude is dedicated to advancing cures and means of prevention through groundbreaking research and compassionate care. Through global collaborations and innovative science, St. Jude is working to ensure that every child, everywhere, has the best chance at a healthy future.  To learn more, visit stjude.org, read St. Jude Progress, a digital magazine, and follow St. Jude on social media at @stjuderesearch.