H5N1 continues to circulate in U.S. dairy cattle, with a new cluster of detections in Washington adding to the outbreak’s geographic reach. Last week, the Washington State Department of Agriculture confirmed 34 affected dairy herds in the state: 33 in Yakima County and one in neighboring Benton County. The first two detections were confirmed Oct. 8, marking Washington’s first known cases in dairy cattle.
The Washington detections bring renewed attention to an outbreak that has affected dairy herds in 19 states since March 2024, according to the USDA. The virus identified in the initial Washington herds was the B3.13 strain, which has circulated in U.S. dairy cattle since spring 2024.
The outbreak has also had human health consequences: on Nov. 1, the Yakima Health District reported that a county resident hospitalized with severe avian influenza after exposure at a dairy with infected cattle had died. The district also reported a third confirmed human case in the county.
As H5N1 remains a concern for dairy producers and veterinarians, researchers continue to explore additional ways to protect cattle. Vaccination is one area of investigation, with studies evaluating different platforms and, increasingly, looking beyond infection itself to what protection could mean for milk production and udder health.
A new study from the University of Pennsylvania evaluated an mRNA–lipid nanoparticle (mRNA–LNP) vaccine designed to help dairy cows fight off H5N1. The vaccine carries instructions for making the virus’s hemagglutinin (HA) protein, giving the immune system a target to recognize. The findings add to earlier research showing that mRNA vaccination can protect dairy cows against experimental H5N1 infection, with a closer look at what that protection could mean for milk production and udder health.
Researchers are exploring several approaches to H5N1 vaccination in dairy cattle. A January 2026 study from the Chinese Academy of Agricultural Sciences found that an HA-based mRNA vaccine protected lactating cows against experimental infection, with some protection still evident 19 weeks after the first dose. Meanwhile, researchers at the University of Nebraska evaluated an adenovirus-vectored vaccine designed to stimulate immune responses in both the bloodstream and respiratory tract. The vaccine protected mice against experimental challenge with different H5N1 strains, including a bovine isolate, and prompted immune responses in dairy calves, although its ability to protect cattle against infection has not yet been tested.
Vaccinated Cows Had Less Virus in Milk and Fewer Signs of Illness
To test their mRNA vaccine, Pennsylvania researchers gave four lactating Holstein cows two doses, three weeks apart. Four additional cows received saline injections as controls. Seven weeks after the first dose, all eight cows were exposed to H5N1 through direct inoculation into the mammary gland. The animals were followed for two weeks.
The difference in milk samples was striking. Vaccinated cows had peak levels of H5N1 viral RNA approximately 1,000 times lower than unvaccinated cows. In the unvaccinated group, viral RNA appeared in milk just one day after inoculation and remained detectable throughout the study. Viral RNA levels were significantly lower in vaccinated cows at days 1 to 3, day 5 and days 8 to 13 after inoculation.
The researchers also looked at whether the virus spread beyond the mammary quarters that were directly exposed. In two unvaccinated cows, they found evidence that infection had reached other quarters. Viral RNA was not found in milk from uninoculated quarters in vaccinated cows, although viral antigen was detected in tissue from one uninoculated quarter in a vaccinated animal. This suggests vaccination may help limit spread within the udder.
The benefits were also apparent in the cows’ day-to-day performance. Unvaccinated animals experienced drops in milk production beginning two days after infection, while vaccinated cows maintained production more consistently. The unvaccinated cows also showed greater declines in rumination, more noticeable changes in milk color and consistency, and higher California Mastitis Test scores, pointing to more inflammation in the mammary gland.
When researchers examined the mammary glands, they found substantially less fibrosis in vaccinated cows. Fibrosis occurs when normal tissue is replaced by connective tissue, potentially affecting how the gland functions. Whether preventing this damage helps preserve milk production over the long term remains unknown.
Taken together, the findings offer a broader picture of what H5N1 vaccination could do for dairy cows: reduce the amount of viral RNA found in milk, lessen the drop in production and limit damage to mammary tissue.
What the H5N1 Vaccine Findings Mean for Dairy Herds
All four vaccinated cows developed antibodies against H5N1 in their blood, and researchers also detected H5-reactive antibodies in milk after the second dose. They found no detectable vaccine-derived H5 mRNA in milk samples collected for up to eight days after vaccination.
The results are encouraging, but several questions remain before veterinarians can assess how vaccination might fit into H5N1 control programs:
- Field effectiveness: Would the vaccine protect cows exposed to the virus under normal farm conditions?
- Duration of protection: How long does immunity last, and could a different dosing schedule provide similar protection?
- Transmission: Does reducing viral RNA in milk translate into less spread between cows or farms?
With H5N1 continuing to circulate in dairy cattle, finding additional ways to protect herds remains a priority. Vaccination is not yet an established control tool, but these studies are helping researchers determine whether it could eventually complement biosecurity and surveillance efforts while reducing the effects of infection on cattle health and milk production.
