Marine Mammal Center tests HPAI vaccine

Cow vaccine could help protect rare Hawaiian monk seal

The Marine Mammal Center is testing Highly Pathogenic Avian Influenza vaccine on elephant seals at its Sausalito hospital. If it is effective in producing antibodies, TMMC plans to vaccinate Hawaiian monk seals. TMMC’s Ke Kai Ola monk seal hospital is on the island of Hawaii.

Hawaiian monk seals are classified as Endangered, with only 1,600 seals surviving. They are “one of the rarest seal species in the world, and conservation efforts are critical to their survival,” according to the TMMC website.

HPAI infects birds and mammals

HPAI, caused by the H5N1 virus, is highly contagious and deadly to several species. It has now infected species from wild birds to domesticated birds and even to mammals. That species-to-species transmission makes a threat to global biodiversity.

Monk seals may be vulnerable to HPAI, as may the Central Coast’s herd of Northern Elephant Seals. In 2023, HPAI wiped out Southern Elephant Seals in Argentina, killing more than 96 percent of the pups, 17,500, born there, and an undetermined number of adult seals. Scientists studying the colony say it may take 100 years for the colony to recover to the numbers it had in 2022, 18,000. 

Elephant seals try the vaccine

Veterinarians advised testing the vaccine on elephant seals first, to avoid risk to the already precarious status of monk seals. National Oceanic and Atmospheric Administration officials, which have jurisdiction over National Marine Sanctuaries, also consulted on the trial.

“Northern elephant seal research has taken tremendous steps forward over the past decade within our shared network of west coast research partners,” says Dr. Sophie Whoriskey, Associate Director, Hawai’i Conservation Medicine at The Marine Mammal Center. “This vaccine trial on six in-care elephant seal pups at our Sausalito based hospital is especially significant given the great risk that Avian Influenza actively poses for marine mammals.”

The Marine Mammal Center’s Dr. Dane Whitaker (left), Associate Veterinarian, and Sarah Pattison (right), Director of Hospital Operations, collect a blood sample from a northern elephant seal patient during an Avian Influenza vaccine trial at the Center’s Sausalito, California, based hospital on July 14, 2025. Photo by Bill Hunnewell © The Marine Mammal Center

The six young elephant seals who were in the vaccine trial had been rescued and were already at the hospital for treatment. The vaccine is one that was reformulated by Zoetis, a veterinary pharmaceutical company, from the HPAI poultry vaccine for use on cows. HPAI in California dairy herds has caused milk production to decline more than 10 percent, and caused other costly herd problems. HPAI has been found in poultry flocks and dairy herds in other states and countries.  

HPAI has also affected egg production across the country. The only strategy for controlling HPAI in poultry flocks is to depopulate, kill, all the birds in the infected flock. The reduced production of eggs and milk has had uneven effectson consumer prices. 

Vaccinated and Placebo groups

The vaccine trial started in July, when three of the seals got the vaccine, and three got a placebo. Some briefly developed hives, including one who got the placebo, but the hives lasted only a few hours. One of the seals in the placebo group died. That seal’s death is under investigation. The seals were already at risk, hospitalized for other reasons.

All else proceeded well, with no other symptoms in either group. In late August, the researchers collected blood samples from the five seals, who had all recovered from the problems that caused them to strand and be treated at the hospital.

Healthy and able to survive in their wild home, they were released to resume their elephant seal lives.

Veterinary experts at The Marine Mammal Center’s hospital and visitor center in Sausalito, California, carefully place a post-release tracking tag on the head of a northern elephant seal pup as part of an Avian Influenza vaccine trial on August 27, 2025. Photo by Bill Hunnewell © The Marine Mammal Center

The blood samples they left behind will be evaluated for HPAI antibodies. With those results in hand, the TMMC team will decide whether vaccinating wild monk seals is worthwhile.

“These individual elephant seal patients are providing valuable information to inform any future vaccination efforts to a related species, Hawaiian monk seals, that are endangered and at heightened risk due to their current population size,” Dr. Whoriskey said. “This initial pilot study has showed us encouraging signs that this vaccine is safe and we are in the early stages of measuring the antibody response produced to determine whether it is also effective. As more information is gathered post-release on these seals, consultation and discussions with study partners will be key in finalizing any future vaccination plans.”

If they decide to vaccinate the monk seals, the veterinarian will use a pole syringe to inject seals on the beach, to keep distance between the veterinarian and the seal.

“We may decide to go forward even if we’re not seeing a very strong antibody response,” said Dr. Whoriskey, noting that the seals did not have any pre-existing immunity to the virus. “Something is probably better than nothing in this case.”

Immunizing wildlife

Vaccinating wildlife sounds impossible, but Ventana Wildlife Society has done it for the Central Coast condor flock. The research team captures every condor at least once a year, to check for lead poisoning, so they have the birds in hand to vaccinate.

Andrea Goodnight, DVM, came to Big Sur to vaccinate condors against HPAI. Phoo credit Ventana Widlife Society. 

Ninety-eight condors of the Central Coast flock, 89 percent, have received at least one dose of the HPAI vaccine, and 72 of those 98 have received the second, booster, shot, and are fully vaccinated. 

In Arizona in 2023, an HPAI outbreak affected 25 California condors, killing 21 of them. California condors are also classified as Endangered.

The main threat to condor survival as a species is lead poisoning from consuming lead ammunition in carcasses shot by hunters. HPAI is another threat to the condor species’ tentative recovery from near-extinction in the 1980s.

California condor. Photo credit Tim Huntington