Wildlife veterinarians have begun testing bird flu vaccines in marine mammals, which have suffered enormous losses in the ongoing global outbreak. The first trial, which began in July, is tiny, enrolling just six northern elephant seals that were already being rehabilitated at the Marine Mammal Center in Sausalito, Calif.
But if the results are promising, the researchers hope to quickly begin vaccinating wild Hawaiian monk seals, an endangered species that they fear could be wiped out by the virus.
Just 1,600 of the seals remain in the wild, living primarily around a remote chain of Hawaiian islands. Over the coming weeks, scores of migrating birds will arrive in the Aloha State, potentially bringing the virus with them.
“There is some urgency,” said Dr. Sophie Whoriskey, the associate director of Hawai’i Conservation Medicine at the Marine Mammal Center, which is leading the trial in consultation with the National Oceanic and Atmospheric Administration. “This virus is on the move.”
A decision about whether to vaccinate monk seals will be made in consultation with the agency, Dr. Whoriskey said.
Rachel Hager, a spokeswoman for NOAA, referred questions about the trial to the Marine Mammal Center. “We will take the center’s results into consideration as we work to ensure the survival of our nation’s at-risk marine species,” she said in an emailed statement.
Over the last few years, a new version of the bird flu virus known as H5N1 has spread around the world, decimating wild birds and farmed poultry, jumping into dairy cows and infecting dozens of U.S. farmworkers.
It’s also taken a toll on marine mammals. In late 2023, the virus tore through Valdés Peninsula in Argentina, killing an estimated 17,400 southern elephant seal pups.
It was an unfathomable loss, of a scale that the tiny Hawaiian monk seal population,would be unlikely to survive.
The monk seals’ remote location bought them some time, but after the virus finally arrived in Hawaii in late 2024, experts began discussing vaccination.
Vaccination is not a practical strategy for protecting all the wild animals that are vulnerable to bird flu. But in select circumstances, it can be helpful. In 2023, federal officials began administering bird flu vaccines to California condors, another endangered species at grave risk from the virus.
Monk seals are another “rare instance” of a wild population that is sufficiently small and closely monitored for vaccination to be feasible, and “at risk enough” that the intervention might make a difference, said Dr. Dominic Travis, a wildlife veterinary epidemiologist and the chief programs officer at the Marine Mammal Center.
There is no bird flu vaccine specifically for marine mammals. But Zoetis, a veterinary pharmaceutical company, donated some doses of its bird flu vaccine, which it had recently reformulated for cows.
The researchers didn’t want to jump straight to giving a new vaccine to an endangered species. So they designed a small trial for northern elephant seals, which were also vulnerable to the virus but far more numerous than Hawaiian monk seals.
In July, the Marine Mammal Center rounded up six patients and began administering shots. Three of the seals got the first dose of the two-dose vaccine; the others received a placebo. A second round of shots was administered three weeks later.
It was not entirely smooth sailing. Three seals developed some hives after their shots, but it was not clear whether the vaccine was responsible, said Dr. Cara Field, the center’s director of conservation medicine. One of the affected seals had received the placebo, she noted, and the hives could have been caused by the sedation drug that the seals received or the wasps that were circling that morning. “They were back to normal within a few hours,” Dr. Field said of the seals.
More seriously, in early August, one of the seals in the trial suddenly died. But the animal was in the placebo group, so it had never received the vaccine. The team is still investigating the cause of death.
The other seals remained healthy and robust, and the researchers said that they saw no signs of serious safety problems. “There hasn’t been something that made major alarm bells go off,” Dr. Field said.
In late August, the researchers collected blood samples from the five remaining seals and then released them back into the wild. In the coming weeks, they will analyze the samples to determine whether the seals developed antibodies to the virus and meet with their partners to discuss whether to move on to monk seals.
Administering the shots to seals in the wild will require some finesse. “You basically sneak up on them using a syringe that’s attached to a longer pole,” Dr. Field said. “They can be quite large, and they don’t appreciate being disrupted with a needle in the bum.”
But as long as the shots seem safe, the bar for efficacy may be low. “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.”
Emily Anthes is a science reporter, writing primarily about animal health and science. She also covered the coronavirus pandemic.
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