Some vaccines are making progress in protecting vulnerable species

Southern elephant seal pups were among the first to die when a deadly strain of avian influenza arrived in the Crozet Islands in 2024. But as the virus spread across the sub-Antarctic archipelago, a handful of penguin chicks had a potential advantage: They’d gotten a flu shot.

Disease ecologist Thierry Boulinier and his colleagues were poised to wrap up a small vaccine trial in young king penguins on the archipelago’s Possession Island when the virus arrived in October. A slew of H5N1 outbreaks that swept the globe in 2022, killing birds and mammals including bald eagles and red foxes, was a “clear motivating factor” in starting the trial, says Boulinier, of the Centre d’Ecologie Fonctionnelle et Evolutive in Montpellier, France. Vaccinating vulnerable wild birds such as the Northern rockhopper penguin or the Amsterdam albatross might safeguard them from the deadly virus.

The project is one of many that seek to leverage vaccines to protect endangered species from devastating diseases. In September, Australian officials approved a vaccine against chlamydia for use in wild koalas. Shots for a deadly herpesvirus that causes hemorrhagic disease in elephants are showing promising results in a few zoos. And researchers are vaccinating bats in the western United States against white-nose syndrome, a fungal disease that has killed millions of bats nationwide.

Vaccines can be an essential conservation tool, says Tonie Rocke, a wildlife biologist with the United States Geological Survey’s National Wildlife Health Center in Madison, Wisc. As habitats shrink, animals interact with each other more frequently, offering new opportunities to spread pathogens. “Their environment has changed, as has ours,” Rocke says, “and there are diseases moving all over the world at a pace that wouldn’t have happened in the past.”

Vaccines aren’t a silver bullet to stop outbreaks, in part because they can be expensive to develop and deploy. Still, researchers are making strides toward using the technology to protect wildlife from lethal infectious diseases. 

Early trials hint at protection as bird flu spreads in the Antarctic

H5N1 first arrived in the Antarctic region in October 2023, on Bird Island off South America’s Atlantic Coast. A year later, the virus popped up roughly 5,800 kilometers west on Possession Island in the southern Indian Ocean, where Boulinier’s team was testing its vaccine.

Dozens of southern elephant seal pups and adults across the rainy and windy archipelago died, as well as brown skuas, snowy albatrosses and gentoo penguins. The virus also killed hundreds of king penguins, the team reported in September in Nature Communications. While that’s a small fraction of the tens of thousands of king penguins that live in three colonies across the island, Boulinier says, “we cannot tell how many may die in the future if the virus reemerges.”

In February 2024, the team vaccinated 30 king penguin chicks and followed up with a booster dose a month later. The results are promising: The immunized penguins mounted an immune response without any dangerous side effects, the researchers reported in a paper posted in September to bioRxiv.org and to appear in Nature Communications.

Whether the shot protects the seabirds from disease remains unclear, as none of the vaccinated chicks got infected during the outbreak. What’s more, the need for two doses makes it “less than ideal” to vaccinate lots of animals at once, Boulinier says. But the team plans to test single doses and is kickstarting a new trial in adult king penguins to find out how long immune protection might stick around. 

With vaccination, king penguins join a short list of creatures that researchers have sought to protect from bird flu. Critically endangered California condors and New Zealand’s kākāpōs, the only flightless parrot, are among the avian species that have mounted immune responses against the virus in small vaccine trials.

Bird flu shots have also shown early promise in marine mammals, says Dominic Travis, a veterinary epidemiologist with the Marine Mammal Center in Sausalito, Calif. He and his colleagues have vaccinated northern elephant seals and endangered Hawaiian monk seals, with the animals mounting an immune response that the team believes is protective. So far, Travis says, “it’s all good news.”

A long-awaited shot may reduce a major threat to koala survival

Last year, Australia’s veterinary medicine regulator approved a vaccine to protect endangered koalas from chlamydia, a milestone a decade in the making. 

Chlamydia pecorum, a bacterial infection that can cause blindness and infertility, is just one of the many threats koalas face, says molecular biologist Nina Pollak of the University of the Sunshine Coast in Queensland, Australia. But other stressors such as habitat loss and climate change can make chlamydial infections worse. “If [koalas] get stressed, they are less resilient for disease,” Pollak says. Vaccination can give them a better chance of survival. 

Antibiotics are typically the first choice for treating bacterial infections. But the drugs can kill off the gut bacteria that koalas rely on to detoxify poisonous eucalyptus leaves, their only food source. “They became weak and that’s, unfortunately, pretty much a death sentence,” Pollak says. 

The vaccine aims to prevent the marsupials from developing severe chlamydial infections in the first place. “It’s not a magic cure,” Pollak says. For one, the vaccine is not 100 percent effective; the shot reduced koala mortality by 64 percent, researchers reported in npj vaccines in August. For another, it can be hard to reach populations that need the vaccine most.

Wild koalas admitted to hospitals and sanctuaries for treatment can receive the shot upon arrival, but finding koalas in their natural habitat is not easy. “They live up trees, and it’s terrain that is not easily accessible,” Pollak says. Finding them would take many people, traps, detection dogs and possibly drones, which can be costly. The team is also seeking funding to make and distribute the vaccine. 

While the first doses could become available this year, Pollak says, “there might not be that many doses. Not everyone will likely get it, but we will try to fairly distribute it.” 

Young elephants could one day be shielded from a deadly virus

In February 2024, two Asian elephants at the Cincinnati Zoo contracted a deadly virus. Both survived.

Elephant endotheliotropic herpesvirus, or EEHV, kills 60 to 85 percent of animals that develop a grisly viral hemorrhagic disease, making it the leading cause of death in young Asian elephants in North America and Europe. But a few months before the Cincinnati Zoo’s elephants got infected, they received a new vaccine. 

Both elephants had mild infections and neither required treatment, the zoo reported in July. “These cases mark the first documented instances of natural exposure following vaccination, suggesting that the vaccine can prevent severe disease.” 

Various forms of EEHV naturally infect nearly all Asian and African elephants, says Lauren Farris, an immunologist at Baylor College of Medicine in Houston. “It’s not really if they’re going to get it or what chance they might catch it. They will have it eventually.” Not all elephants fall severely ill, but infections can be particularly risky for young Asian elephants between 2 and 8 years old. 

Calves younger than 2 have antibodies from their mother that attack the virus, says virologist Paul Ling, whose lab at Baylor developed the vaccine. But that protection eventually fades away. It’s possible that without their mother’s antibodies, EEHV causes a “runaway infection” that the young elephants’ immune systems scramble, and fail, to control. A vaccine could help their bodies prepare and make infections less deadly. 

The long-term goal is to protect wild, free-roaming elephants, Ling says. It’s unclear whether the hemorrhagic disease is as lethal in the wild as it is in captivity, though some wild elephants have died from it. “This [vaccine] is a part of the toolbox that we’re going to need in order to help preserve this species and keep it around.” 

But EEHV vaccines aren’t yet ready for widespread use. Ling plans to monitor the elephants that have been vaccinated so far — some of which have not responded as well as the Cincinnati Zoo’s elephants — and get the shot to other elephants in human care. A different vaccine prompted an immune response and proved safe in the first completed trial in captive adult Asian elephants, researchers reported in October in Nature Communications. The next step is to test that vaccine in its target population: calves.

Success against a fungal disease offers hope for endangered bats

A fungal infection has northern long-eared bats facing an extinction threat. White-nose syndrome, caused by the fungus Pseudogymnoascus destructans, has killed millions of bats across North America since it was first detected in 2006. The fungus grows on the mammals’ skin and intermittently wakes them as they hibernate, forcing the bats to burn the energy they need to survive winter. Among the myriad tools researchers are developing to protect bats, an oral vaccine undergoing field tests shows promise. 

“The fact that we were able to actually develop a vaccine against a fungal disease is pretty remarkable,” says Rocke, the USGS wildlife biologist. “There aren’t even any approved for humans at this point.” 

Rocke and colleagues reported in 2019 that vaccinated little brown bats were less likely to develop skin lesions or die compared with control bats. Since then, the team has vaccinated more than 5,000 wild bats of various species. In 2023, they vaccinated a Wyoming colony of northern long-eared bats, a species listed as endangered because of white-nose syndrome. This year, more northern long-eared bats in South Dakota and Montana will get the vaccine, too. “We may lose that species,” Rocke says. “It’s not clear. But everybody’s making an effort to prevent that.”

These field trials, conducted mostly across the western United States, suggest that the vaccine can protect wild bats, Rocke says. While bats in the East and Midwest are developing resistance to the disease, bat populations in the West are more vulnerable. That’s because the populations, and the bats themselves, are much smaller. “The disease takes a really large energetic toll during hibernation,” Rocke says. “Those really small bats suffer the most.” 

Signs that the shot is working are giving Rocke some hope, and the team will vaccinate as many bats as they can. “Sometimes these kinds of interventions are really necessary if we’re going to conserve a species,” Rocke says. “There’s good conservation reasons for vaccinating animals, and we wouldn’t do it if it would harm them more.”