The troop of gorillas was moving slower than usual. Then came the coughing. When concerned keepers at the San Diego Zoo tested the animals’ fecal matter in early January, they learned that eight gorillas had tested positive for the California variant of the coronavirus.

An asymptomatic keeper wearing protective gear had unknowingly exposed the apes to the virus, they later found. Although the gorillas fully recovered weeks later, the infections were a reality check for the zoo’s wildlife experts — they needed to make sure their endangered animals were protected.

On Wednesday, the San Diego Zoo Wildlife Alliance, the zoo’s nonprofit parent organization, said that four orangutans and five bonobos have now received two doses of a coronavirus vaccine made specifically for animals. They’re the first nonhuman primates to be vaccinated against the virus, which has been shown to infect a number of mammals.

“This isn’t the norm. In my career, I haven’t had access to an experimental vaccine this early in the process and haven’t had such an overwhelming desire to want to use one,” Nadine Lamberski, chief conservation and wildlife health officer at the San Diego Zoo Wildlife Alliance, told National Geographic.

The Zoo partnered with Zoetis, a veterinary pharmaceutical company, which sent 27 extra doses of the experimental vaccine to the zoo last month, the San Diego Union-Tribune reported. The zoo decided to use those doses on the apes, even though they hadn’t been tested for those animals yet. The risk was worth it given the apes’ high vulnerability to the virus, the zoo said.

Pascal Gagneux, a zoologist at the University of California at San Diego, told the Union-Tribune that the zoo made the right decision.

“It makes quite a bit of sense. These animals are incredibly precious,” he said. “There’s a very finite number of great apes in captivity.”

The apes have responded well to the vaccines, the zoo said. Among the troop that received the doses, according to National Geographic, is an orangutan named Karen, who made history in 1994 as the first ape to undergo open heart surgery.

Zoetis began its research on coronavirus vaccines for animals in March 2020, when a dog in Hong Kong contracted the virus, the firm said in a statement. Its doses work similarly to the vaccine created by Novavax, which uses simulated spike proteins that trigger an immune system response. The company says its studies have shown the vaccine is safe for other animals, including dogs and cats.

Since the beginning of the pandemic, experts have warned that covid-19 could wipe out endangered species such as chimpanzees, gorillas and other great apes. There are fewer than 5,000 gorillas remaining in the wild, according to National Geographic.

Last year, the International Union for Conservation of Nature urged people to stay at least 33 feet away from great apes in the wild and at zoos to avoid transmitting the coronavirus. In a letter published in Nature in March 2020, a group of experts also asked countries with wild ape populations to reduce field research and shut down wildlife tourism.

Other animals have also contracted the virus. In April, the Bronx Zoo said four tigers and three lions tested positive for coronavirus. Around the same time, two house cats in New York also tested positive. In Pakistan, officials said last month they suspect two white tiger cubs died of covid-19.

Mink, in particular, have had a high infection rate. Scientists grew so concerned about a virus mutation in the animals in Denmark, which is one of the world’s largest fur producers, that the government made plans to euthanize every mink in the country.

Zoetis is conducting vaccine trials on mink, the company said in a news release. A spokesperson told National Geographic that many zoos have requested doses of the vaccine for their great apes. The company expects to distribute those by June. Zoetis has also shared research with zoos throughout the country and with a zoo in Prague, where a gorilla and two lions recently contracted the virus.

The San Diego Wildlife Alliance said it hopes to administer vaccines to other animals at the San Diego Zoo, including the gorilla troop at the safari park and possibly big cats at the zoo, Lamberski told National Geographic.

The vaccine has offered a glimmer of hope for wildlife experts. But the work isn’t over, Lamberski said in an interview with Insider.

“That big sigh of relief isn’t going to come until our entire community is vaccinated, until the vaccine gets to, you know, remote communities all over the world, to areas where gorillas live in the wild,” she said.