Gorillas, otters, aye-ayes, and more will soon be joining the ranks of those vaccinated against COVID-19.

The Philadelphia Zoo is gearing up to vaccinate its highest-risk animals with an experimental vaccine developed by Zoetis, a former subsidiary of Pfizer that develops drugs for animals. While animals are not a major concern for spreading the virus to humans, according to the Centers for Disease Control and Prevention, they may still get infected. Cases have been reported in some big cats and gorillas at zoos, household pets, and farmed minks, motivating zoos nationwide to help their animals build up immune defenses.

“We were concerned that animals would be susceptible and we started taking precautions pretty early on,” said Keith Hinshaw, director of animal health at the Philadelphia Zoo, which is awaiting state approval before it proceeds. “This vaccine is really what we were looking for all along.”

The zoo’s precautions included face shields and gloves for zookeepers and minimizing the time they spent within six feet of the animals — which wasn’t always easy for animals used to interacting with their human companions. “When the state was in lockdown, I would walk through the zoo and the animals would be very excited to see one person,” Hinshaw said.

But there was good reason to be careful. Gorillas at the San Diego Zoo Safari Park not only got infected with COVID-19, but also developed unpleasant symptoms like coughing and congestion, which alarmed Hinshaw. When he heard about an experimental vaccine from Zoetis that the San Diego Zoo had administered to their great apes, he immediately reached out to the New Jersey-based company to see how the Philadelphia Zoo could get its own supply.

Zoetis originally thought there might be demand for a dog or cat COVID-19 vaccine, but switched gears to minks when outbreaks on farms prompted the U.S. Department of Agriculture to put out a call for mink COVID-19 vaccines. Now that vaccine, still under development, is being distributed for zoo animals.

“We can’t make a new vaccine for every species,” said Mahesh Kumar, senior vice president of global biologics at Zoetis. But the company thought its prototype, which seemed to be safe and effective in cats, dogs, and other species, might work for a wider menagerie.

Zoetis is donating more than 11,000 doses to more than 70 zoos and other animal organizations to be administered to everything from meerkats to orangutans. To maximize efficacy, Zoetis instructs zoos to administer two doses: the first to prime the immune system, then a second to boost the response weeks later, like human vaccines. “Even in a large animal, you’ll get some level of protection,” Kumar said.

The animal vaccine isn’t quite the same as the jabs that humans receive. Both are designed to teach the immune system to recognize the spike protein that gives the coronavirus its characteristic crownlike halo. That way, if the immune system encounters the protein again — from an infection — it knows to defend the body. But unlike the human COVID-19 vaccines that use RNA to deliver genetic instructions to make the spike protein, the Zoetis vaccine delivers pieces of the spike protein made in the lab. Whatever the species getting jabbed, it’s impossible to actually get the disease from the vaccine as it doesn’t deliver the entire functional virus.

The Zoetis vaccine also uses an adjuvant — material delivered with the spike protein to get the immune system’s attention — designed for cats and dogs that seems to work in other animals too.

Zoetis doesn’t yet know how all species will respond to the vaccine, but zoos will be encouraged to share any adverse events that occur. Some zoos have been wary, and a research study is in the works to measure effectiveness and safety, said Michele Goodman, director of veterinary services at the Elmwood Park Zoo in Montgomery County. The Elmwood Park Zoo has not decided if it will participate.

Hinshaw knows that administering an experimental vaccine might have risks, but feels more comfortable because the vaccine doesn’t contain a live virus and because it has already been safely administered by other zoos. The benefits are substantial, he said: keeping animals and zoo staff healthy, and minimizing opportunities for the virus to replicate and pick up mutations that make it more contagious, like the delta variant that now is causing cases to rise around the nation and world in unvaccinated people.

“It’s a little bit of extra assurance that we are trying to do our best caring for the animals,” Hinshaw said.

As with the human vaccine rollout, in which some groups were prioritized, some animals will have to wait their turn. Primates are at the top of the list, starting with great apes such as gorillas and orangutans, because their biological similarity to humans might make their cells an easier target for the virus. The rest of the monkeys will follow, including gibbons and mangabeys. There are also around 40 carnivores slated to be vaccinated, beginning with big cats — lions and tigers — as well as otters, bears, and more.

Many of the animals are seasoned vaccine recipients and have already been trained to cooperate with routine rabies and canine distemper shots, Hinshaw said. Tigers, for example, will walk to the front of their enclosure, turn sideways, lie down, and press their shoulder or thigh up against a mesh panel. There’s a treat on the line, of course: A keeper stands by with pieces of meat or a squirt bottle of milk as a reward.

The Philadelphia Zoo is waiting for the state to approve the vaccine’s use before it can receive and administer the doses, but Hinshaw expects this to happen within the next two months.

Pet vaccines against COVID-19 haven’t been approved by regulatory agencies yet, so don’t even try calling your veterinarian. For now, Kumar has some simple advice for worried pet owners: “The best thing you can do is vaccinate yourself.”