As the COVID vaccine rolls out to Americans across the country, doses of an experimental COVID shot are being administered to a number of monkeys at the San Diego Zoo Safari Park.

The milestone vaccine trial is the first time that non-humans have received the COVID shot. The first animal to receive the COVID vaccine at the zoo was an orangutan named Karen, who was also the first ape to undergo heart surgery in 1994, National Geographic reported.

The news outlet also said that three other orangutans and five bonobos have also received two doses of the shot in February. The vaccine was specifically developed for animals by the veterinary pharmaceutical company Zoetis.

San Diego Zoo Safari Park has seen a troop of eight gorillas test positive for the coronavirus, including a 49-year-old named Winston. He developed heart disease and pneumonia but is recovering after an experimental antibody treatment, Nat Geo said.

Monkeys are not the only animals that have been impacted by the virus. COVID has been confirmed in tigers, lions, mink, snow leopards, cougars, a ferret, dogs, and domestic cats, but apes have been a concern for scientists due to their close living arrangement and the possibility of the virus spreading rapidly among them.

All species of gorillas are also listed on the endangered or critically-endangered list compiled on the IUCN Red List.

While little information is known about COVID and its effect on animals, Zoetis began developing a COVID vaccine to inoculate dogs and cats in Hong Kong last February. In October, the company believed the vaccine was safe and effective for the cats and dogs in its test trials.

However, following the January outbreak of the virus among apes at the San Diego Zoo Safari Park, it was decided that the risk of using the COVID vaccine on the animals was necessary.

Nadine Lamberski, chief conservation and wildlife health officer at the San Diego Zoo Wildlife Alliance, told National Geographic that the apes had no reaction to the vaccine and are doing fine. She continued by saying that Karen and one of the bonobos are already developing antibodies, which is an indication that the COVID vaccine may be working.

"It's not like we randomly grab a vaccine and give it to a novel species," she told the news outlet. "A lot of thought and research goes into it — what's the risk of doing it and what's the risk of not doing it. Our motto is, above all, to do no harm."

Another three doses will be given to more bonobos and gorillas that did not contract the virus, Nat Geo said. Zoetis told the news outlet that additional doses of the COVID vaccine have been requested by other U.S. zoos to vaccinate their own great apes, with more doses of the drug expected to be available in June.

A gorilla troop at the San Diego Zoo Safari Park that tested positive for Covid-19
A gorilla troop at the San Diego Zoo Safari Park that tested positive for Covid-19 San Diego Zoo Global / Ken Bohn