Four orangutans and five bonobos have been vaccinated so far, with the zoo planning to immunise another three bonobos and a gorilla soon.

These species, along with chimpanzees, are the closest cousins to humans, placing them at risk of contracting a virus that has spread rapidly from person to person.

In mid-January, the zoo's Safari Park reported that its troop of eight gorillas developed COVID-19 after exposure to a keeper who had the virus, even though the employee had no symptoms and wore protective equipment.

Nadine Lamberski of the San Diego Zoo Wildlife Alliance said that incident sparked concerns for other apes at the zoo.

"We wanted to do our best to protect them from this virus because we don't really know how it's going to impact them," she said.

Many of the zoo's veterinarians and other staffers who work with animals have now been vaccinated against the coronavirus, but Lamberski says that wasn't the case in January.

Zoo staff inoculated the apes between late January and early February with two doses of an experimental vaccine, specially formulated for animals, three weeks apart.

These were voluntary injections. In other words, the animals had to willingly sit down, keep still and let a zookeeper prick them with a needle.

The zoo's apes are used to receiving flu and measles vaccines.

Some of the younger and more energetic apes, however, have been less keen to sit through getting their shots.

Lamberski says staff will try again when the animals are more comfortable rather than risk any complications from anaesthetising them.

She adds that most of the animals haven't had any visible side effects. One or two, though, have rubbed their head or the area around where they got the injection.

Australian Associated Press