Several apes at the San Diego Zoo were given an experimental Covid-19 vaccine for animals after an outbreak among gorillas there.
Four orangutans and five bonobos each had two doses of a jab made by Zoetis, a veterinary pharmaceutical firm.
Eight gorillas at the zoo became the first great apes in the world to test positive for Covid-19 in January. They are now recovering.
Conservationists are concerned about the threat of Covid-19 to great apes. They are particularly worried about the danger to gorillas, which have populations that are listed as critically endangered.
Karen, the first orangutan in the world to have open-heart surgery in 1994, was among those to get a jab.
The apes had not suffered any adverse reactions and would soon be tested for antibodies to determine if the shots were a success.
Zoetis started developing a Covid-19 vaccine for cats and dogs in February last year after a dog tested positive for the virus in Hong Kong.