Nine great apes at the San Diego Zoo - four orangutans and five bonobos - made veterinary history in recent weeks as the world's first non-human primates known to be vaccinated against Covid-19, zoo officials said on Thursday.

One of the recipients was a 28-year-old female Sumatran orangutan named Karen who had garnered headlines at the zoo when she became the first ape to undergo open-heart surgery in 1994.

Each of the nine animals received two doses of an experimental vaccine originally designed for dogs and cats, with the apes exhibiting no adverse reactions, and are all doing well, zoo spokeswoman Darla Davis said in an email to Reuters.

Zoo officials went ahead with the shots over concerns about the animals' wellbeing after a troop of eight gorillas at the affiliated San Diego Zoo Safari Park fell ill with Covid-19 in January, marking the first known transmission of the virus to great apes.

The eight gorillas, including a 48-year-old male ‘silverback’ named Winston who suffered from pneumonia and heart disease, have since improved and appear to be on their way to a full recovery, Davis said.

Winston was treated with a variety of medications, including a coronavirus antibody therapy for non-humans.

The gorillas were not vaccinated because veterinarians assumed their immune systems had already developed antibodies to the virus. They were thought to have caught the illness from an asymptomatic staff member.

The orangutans and bonobos selected for immunization were among the great apes at the zoo considered the most at risk of catching the virus and among the easiest to inoculate. Staff vaccinated the animals by distracting them from the needle with treats.

Zoo staff began administering the shots to some of the animals in January and continued through February, with the last few given in March, Davis said.

The vaccine developed by veterinary pharmaceutical company Zoetis was not tested on apes. But cross-species use of vaccines is not uncommon, and apes at the zoo get human flu and measles vaccines, according to Nadine Lamberski, chief conservation and wildlife officer for the San Diego Wildlife Alliance, the entity that owns the zoo and safari park.

She said the nine great apes were the first non-human primates known to have received a Covid-19 vaccine of any kind.