Thursday, zoo officials at the San Diego Zoo said that nine great apes, including four orangutans and five bonobos, made veterinary hisotry as the world's first non-humans known to be vaccinated against the coronavirus.

One of the vaccine recipients was a 28-year-old female Sumatran orangutan called Karen who had previously made headlines when she became the first ape to undergo open-heart surgery almost 27 years ago in 1994.

All of the apes received both jabs of an experimental COVID-19 vaccine made originally for dogs and cats. 

The apes are exhibiting no adverse reactions, said zoo spokeswoman Darla Davis to Reuters.

The apes were vaccinated following concerns from zoo officials about their wellbeing after eight gorillas at the San Diego Zoo Safari Park were infected with the coronavirus in January, therefore accounting for the first known transmission of COVID-19 to apes.

One of the aforementioned gorillas, 48-year-old Winston, suffered symptoms as a result of his infection, such as pneumonia and heart disease. However, he is on his way to a full recovery, said Davis.

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

Notably, the infected gorillas were not vaccinated because veterinarians concluded that their immune systems had already developed antibodies to the coronavirus.

These gorillas were thought to have caught the infection from an asymptomatic staff member.

The apes selected to receive vaccinations were among the great apes at the zoo considered most vulnerable to catching the coronavirus and among the easiest to vaccinate. 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, which was developed by the veterinary pharmaceutical company Zoetis, was not tested on apes.

However, cross-species use of vaccines is not uncommon, and apes at the zoo get human flu and measles vaccines, according to the chief conservation and wildlife officer at the San Diego Wildlife Alliance, Nadine Lamberski.