NEW YORK (NYTIMES) - Over the past year, Covid-19 vaccines have gone into billions of human arms - and into the fuzzy haunches of an ark's worth of zoo animals. Jaguars are getting the jab. Bonobos are being dosed. So are orangutans and otters, ferrets and fruit bats, and, of course, lions and tigers and bears.
Largely left behind, however, are two creatures much closer to home: domestic cats and dogs.
Pet owners have noticed.
"I get so many questions about this issue," said Dr Elizabeth Lennon, a veterinarian at the University of Pennsylvania. "Will there be a vaccine? When will there be a vaccine?"
Technically, a pet vaccine is feasible. In fact, several research teams say they have already developed promising cat or dog vaccines; the shots that zoo animals are receiving were initially designed for dogs.
But vaccinating pets is simply not a priority, experts said. Although dogs and cats can catch the virus, a growing body of evidence suggests that Fluffy and Fido play little to no role in its spread - and rarely fall ill themselves.
"A vaccine is quite unlikely, I think, for dogs and cats," said Dr Will Sander, a veterinarian at the University of Illinois Urbana-Champaign. "The risk of disease spread and illness in pets is so low that any vaccine would not be worth giving."
In February 2020, a woman in Hong Kong was diagnosed with Covid-19. Two other people in her home soon tested positive for the coronavirus, as did one unexpected member of the household: an elderly Pomeranian. The 17-year-old dog was the first pet known to catch the virus.
But not the last. A German shepherd in Hong Kong soon tested positive, too, as did cats in Hong Kong, Belgium and New York. The cases were exceedingly mild - the animals had few or no symptoms - and experts concluded that humans had spread the virus to the pets, rather than vice versa.
"To date, there hasn't been any documented cases of dogs or cats spreading the virus to people," Dr Lennon said.
But the prospect of a pet pandemic sparked interested in an animal vaccine. Zoetis, a veterinary pharmaceutical company based in New Jersey, began working on one as soon as it heard about the Hong Kong Pomeranian.
"We figured, 'Wow, this could become serious, so let's start working on a product'," said Dr Mahesh Kumar, senior vice-president at Zoetis, who leads vaccine development.
By the fall of 2020, Zoetis had four promising candidates for a vaccine, each of which elicited "robust" antibody responses in cats and dogs, the company announced. The studies, which were small, have not been published.
But as vaccine development progressed, it became increasingly apparent that the infection of pets was unlikely to pose a serious threat to animals or people.
In one study of 76 pets living with people who had the virus, 17.6 per cent of cats and 1.7 per cent of dogs also tested positive. Studies have consistently shown that cats are more susceptible to infection than dogs, perhaps for both biological and behavioural reasons. Of the infected pets, 82.4 per cent had no symptoms.
"It doesn't look like cats or dogs would ever be a reservoir for this virus," Dr Jeanette O'Quin, a veterinarian at Ohio State University, said. "We believe that if there weren't sick people around them, they would not be able to continue spreading it from animal to animal - it would not continue to exist in their population."
Together, these factors convinced experts that a vaccine for pets was not necessary.
In November 2020, the United States Department of Agriculture (USDA), which regulates veterinary medicines, said it was not accepting any applications for cat or dog vaccines because data does not "indicate such a vaccine would have value".
Keeping mink in the pink
As the pet threat was receding, another problem was coming into focus: mink. The sleek, svelte mammals, which are farmed in large numbers, turned out to be highly susceptible to the virus. And not only were they dying from it, but they were also spreading it to one another and back to humans.
"I think that the situation in mink absolutely warrants a vaccine," Dr Lennon said.
The USDA thought so, too, and in the same November notice in which the agency said it was not considering cat or dog vaccines, it declared itself open to applications for a mink vaccine.
Zoetis pivoted, deciding to repurpose one of its dog vaccines for mink ones.
Several other teams are also developing mink vaccines, and Russia has already approved a shot for all carnivores, including mink, and has reportedly started administering it to animals.
Studies in mink are ongoing, but when word got out about Zoetis' work, zoos came calling. Some of their animals - including gorillas, tigers and snow leopards - had already caught the virus, and they wanted to give the mink vaccine a whirl.
"We got a huge number of requests," Dr Kumar said.
Zoetis, which decided to supply the vaccine to zoos on an experimental basis, has now committed to donating 26,000 doses - enough to vaccinate 13,000 animals - to zoos and animal sanctuaries in 14 countries.
The development means that many zoo-dwelling cats, like lions and tigers, are getting vaccinated, while their domestic cousins are not. In part, that is because these species appear to be more susceptible to the virus; some have died after becoming infected, although the cause of death is often difficult to conclusively determine.
"The big cats seem to be getting sicker than the house cats," Dr Lennon said.
The cat vaccine calculus
Although the evidence so far suggests that the virus is not a major threat to pets, there is a lot left to learn, scientists acknowledge.
It is still not clear how frequently infected humans pass the virus to their pets, especially because officials do not recommend routine testing for companion animals, and the virus may have health effects in pets that have not yet been identified.
In a paper published this month, scientists raised the possibility that the Alpha variant, which was first identified in Britain, might cause heart inflammation in dogs and cats. The evidence is circumstantial, but the virus has been linked to the same problem in humans, and the connection is worth exploring, experts said.
"We need to do more research in this area to find out if this is a real association," Dr O'Quin said.
There may be individual pets that are at especially high risk from the virus. Dr Lennon and her colleagues recently identified an immunocompromised dog that appeared to become severely ill from the virus. Unlike most infected dogs, this one also shed high levels of the virus for more than a week.
"Of course, that's one case, but it really does illustrate that Covid-19 isn't the same in all pets, just like it isn't in all people," Dr Lennon said.
It is certainly possible that future research - or changes in the virus - could change the calculus on a pet vaccine.
If the virus turns out to be more prevalent, virulent or transmissible in dogs or cats than is currently known, that would make the case for a vaccine more compelling, scientists said. The USDA has said it may reevaluate its position if "more evidence of transmission and clinical disease" emerges in a particular species.
If that time comes, Zoetis is prepared to pick up where it left off with its pet vaccines, Dr Kumar said. He said that if the company's mink vaccine is licensed, veterinarians might be able to use it off-label in the event of an unexpected outbreak in cats or dogs.
Applied DNA Sciences, a New York-based biotech company, has also developed a promising cat vaccine "as a just in case", chief executive James Hayward said.
Like Zoetis, the company, which is working in partnership with Italian company Evvivax, is now more focused on a mink vaccine.