The alpha and gamma variants of the coronavirus continued to circulate and evolve in whitetail deer, even after the variants stopped spreading widely among people, a study suggests.

Whether the variants are still circulating in deer remains unknown, said Dr. Diego Diel, a virus expert at Cornell University and an author of the study, which was published in January in Proceedings of the National Academy of Sciences.

But the findings, which are based on samples collected through December 2021, provide more evidence that deer could be a reservoir of the virus and a potential source of future variants, which could spill back into human populations.

"It is a very large wildlife population in North America that has constant and very intense contact with humans," Diel said.

Previous studies of deer have suggested humans have repeatedly introduced the coronavirus into whitetail populations in the United States and Canada and that deer can spread the virus to one another. Scientists are not sure how people are passing the virus to deer, but they have speculated that it might happen when people feed deer or deer encounter human trash or waste.

The scale of the risk that infected deer pose to humans remains unclear. Scientists have documented one case that most likely resulted from deer-to-human transmission in Ontario, and they note that hunters and others who have regular contact with the animals could catch the virus from them.

For the new study, Diel and his colleagues analyzed about 5,500 tissue samples collected from deer killed by hunters in New York state from September through December in 2020 and 2021.

During the 2020 season, 0.6% of the samples tested positive for the virus, a figure that rose to 21% during the 2021 season.

Genetic sequencing revealed that three variants of concern — alpha, gamma and delta — were all present in deer during the 2021 season. At the time, delta was still prevalent among New York's human residents. But alpha and gamma had practically vanished.

The CDC recommends that deer hunters take precautions to reduce the risk of infection, including wearing masks while handling game and washing hands thoroughly.