Researchers suspect humans gave COVID to deer. And it once spilled back into a person Two studies still out for review show the latest evidence for COVID spillover from humans into white-tailed deer. The strains in the animals had been circulating for months, picking up mutations.

Researchers suspect humans gave COVID to deer. And it once spilled back into a person

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Two new studies provide further evidence that COVID spreads from humans to animals but with a couple of important twists. NPR's Ari Daniel reports.

ARI DANIEL, BYLINE: Late last year, researchers swabbed the noses of 93 dead deer from across Pennsylvania. Nearly a fifth tested positive for COVID.

ANDREW MARKS: These aren't deer that died from COVID. These are deer that died with COVID.

DANIEL: Andrew Marks is a Ph.D. student at University of Pennsylvania's Perelman School of Medicine. He suspects the deer originally got COVID from humans but doesn't know how exactly.

MARKS: Everything is basically speculation at the moment.

DANIEL: But the question Marks had was, what did these viruses look like? And how had they evolved? Marks is lead author on a study still out for review that found that some of these samples had variants resembling the ones circulating in people at the time. But others - and this is twist No. 1 - resembled alpha, the first variant of concern, months after it was no longer dominant in people.

MARKS: Which is strange.

DANIEL: And problematic because after alpha had fallen by the wayside in people, it lingered in deer, meaning we can't quite forget about variants no longer affecting us.

MARKS: They're still present in the environment around us.

DANIEL: And then Marks and his collaborators noticed something else. Some of the viral samples had picked up a few dozen mutations. He likens it to subbing out a handful of individuals in a 30,000-person parade.

MARKS: Some of these people play really important roles in the parade, and by changing those people, that can change the way that the parade moves, the way that the parade sounds. And we can think of that as an analogy of the virus where it changes the way that the virus might spread.

DANIEL: So that's twist No. 2 - the sheer quantity of mutations. And the more mutations, the greater the possibility of a more transmissible or virulent variant. Another study, also pending review, considered nearly 300 deer across southwestern and eastern Ontario in Canada. Some 6% had a COVID strain that was really retro.

SAMIRA MUBAREKA: This one is from an even older lineage. So it probably spilled over into deer at some point in 2020, maybe early 2021.

DANIEL: Samira Mubareka is a virologist at the Sunnybrook Research Institute and the University of Toronto. She says that spillover predates any of the variants of concern - delta, omicron, even alpha, which means it was moving around in the deer for a relatively long time.

MUBAREKA: So the number of mutations that have accumulated are fairly substantial.

DANIEL: But this constellation of mutations didn't stay confined to the deer. And here's twist No. 3. It turned up in a person.

MUBAREKA: The most likely scenario is deer-to-human transmission.

DANIEL: Now, the vaccines seem to hold their own against this particular lineage of the virus. But here's the concern - that after finding its way into a different species, a different reservoir where it gathers additional mutations, a new variant or strain of COVID can slosh back into people and create more trouble. Barbara Han wasn't involved with the deer research. She's a disease ecologist at the Cary Institute of Ecosystem Studies, and she worries that once we see a spillover, we're in it for the long haul.

BARBARA HAN: I can't think of a single zoonotic disease that has established in an animal reservoir in the wild that we have been successful at eradicating. So the fact that we now have a semipermanent reservoir species - and not just in white-tailed deer but in mink and in deer mice and in all these other species around the world - suggests to me that we now have to keep track of how it's evolving in these species and constantly update our calculations of what the risk is for humans.

DANIEL: Ari Daniel, NPR News.

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