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The same team of French, Russian and German researchers previously isolated ancient viruses from the permafrost and published their findings in 2015. This concentration of fresh viruses suggests that such pathogens are probably more common in the tundra than previously believed, they suggest in a preprint study they published last month on the BioRxiv website, a portal where many scientists circulate their research before it is accepted in a scientific journal.

“Every time we look, we will find a virus,” said Jean-Michel Claverie, a co-author of the study and an emeritus professor of virology at Aix-Marseille Université in France, in a phone interview. “It’s a done deal. We know that every time we’re going to look for viruses, infectious viruses in permafrost, we are going to find some.”

Although the ones they studied were infectious only to amoebas, the researchers said that there was a risk that other viruses trapped in the permafrost for millennia could spread to humans and other animals.

Virologists who were not involved in the research said the specter of future pandemics being unleashed from the Siberian steppe ranks low on the list of current public health threats. Most new — or ancient — viruses are not dangerous, and the ones that survive the deep freeze for thousands of years tend not to be in the category of coronaviruses and other highly infectious viruses that lead to pandemics, they said.

The European team’s findings have not yet been peer-reviewed. But independent virologists said that their findings seemed plausible, and relied on the same techniques that have produced other, vetted results.

The risks from viruses pent up in the Arctic are worth monitoring, several scientists said. Smallpox, for example, has a genetic structure that can hold up under long-term freezing, and if people stumble upon the defrosted corpses of smallpox victims, there is a chance they could be infected anew. Other categories of virus — such as the coronaviruses that cause covid-19 — are more fragile and less likely to survive the deep freeze.

“In nature we have a big natural freezer, which is the Siberian permafrost,” said Paulo Verardi, a virologist who is the head of the Department of Pathobiology and Veterinary Science at the University of Connecticut. “And that can be a little bit concerning,” especially if pathogens are frozen inside animals or people, he said.

But, he said, “if you do the risk assessment, this is very low,” he added. “We have many more things to worry about right now.”

For the most recent research, the European team took samples from several sites in Siberia over a series of years starting in 2015. The viruses they found — of an unusually large type that infects amoebas — were last active thousands, and in some cases, tens of thousands of years ago. Some of the samples were in soil or rivers, although one of the amoeba-targeting viruses was found in the frozen intestinal remains of a Siberian wolf from at least 27,000 years ago, the team said.

The researchers used amoebas as “virus bait,” they said, because they thought it would be a good way to search for viruses without propagating ones that could spread to animals or humans. But they said that didn’t mean these viruses didn’t exist in the frozen tundra.

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Siberia is warming at one of the fastest rates on Earth, about four times the global average. For many recent summers it has been plagued by wildfires and temperatures reaching 100 degrees Fahrenheit. And its permafrost — soil that is so thoroughly cold that it remains frozen even through the summer — is rapidly thawing. That means that organisms that have been locked away for thousands of years are now being exposed, as longer periods of defrosting at the soil surface enables objects that had been trapped below to rise upward.

Researchers say the chance of humans stumbling upon the carcasses of humans or animals is increasing, especially in Russia, whose far-north reaches are more densely settled than Arctic regions in other countries. The team gathered some of their samples in Yakutsk, a regional capital and one of Russia’s fastest-growing cities due to a mining boom.

The warming permafrost has been blamed for outbreaks of infectious disease before. A 2016 outbreak of anthrax hit a remote Siberian village and was linked to a 75-year-old reindeer carcass that had emerged from the frozen ground. But anthrax, which is not a virus, isn’t unique to Siberia and is unlikely to cause widespread pandemics.

Many virologists say they are more worried by viruses that are currently circulating among humans than the risk of unusual ones from the permafrost.

New microbes emerge or reemerge all the time, Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, told The Washington Post in 2015, when the permafrost researchers’ first findings came out.

“This is a fact of our planet and our existence,” he said. “The finding of new viruses in permafrost is not much different from all of this. Its relevance will be dependent on a sequence of unlikely events: The permafrost virus must be able to infect humans, it must then [cause disease], and it must be able to spread efficiently from human to human. This can happen, but it is very unlikely.”

More problematic, many virologists say, are modern-day viruses that infect people and lead to diseases that are sometimes hard to control, such as Ebola, cholera, Dengue and even the ordinary flu. Viruses that cause disease in humans are unlikely to survive the repeated defrosting and freezing cycle that happens at the surface level of the permafrost. And the spread in mosquitoes and ticks that has been linked to global warming is more likely to infect humans with pathogens, some experts say.

An extinct virus “seems like a low risk compared to the large numbers of viruses that are circulating among vertebrates around the world, and that have proven to be real threats in the past, and where similar events could happen in the future, as we still lack a framework for recognizing those ahead of time,” said Colin Parrish, a virologist at Cornell University who is also the president of the American Society for Virology.

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Michael Birnbaum is a climate reporter for The Washington Post. He previously served more than a decade in Europe as the newspaper’s bureau chief in Brussels, Moscow and Berlin, reporting from more than 40 countries. He joined The Post in 2008.

Ellen Francis is a reporter covering breaking news for The Washington Post in London.

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