permafrost, viruses, and silly zombie headlines

Permafrost is “any ground that remains completely frozen – 0°C or colder – for at least two years straight”, and as you’d expect is found at high altitudes or in polar regions. It acts like a deep-freezer – scientists have found mammoths buried in permafrost that were so well preserved that at least one field researcher is reputed to have tried eating it. Researchers have learned a great deal about prehistoric high-latitude environments by studying not only the mammoths but plants and other animals encased in the frozen soil.

It’s not just plants and animals that are preserved by permafrost. In 2016 there was an outbreak of the bacterial disease anthrax in Northwest Siberia, which killed reindeer in their thousands and sickened humans. This was the first outbreak in the area in 70 years, and was put down to anthrax spores reactivating as the region’s permafrost thawed during an unusually hot summer. And in 1997 US researchers successfully isolated the virus that caused the 1918-19 influenza pandemic from the body of a woman buried in Alaskan permafrost.

So when a friend asked me, “What’s this thing going around about the climate causing a zombie virus?” my first thought was, melting permafrost, and of course I had to look into it.

First up, I do have to say several of the headlines were of the somewhat alarming variety, presumably written to gain ‘clicks’ for the article. I think that’s reprehensible, particularly at a time when the world is still living through a pandemic – are headlines like this really helpful?

But the backstory itself is really interesting. In a paper published this February, a research team led by Professor Jean-Michel Claverie describes how they had searched for the presence of deep-frozen viruses in samples from several several ancient permafrost sites in Siberia, dating back more than 48,500 years. Noting how the Arctic seems particularly badly affected by climatic warming, they comment that “the thawing of permafrost has significant microbiological consequences.”

This is partly because once liquid water (rather than ice) is available, many soil microbes can be reactivated, which in turn can lead to microbial breakdown of plant & animal material and the release of additional CO2 & methane into the air. And it’s partly because, with further permafrost melting, there’s the possibility that bacteria & viruses trapped for up to half a million years may be released from their deep-freeze stasis.

The researchers noted that while there is a lot of literature about bacteria found in the permafrost, there’s not a lot on viruses, and this might give the impression that viral survival is rare. So the rationale for their study was to probe this impression. For obvious safety reasons¹ they chose to look for viruses that survive in amoebae (single-celled creatures), and used primers for known amoeba-infecting viruses to find out what they had. In the paper, they announce that they’d identified 13 new, still-infectious viruses² from their 7 ancient permafrost sites. They went on to infer that many other viruses that could infect a range of different hosts “may also remain infectious in similar conditions,” something that they warned could pose a future threat to human health as more people move into warming Arctic regions.

However, other scientists have commented that for a frozen virus to have the ability to infect humans, you’d surely have to source it from similarly frozen humans that died from it³, in the same way as the 1918 flu virus was. EDIT: unless, of course, as my friend Grant pointed out, it happened to be an intact sarbecovirus with an appropriate mode of transmission, as they have a wider host range than some other viruses.

In any event, better surveillance would be good (& with the likelihood of further zoonotic spillovers in the future, it would be nice to think governments would put funding into that), but an alarmist approach isn’t helpful.


J-M.Alempic, A.Lartigue, A.E.Goncharov, G.Grosse, … & J-M.Claverie  Viruses 202315(2), 564;


¹ “When we use Acanthamoeba spp. cultures to investigate the presence of infectious unknown viruses in prehistorical permafrost (in particular from paleontological sites, such as RHS [46,47]), we are using its billion years of evolutionary distance with human and other mammals as the best possible protection against an accidental infection of laboratory workers or the spread of a dreadful virus once infecting Pleistocene mammals to their contemporary relatives. The biohazard associated with reviving prehistorical amoeba-infecting viruses is thus totally negligible compared to the search for “paleoviruses” directly from permafrost-preserved remains of mammoths, woolly rhinoceros, or prehistoric horses” (J-M.Alempic, A.Lartigue, A.E.Goncharov, G.Grosse, … & J-M.Claverie, 2023).

² They’ve been dubbed “zombie viruses” because they’ve been revived after so long in sub-zero conditions (although I suspect my friend had initially put a different interpretation on the name).

³ There are huge numbers of viruses all around us (& living in us), & the great majority don’t cause us harm.

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