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Denmark set off alarm bells this week with its announcement that it is culling the nation’s entire mink herd — the largest in the world — to stop spread of the SARS-CoV-2 virus in the prized fur species because of potentially dangerous mutations.

Inter-species jumps of viruses make scientists nervous — as do suggestions of potentially significant mutations that result from those jumps. In this case, Danish authorities say they’ve found some genetic changes that might undermine the effectiveness of Covid-19 vaccines currently in development.

But is this latest twist in the Covid-19 saga reason to be deeply concerned? Several experts STAT consulted suggested the answer to that question is probably not.


“This hits all the scary buttons,” noted Carl Bergstrom, an evolutionary biologist at the University of Washington. But Bergstrom and others argued that while the virus’s penchant for infecting mink bears watching, it isn’t likely to lead to a nightmare strain that is more effective at infecting people than the current human virus.

“I don’t believe that a strain which gets adapted to mink poses a higher risk to humans,” said Francois Balloux, director of University College London’s Genetics Institute.


“We can never rule out anything, but in principle it shouldn’t. It should definitely not increase transmission. I don’t see any good reason why it should make the virus more severe,” he said.

Let’s take a look at what’s known about the Danish situation, why inter-species jumps make scientists nervous, whether the mutations are likely to affect vaccine effectiveness, and why Balloux thinks this situation is “fantastically interesting.”

What’s happening in the state of Denmark?

Denmark is the world’s largest producer of mink — by some estimates 40%.

Unfortunately, mink are susceptible to the SARS-2 virus, a fact that came to light in April when the Netherlands reported outbreaks on mink farms there. Infected humans who work in the farms transmit the virus to captive minks, which are housed in close quarters ideal for rapid transmission from mink to mink.

Occasionally, the mink infect people — a phenomenon recorded in both the Netherlands and in Denmark. In a statement, the Danish Ministry of Environment and Food said the country would cull its entire herd — estimated to be about 17 million animals — after finding mutations in the viruses from the mink that it believes would allow those viruses to evade the immune protection generated by Covid-19 vaccines.

Why do they think the mutated viruses would evade the vaccines?

Experts outside the country are not clear what that claim is based on. While there has been some information released about the mutations that have been recorded, it isn’t yet enough to support such a bold claim, said Marion Koopmans, head of virology at Erasmus Medical Center in Rotterdam, the Netherlands, where a lot of the analyses from viruses from the Dutch mink outbreaks have been conducted.

“That is a very big statement,” said Koopmans. “A single mutation, I would not expect to have that dramatic an effect.”

Outside experts haven’t had genetic sequencing data to peruse, said Emma Hodcroft, a molecular epidemiologist at the Institute of Social and Preventive Medicine in Bern, Switzerland. But Denmark uploaded 500 genetic sequences into databases open to scientists around the globe on Thursday, and is expected to add hundreds more in the days to come.

Experts will pore over those sequences looking for what the Danes saw and to try to determine what impact these mutations may have if viruses containing them infect people.

For now, however, Hodcroft agrees with Koopmans. “It’s almost never the case that it’s such a simple story of one mutation and all your vaccines stop working.”

She, frankly, is more concerned with how the announcement was handled than about the findings themselves. “It puts scientists and the public in a really difficult position when we have statements like this out there for which we have very little information or context,” Hodcroft said. “These things are essentially never black and white.”

What’s the big deal of species jumps anyway?

Species jumps always make scientists nervous. One such event, after all, is how we ended up with the Covid-19 pandemic.

Viruses that typically infect one kind of animal — let’s use bats as an example — that find their way into another species can trigger severe illness in the new species if the virus is able to transmit efficiently. Viruses can become entrenched — endemic — in the new species.

It’s thought, for instance, that the four coronaviruses — cousins of SARS-2 — that cause common colds spilled over from other species into humans at some point in the past. Flu virus spillover events — from poultry or from pigs — occur from time to time. The 2009 H1N1 pandemic was triggered when a flu virus that had been circulating in pigs started infecting people.

After years of coping with viral spillovers like Ebola outbreaks, flu pandemics plus the earlier coronavirus jumps like the 2003 SARS outbreak, people are primed to be worried about these events, Bergstrom said.

But this is a different situation, he said. It’s not a virus unknown to humans that has jumped from an animal species. In this case, a virus that has already adapted to spreading among people jumped to minks and is now occasionally jumping back.

Bergstrom thinks it’s prudent of the Danish government to cull the mink herd. But he’s not sure the changes that have occurred in the mink are likely to make the virus worse for people.

“We’re used to being scared before a pandemic when something from a distant species comes into a nearer species. And our intuitions aren’t quite right for what happens in the middle of a pandemic when something goes from us into a distant species and then comes back,” he said.

Balloux and others suggested the changes seen in the mink viruses may be a sign of the virus adapting to infect minks — which might make the viruses less effective in people over time.

Can you capture a spillover in real time?

Balloux pegs the risk the spillover poses to humans as being “really, really small.”

But he said it is exceptional to actually be able to capture in real time what happens when a spillover happens, and chart the genetic changes from the start.

Typically when such events occur, humans only recognize what’s going on when a virus has adapted to spread in people. For example, the early changes that made SARS-2 capable of transmitting from a still unknown animal species to people were never observed.

“It’s completely exceptional,” Balloux said. “We are always [too] late.”

An earlier version of this story stated that Denmark produces 28% of the world’s mink supply. In fact, estimates vary.

  • What strikes me is how this supports the idea that the original strain did not spillover from bats but was the result of gain of function research on ferrets. Minks are just cousins of ferrets that are just larger and have more expensive coats.

  • It is a despicable industry and any animal bred and or held in captivity for human consumption or vanity should be banned worldwide….no matter where in the world and for whatever reason given!!!!!!!!!

  • This news has hit major international news outlets. Reading this article leaves one still wondering what the observed mutations were (where, how many) and what possible properties may be conferred to the virus as a result of them. There needs to be more trasparency in science as govts. announce such radical measures, and finally, who needs mink fur?

  • This article is so, so wrong and not seeing the point at all. We’re talking about killing millions upon millions of living, breathing animals, not them effing infecting people.

    • They were all going to eventually die anyways, ya know because that’s why they’re farmed, to be killed for their fur. What’s your point?

  • who are the Danish experts who examined this and caused the decisions for culling and lockdown ?
    They probably have more info, maybe did some tests.
    500 sequences is a lot, indicating that they spent quite some effort on this.
    The secrecy,delay is it maybe because they want to publish a paper
    -exclusively- in a scientific journal ?
    I’m not into the paper-publishing business, but that seems to be how
    researchers get reputation and jobs and money

  • There is good thinking behind not taking any chances with a mutated virus from SARS COV 2 that we do not know about but has proven to jump back to humans and transmit between them (read the article the dutch did).
    We are dealing with one pandemic which we barely control at present and the good old “precaution principle” of public health notoriety should and has to apply. An industry like mink fur (which I am sure many people like me will be surprised to realise is going on to such large scale in several countries) does not weight anything in the balance compared to even a very slight risk to confuse, multiple et render close to impossible to control the current pandemic.

    We can study and learn from a few of those mink-rearing places, carefully quarantined, to advance our knowledge on the virus and its mutations and risk of this happening with other animal species and unfortunately destroy 99% of the minks.

    • You are talking about exterminating millions of lives. Innocent lives, that have done nothing wrong. “Unfortunate” does not even begin to describe the horrificness of this idea. You can not measure the weight of millions of lives versus anything, because at that point, nothing can be worth the consequences. Stop being an arrogant human and consider that other lives matter too.

  • “That is a very big statement,” said Koopmans. “A single mutation, I would not expect to have that dramatic an effect.” But haven’t multiple mutations been found in some strains? That’s my understanding — but I’m in the U.S., getting all of my information from translated excerpts in various news articles, so perhaps I am not getting the full picture. Now that we seem to be getting closer to a vaccine (and, we hope, effective therapeutics as well), it would be beyond catastrophic to have to “start all over again” with the R&D process as a new “COVID-20” began searing its way through the world’s population. I think a global depression (economic and psychological) would be the almost inevitable result.

    • Well it’s hard to say from the scarce info we get. Farms with a lot of animals where the disease can pass freely (and these absolute fools have known that for months now) are perfect breeding and mutation grounds.

      BUT that said, usually when a human virus spends time in a different organism it becomes less infectious. In fact, this is how live vaccines are made – viruses are bred in other organisms, thus weakening their reproduction, and then injected into a human. The immune system then has an easy job.

      Then there is the fact that everybody builds different antibodies for the same target. Say, some have antibodies that attack the left side of the virus, some attack the right side. Now if you have a mutation that changes the right side, everybody who has that antibody from a previous infection (or vaccine) will have a reduced defense ability. Everybody who happened to attack the left side – no change. I believe this is the logic behind these concerns. It’s not that the virus suddenly resists “all” antibodies.

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