Millions of minks have been killed since June because they can pass the coronavirus to people. But fears of a mutated strain are overblown.

• Denmark is culling its entire population of minks, up to 17 million across more than 1,000 fur farms, after finding a mutated coronavirus strain circulating among the animals.
• A dozen people have been infected with the strain. More than 280,000 Danes in the region where the strain was found went into lockdown Friday.
• Although Danish officials warned that the observed mutation could undermine a future vaccine, scientists said there's no reason to panic yet.

Danish government officials and scientists announced Wednesday that they'd identified a mutated strain of the new coronavirus in minks. The animals passed the strain to a dozen people, they said. More than 280,000 Danes in northern Denmark, where the strain was found, went into lockdown Friday.

Prime Minister Mette Frederiksen has ordered armed forces to cull every mink in Denmark — up to 17 million across more than 1,100 fur farms. It's the latest and largest in a series of mink cullings across Europe since June. Once the order is fully carried out, nearly 20 million minks will have been killed this year to stop potential coronavirus spread.

"We have known since spring that SARS-CoV-2 can spread easily in mink, thanks to outbreaks in different places across Europe," Emma Hodcroft, a geneticist at the University of Basel, Switzerland, told Business Insider.

Denmark's mink troubles started five months ago, when the government culled 11,000 coronavirus-infected animals. In October, more than 1 million additional Danish mink were killed; infected minks have been found on 207 farms.

However, this is the first time Danish government officials confirmed that the had virus jumped from minks to people there. The mutated strain in question, which was traced to five farms, has been been found in 12 people and linked to 400 cases in Denmark.

Virus strains in minks are already circulating in people

Hodcroft, who studies the coronavirus's changing genome, said Danish scientists have yet to publicly share the genetic code of the mutated strain the prime minister is referring to. But the other strains known to circulate in minks so far, she added, do not seem different or more pernicious than the ones humans are already dealing with.

"As far as I'm aware, we haven't seen signs that infections in minks are somehow related to generating mutations that are functionally different or more worrying than we see arise in humans," Hodcroft said.

All viruses accumulate genetic errors over time, but in the case of the new coronavirus, no mutation so far has been definitively shown to affect how well the virus infects hosts.

"SARS-CoV-2 mutates in minks as it does in humans, cats, dogs or anything else, and there is little evidence for mutations altering viral transmissibility," Lucy van Dorp, a researcher at the Genetics Institute of the University College London, told Business Insider.

Similarly, Angela Rasmussen, a virologist from Columbia University, tweeted on Thursday that "there have not been any data demonstrating that COVID-19 caused by mink-derived variants is substantially different from human-derived variants."

That's not for lack of investigation: Scientists in Denmark published two papers in September and October highlighting key mutations found in coronavirus strains among Danish minks. But so far, all known mink coronavirus genomes "fall into the diversity" of human ones, van Dorp tweeted Wednesday.

Concerns about a weaker antibody response

Denmark's State Serum Institute, which is run by the Danish Ministry of Health, said the 12 Danes infected by the newly discovered mink coronavirus strain appeared less able to produce virus-fighting antibodies. Frederiksen warned that the mutated strain could therefore threaten the efficacy of a future vaccine.

"This means that the mutated virus — via mink — can pose the risk that the upcoming vaccine will not work the way it should," Frederiksen said on Facebook Wednesday.

But experts said that concern is premature.

"'Weak' antibody responses are seen in mild or asymptomatic COVID-19 cases too," Rasmussen tweeted Thursday.

Plus, antibodies aren't the only component of the body's immune respons. White blood cells called memory T cells can identify and destroy infected cells, too, and mount another attack should the coronavirus ever return. An August study of 206 recovered coronavirus patients from Sweden found that even people who did not test positive for antibodies developed memory T cells.

A summer of cullings across Europe

This isn't the first time millions of minks have been marked for death because of a coronavirus outbreak.

Spain killed nearly 100,000 minks in July, after animals at a farm northeast of Valencia tested positive. Experts weren't able to discern whether any mink-to-human transmission happened there, however. The US, too, saw coronavirus outbreaks among mink farms in Utah, Michigan, and Wisconsin. The government has not recommended cullings, however, since there's no evidence of minks infecting American farmers.

But preliminary Dutch research published in September confirmed that the virus was jumping back and forth between minks and people at farms there.

The research came after scientists in the Netherlands found in April that minks at two Dutch fur farms had the coronavirus. Further analysis showed 15 more farms were affected. Dutch officials announced in May that the minks had passed the virus to a farm worker, then ordered the culling of more than 1 million Dutch minks in June.

That kind of to-and-from transmission is why cullings may be necessary.

"With extensive outbreaks in mink farms, and transmission back to humans, mass cullings are, unfortunately, a useful infection control measure," van Dorp said.

Mass cullings could prevent future outbreaks

When a virus jumps from animals to humans, that's called a spillover event. Species known as zoonotic reservoirs, meanwhile, are animal hosts in which a virus can live then eventually infect people.

Right now, minks don't seem to be a reservoir species — they're just other virus victims. They can get infected by farmers or other minks. (Cats, dogs, and lions and tigers in zoos have also tested positive.)

"As farmed minks are kept in close proximity, they have been particularly susceptible to outbreaks in the Netherlands, Spain, and Denmark," van Dorp said.

On fur farms, where minks are bred for their pelts, multiple minks often share a single cage. Their coronavirus symptoms appear similar to those in humans: Minks who get severely sick have trouble breathing and can die within days.

"It is amazing how easily this virus spreads in mink," Marion Koopmans, a virologist in Rotterdam and a co-author of the Dutch research released in September, told CNN.

She added that she supports cullings, since one day minks could become a new reservoir species that kickstarts subsequent coronavirus outbreaks. Rasmussen said that risk is more concerning than a mutated strain.

"The thing I'm most concerned about was not really addressed by the Danish authorities: spillback into a new reservoir species," Rasmussen tweeted on Thursday, adding,"if SARS-CoV-2 starts circulating in wild animals, then we risk future outbreaks caused by zoonotic transmission from exposure to these new reservoirs."