Denmark has announced special restrictions for more than 280,000 people in the country's northwest after a mutated version of the new coronavirus linked to mink farms was found in humans.
Copenhagen warned that the mutation could threaten the effectiveness of any future vaccine.
"From tonight, citizens in seven areas of north Jutland are strongly encouraged to stay in their area to prevent the spread of infection," Prime Minister Mette Frederiksen told a news conference.
She said people were being ordered not to travel there, while bars and restaurants would also shut.
"We are asking you in north Jutland to do something completely extraordinary," Frederiksen said, talking of a "real closure" of the region.
"The eyes of the world are on us," she added.
Public transport in the region will be shut down with buses and trains stopped from entering or leaving.
Some schoolchildren will have to follow their classes online in restrictions that are due to last a month.
Denmark, the world's largest exporter of mink fur, raised concerns on Wednesday by announcing the slaughter of all mink in the country - numbering 15 to 17 million spread over 1,080 farms - following the discovery of the mutation which can be passed to humans.
The mutation has already been detected in 12 people - 11 cases in the region being closed down, and one in another.
Department in contact with Irish mink farms
Here, the Department of Agriculture says no animal in Ireland has tested positive for Covid-19.
It says it has been following developments worldwide in relation to Covid in animals, including mink.
Responding to a query from RTÉ News, the Department said it has written to mink farms in Ireland, on a number of occasions this year.
There are three mink farms in Ireland; in Donegal, Laois and Kerry.
It says it is continuing to engage with the farms to provide information on SARS-CoV-2 infection in mink and to advise on the implementation of biosecurity measures to prevent their mink being exposed to the virus.
No mink have been imported into Ireland during 2020.
Virus mutations 'often harmless'
Scientists say virus mutations are common and often harmless.
Some experts have nevertheless called on Denmark to release more scientific data to better evaluate this one.
According to Danish authorities, this virus mutation does not cause a more severe illness in humans.
But it is not inhibited by antibodies to the same degree as the normal virus, which they fear could threaten the efficacy of coronavirus vaccines that are being developed around the world.
In north Jutland, health authorities believe around 5% of coronavirus patients could be carrying this mutated strain, but no recent case has been reported.
As such, Viggo Andreasen, epidemiology professor at Roskilde University, said the mutation had "quite a good chance" of disappearing, as long as it is effectively contained.
Denmark has been relatively spared from the ravages of Covid-19 with 733 deaths reported.
However, new national restrictions were imposed in October to curb a rapid spike in cases.