The Västmanland region, west of Stockholm and home to the city of Västerås, on Wednesday confirmed that 20 people had tested positive for the B.1.351 variant.
None of the cases can be linked to international travel, the region wrote in a statement.
“The infection is limited to clusters, that is groups of people who have been in close contact with one another,” it said.
It added that it considered the outbreak of the variant to so far be “limited and controlled, thanks to increased contact tracing”.
It said more people who have also tested positive for coronavirus may be part of the same cluster, but not all samples have so far undergone sequencing to confirm whether or not they are part of the same strain.
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Västmanland is the first region in Sweden to confirm a cluster of B.1.351 which cannot be linked to travel.
The variant, which is also known as B.1.351, is thought to be more infectious than the original coronavirus, but does not appear to cause more serious illness, said the region in its statement. There is however a concern that the current vaccines will be less effective against this variant, although it may be too early to say.
Another variant of the coronavirus, B117, which was first discovered in the UK, was detected in Västmanland in January and now makes up around 25 percent of all positive coronavirus self tests in the region.