LONDON—British public-health officials were mystified. Nationwide restrictions were driving down the spread of coronavirus across much of the country by late November. But in one part of southeast England infections were inexplicably surging.

Epidemiologists set out to investigate, at first assuming there had been some kind of superspreader event or that people were ignoring social-distancing rules at work, in illegal house parties, or elsewhere. They found nothing. Stumped, they turned to a team of scientists monitoring mutations in the virus’ genome.

On Dec. 8, the group, known as the Covid-19 Genomics UK Consortium, found a new variant of the virus, with 23 mutations, in a sample taken from a patient in Kent, near the center of the outbreak in late September. They found the same variant in someone tested in London a day later.

Some of the new mutations had the potential to increase the transmissibility of the virus.