Suddenly the coronavirus seemed to be changing.

For months, Dr. Steven Kemp, an infectious disease expert, a global library of coronavirus genomes. He was studying how the virus mutated in the lungs of a patient struggling to shake a raging infection in a nearby Cambridge hospital and wanted to know if those changes would occur in other people.

At the end of November, Dr. Kemp then came up with a surprising match: some of the same mutations seen in the patient, as well as other changes, kept coming back in newly infected people, mostly in the UK.

Worse still, the changes focused on the spike protein that is used by the virus to attach to human cells, suggesting that a virus that is already wreaking havoc around the world has evolved in ways that could make it even more contagious .

“There are a lot of mutations that go along with the same frequency,” he wrote to Dr. Ravindra Gupta, a Cambridge virologist. He listed the most disturbing changes and added, “ALL of these sequences have the following spike mutants.”

The two researchers didn’t know yet, but they had found a new, highly contagious variant of the coronavirus that has since spread across the UK, shaking scientists’ understanding of the virus and threatening to prevent global recovery from the pandemic.

A consortium of British disease researchers, long-time torchbearers in genomics who had helped track the Ebola and Zika epidemics, became known. They gathered on Slack and video calls, comparing notes as they searched for clues, including a tip from scientists in South Africa about another new variation there. Others have since appeared in Brazil.

For almost a year, scientists had only observed incremental changes in the coronavirus and expected more of them. The new variants forced them to change their thinking, suggesting a new phase in the pandemic where the virus could develop to the point that vaccines will be undermined.