“It’s a real warning that we need to pay closer attention,” Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle. “Certainly these mutations are going to spread and definitely, the scientific community — we need to monitor these mutations and we need to characterize which ones have effects.”

The British variant has 23 mutations, including several that affect how the virus locks onto human cells and infects them. These mutations may allow the variant to replicate and transmit more efficiently, said Muge Cevik, an infectious disease expert at the University of St. Andrews in Scotland and a scientific adviser to the British government.

But the estimate of greater transmissibility — British officials said the variant was as much as 70 percent more transmissible — is based on modeling and has not been confirmed by lab experiments, Dr. Cevik added.