The new, mutation-laden variant of the coronavirus that has been spreading rapidly in the United Kingdom may already be circulating in the United States and other countries, leading infectious-disease experts said Monday. There is no evidence that this variant causes more severe illness from covid-19, the disease caused by the novel coronavirus. And scientists do not know with certainty whether this variant, officially known as B.1.1.7, is truly more transmissible. But it has the appearance of being so.

“We don’t know that for absolutely certain, but it is reasonable to assume that is going on,” Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said Monday.

“Nor is there any indication that the mutation would interfere with the response to the vaccine,” Fauci added. “Having said all of that, when you’re dealing with mutations, you have to follow them very closely and don’t take them lightly.”

Several top infectious-disease experts said Monday the variant may not have originated in the United Kingdom. Instead, it may have been identified there first because the United Kingdom has a robust monitoring system that has examined tens of thousands of genomic sequences of virus samples.

The United States has lagged in sequencing and does not have nearly the same level of virus surveillance.