Scientists around the world are racing to assess the dangers posed by a new variant of the coronavirus that has spread rapidly in Britain, as more countries shut their borders to travelers from the U.K. in an effort to keep it out. Epidemiologists and virologists advising the British government say initial evidence indicates the new strain is more contagious than older variants, but that so far there are no signs that it causes more severe disease.

Transmissibility—how easily the virus spreads from one person to another—and the seriousness of the illness it causes are both important metrics for gauging the potential threat. And scientists say there is still much they don’t know. Viruses mutate naturally, particularly RNA viruses like the new coronavirus. Many variants of the new coronavirus have surfaced since it raced around the world.

Worrisome Mutations

A new coronavirus strain spreading in the U.K. has genetic changes that could make it better at infecting cells.

Several changes, or mutations, were found in genes that code for the spike proteins on the viruses surface.

Respiratory Cell

Coronavirus

Spike protein

Genes

Receptor

Spike proteins bind to receptors on the surface of cells, aiding viral entry.

Scientists are still studying the effects of these mutations on how quickly the virus spreads.

Viral reproduction

Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said he expects the variant will come to circulate in the U.S. if it isn’t in the country already, but said he didn’t believe it has been a factor in the current surge in infections in the U.S. He said he doesn’t believe the U.S. needs to halt flights from the U.K., but that “I think you might want to consider the possibility of requiring people who get on a plane and fly from England to have been tested in the last 48 to 72 hours.”

Dozens of countries around the world have moved to curb travel from the U.K. The U.S. has not. New York has asked airlines to require all passengers from the U.K. to have a negative Covid-19 test before boarding flights to the state.

Health experts and drug industry officials say they expect newly authorized vaccines, which induce virus-busting responses aimed at different areas on the surface of the pathogen, will protect against the new variant.

Ugur Sahin, the chief executive of BioNTech SE, which partnered with Pfizer Inc. on a vaccine, said Tuesday he thought the shot would work against the new version of the virus, but that if it is indeed more transmissible, it could raise the threshold needed to protect the community.

“If the virus becomes more efficient in infecting people, we might need even a higher vaccination rate to ensure that normal life can continue without interruption,” he said.