Anthony Fauci, President Joe Biden’s chief medical adviser, said COVID-19’s omicron variant may well already have arrived in the U.S.

“I would not be surprised if it is,” Fauci said on NBC’s “Weekend Today” on Saturday. “We have not detected yet,” but when a virus shows “this degree of transmissibility” it “almost invariably ultimately is going to go essentially all over,” he said.

Travel restrictions imposed by Biden on South Africa and seven other countries in the region are a way to buy time for the U.S. to prepare defenses against the variant and shouldn’t lead to panic, Fauci said.

Questions about the omicron variant include whether it causes disease that’s more serious than infections with the earlier delta variant, he said. While it’s “conceivable” that the latest variant may diminish vaccine protection against COVID-19, existing vaccination may be able to contain it, Fauci said.

Biden, who is at Nantucket for the long Thanksgiving holiday weekend, has been briefed on the latest and his COVID response team is in regular contact with health officials around the world, according to a White House official.

“It seems to have spread rather rapidly in South Africa,” Fauci said. “Its ability to infect people who have recovered from infection and even people who have been vaccinated makes us say this is something you have to pay really close attention to and be prepared for something that’s serious.”