WASHINGTON (BLOOMBERG) - Dr Anthony Fauci, President Joe Biden's chief medical adviser, has said Covid-19's Omicron variant may well already have arrived in the United States.

"I would not be surprised if it is," Dr Fauci told NBC on Saturday (Nov 27).

He said that the US has not detected the new variant 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 Mr Biden on South Africa and seven other countries in the region are a way to buy time for the US to prepare defences against the variant and should not lead to panic, Dr Fauci said.

Questions about the Omicron variant include whether it causes disease that is more serious than infections with the earlier Delta variant, he said.

While it is conceivable that the latest variant may diminish vaccine protection against Covid-19, existing vaccination may be able to contain it, Dr Fauci said.

Mr Biden, who is at Nantucket for the long Thanksgiving holiday weekend, has been briefed on the latest and his Covid-19 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," Dr 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," he added.