Early indications of the severity of the Omicron Covid-19 variant are "a bit encouraging," top US pandemic advisor Anthony Fauci said Sunday, while cautioning more information was still needed.

"Omicron has a transmission advantage" in South Africa, where the variant was first reported, Fauci said in a CNN interview, noting the country had a low level of cases before it saw "almost a vertical spike upwards, which is almost exclusively Omicron." "Though it's too early to really make any definitive statements about it, thus far, it does not look like there's a great degree of severity to it," he said.

"Thus far, the signals are a bit encouraging." Medical experts have in recent days underscored that the South African population skews young and that more severe cases could emerge in the coming weeks.

Lab tests are underway to determine whether Omicron -- a heavily mutated strain of the virus -- is more transmissible than other strains, resistant to immunity from vaccination and infection or more severe, with results expected within weeks.

"I think that there's a real risk that we're going to see a decrease in effectiveness of the vaccines," Stephen Hoge, president of vaccine producer Moderna, told ABC.

"What I don't know is how substantial that is," he added. "Is it going to be the kind of thing that we saw with the Delta variant, which is, ultimately vaccines were still effective, or are we going to see something like a 50 percent decrease in efficacy, which would mean we need to reboot the vaccines." Moderna, like other pharmaceutical companies, including Pfizer, has already started work to adapt their vaccines if necessary.

Cases of the Omicron variant have so far been confirmed in at least 15 states and some 40 countries.

The United States last week imposed a travel ban on South Africa and seven other southern African countries to stem the variant spread.

Fauci said on Sunday he hoped the restrictions would be lifted "within a quite reasonable period of time."