Early indications of the severity of the new coronavirus variant Omicron are "a bit encouraging," top U.S. infectious disease expert Anthony Fauci told CNN in an interview on Sunday, while cautioning that more information was still needed. 

"Omicron has a transmission advantage" in South Africa where the variant was first reported, Fauci said, 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. "The signals are a bit encouraging."

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

Medical experts have in recent days underscored that the South African population skews toward the young and more severe cases could emerge in the coming weeks.

Lab tests are underway to determine whether Omicron 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 News.

"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."

Cases of the Omicron variant have so far been confirmed in at least 16 U.S. states and some 40 countries.

As U.S. Omicron cases emerge, COVID-19 vaccine makers aim to quickly tweak their shots to target the variant and U.S. regulators have vowed speedy reviews, but that could still take months.

The U.S. Food and Drug Administration and Centers for Disease Control and Prevention (CDC) will move swiftly, CDC Director Rochelle Walensky told ABC News.

Moderna has targeted U.S. approval of an updated vaccine as soon as March, but company officials said on Sunday it will still take time to increase output. Government officials are also working with Pfizer and Johnson & Johnson on updated shots, while Pfizer and Merck & Co., Inc. are pursuing COVID-19 pill treatments.

(With input from AFP, Reuters)