WASHINGTON (REUTERS) - It is too early to know whether the Omicron variant of coronavirus will lead to severe disease, but preliminary information from South Africa indicates it does not result in unusual symptoms, top US infectious disease expert Anthony Fauci said on Tuesday (Nov 30).

Fauci said there were 226 confirmed cases of the variant in 20 countries as of Tuesday morning but Omicron had not been detected yet in the United States.

Fears about the variant have rattled financial markets and sparked concerns about the strength of the global economic recovery as the world continues to fight the coronavirus pandemic.

"It is very difficult to know whether or not this particular variant is going to result in severe disease," Fauci told reporters in a briefing.

"Although some preliminary information from South Africa suggests no unusual symptoms... we do not know, and it is too early to tell."

President Joe Biden and his administration have pressed Americans to take advantage of vaccines and booster shots, but vaccine hesitancy in a segment of the US population has thwarted efforts to tame the virus' spread.

About 69 per cent of Americans aged 12 and up are fully vaccinated.

"We are hoping, and I think with good reason, to feel good that there will be some degree of protection," against the variant from the vaccines, Fauci said. "If you're unvaccinated, get vaccinated. And if you're vaccinated, get boosted."

Biden, whose poll numbers have suffered in part amid frustration that the pandemic is not under control, on Monday urged Americans not to panic about the new variant.

"To beat the pandemic, we have to vaccinate the world as well," the president said.

Asked on Tuesday if the United States was doing enough to vaccinate the rest of the world, Fauci noted the United States was doing more than other nations.

"'Enough' is a tough word. Are we doing a lot? We are doing an awful lot," he said.

Fauci said getting vaccines into people's arms in southern African countries and other low- and middle-income countries had proven difficult logistically and many doses that were shipped went unused.

"Other African countries... have actually told us not to ship any more vaccine because they have not been able to adequately utilise it," he said.