WASHINGTON - U.S. President Joe Biden made a new plea Monday to Americans to get vaccinated against the coronavirus¸ or get booster shots if they have already been inoculated, in the face of what his top medical adviser says is the inevitability that the new omicron variant will enter the country.

"Do not wait," Biden told the estimated 60 million unvaccinated people in the country during a short White House address. "If you are not vaccinated, go get it now."

But the president said he does not currently believe that additional shutdowns of businesses and schools are needed because of the new omicron threat.

Biden said he would "spare no effort" to fight the new variant but health officials expressed the hope that those already vaccinated have a measure of protection against it, even as more scientific studies are being conducted.

Cases of the variant, first identified in South Africa, have now been found in several countries and Biden, starting Monday, has banned flights into the United States from South Africa and seven other countries in Africa.

Dr. Anthony Fauci, the top U.S. infectious disease expert, told ABC's "Good Morning America" show that scientists are hoping to learn in the next week or two how well the existing COVID-19 vaccines will protect people against the omicron variant, and how dangerous it might prove to be.

"We really don't know,' Fauci said.

COVID-19, the disease cause by the coronavirus, has killed more than 750,000 people in the U.S. during the 20-month pandemic, more than in any other country.

Biden said vaccines in the U.S. will always first be available for Americans and for free. He said, however, that the country also has a "moral obligation" to help the rest of the would get vaccinated and that widespread inoculations would help end the global pandemic.

He said 275 million vaccine doses produced in the U.S. have already been shipped to 110 other countries.

"We're throwing everything we can at this virus," he said.