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Dr. Anthony Fauci, the nation's leading expert on infectious diseases, warned Thursday that omicron will become the dominant variant of the novel coronavirus in the United States "very soon."

"It has an extraordinary ability to transmit efficiently and spread," Fauci, the chief medical advisor to U.S. President Joe Biden, told ABC News' George Stephanopoulos in an interview on "Good Morning America."

"It has what we call a doubling time of about three days and if you do the math on that, if you have just a couple of percentage of the isolates being omicron, very soon it's going to be the dominant variant," he explained. "We've seen that in South Africa, we're seeing it in the U.K. and I'm absolutely certain that's what we're going to be seeing here relatively soon."

Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, urged Americans to "absolutely" get vaccinated against COVID-19, if they haven't already, and to also receive a booster shot when they become eligible.

"At this point, we don't believe you need an omicron-specific boost," he added. "We just need to get the boost with what you got originally for the primary vaccination."

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Fauci talks importance of boosters as omicron variant grows in US

Dr. Anthony Fauci shares the latest information about the fast-growing omicron variant as COVID-19 cases continue to rise heading into the holidays.

ABCNews.com

France announced Thursday that it will ban non-essential travel to and from the United Kingdom due to the country's surge in cases of the omicron variant.

Starting Saturday, France will require people to have "a compelling reason" to travel between the two countries. Travel for tourism or work will not be allowed. French citizens, however, can return to France, according to a statement from the French prime minister's office.

All travelers from the U.K. will be required to show proof of a negative COVID-19 test taken less than 24 hours before departure. Upon arrival in France, they must self-isolate for a week, but that period can be ended after 48 hours if they test negative for COVID-19 again.

The new rules apply to people regardless of their vaccination status.

"Faced with the extremely rapid spread of the Omicron variant in the United Kingdom, the Government has chosen to reinstate the need for an essential reason for travel from and to the United Kingdom, and to strengthen the requirement for tests on departure and arrival," the French prime minister's office said in the statement Thursday. "The Government is also calling on travelers who had planned to visit the United Kingdom to postpone their trip."

Indonesia announced Thursday its first confirmed case of the omicron variant.

The case was detected in a janitor who works at the COVID-19 Emergency Hospital of Kemayoran Athletes Village in Jakarta, according to a statement from Indonesian Health Minister Budi Gunadi Sadikin.

The hospital's cleaning staff are routinely tested and the results for three people were positive for COVID-19 on Dec. 10. Those samples were then sent to a genome sequencing lab, which identified the omicron variant in one of the samples on Wednesday, according to the statement.

Indonesia's Ministry of Health has also identified probable cases of omicron among five travelers who were in quarantine -- two Indonesian citizens who had just returned from the United Kingdom and the United States, and three foreigners from China. Their test samples are being sequenced and the results will be known in a few days, according to the statement.

The health minister urged Indonesians "not to panic and to remain calm," and to get vaccinated against COVID-19 if they haven't already.

"The arrival of new variants from abroad, which we identified in quarantine, shows that our defense system against the arrival of new variants is quite good, we need to strengthen it," Sadikin said. "So it's normal to stay 10 days in quarantine. The goal is not to make it difficult for people who came, but to protect the people of Indonesia."

Forecast models used by the CDC suggest weekly death totals and hospital admissions will rise over the next four weeks. 

The U.S. could reach a total of 845,000 deaths by Jan. 8, according to the forecasts from the COVID-19 Forecast Hub at UMass Amherst. 

The COVID-19 Forecast Hub team monitors and combines forecasting models from the nation’s top researchers. They then create an ensemble, usually with a wide cone of uncertainty. Nicholas Reich, a biostatistician who runs the forecasting model, told ABC News Wednesday that he doesn't think the forecasts included omicron in their predictions because the majority of data isn't publicly available yet in a format that can be easily incorporated into a model. 

-ABC News' Arielle Mitropoulos