Nov. 29, 2021 -- The United Kingdom, Italy, Germany, Belgium and Australia reported their first cases of the new Omicron variant during the weekend, joining a growing list of countries that have confirmed cases among travelers returning from southern Africa.

The U.S. hasn’t detected the variant yet, but it’s possible that it could already be spreading across the country, according to Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases.

“When you have a virus that is showing this degree of transmissibility and you’re already having travel-related cases…it almost invariably is ultimately going to go essentially all over,” he told “Weekend TODAY.”

The World Health Organization labeled Omicron, known scientifically as B.1.1.529, as a “variant of concern” on Friday, meaning that there could be a higher risk of transmission, severe disease and evasion from vaccines. Omicron has a large number of mutations, which could make it more contagious and less treatable, Fauci said.

“These are all maybes, but the suggestion is enough,” he said. “This is something we’ve got to pay really close attention to and be prepared for something that’s serious. It may not turn out that way, but you really want to be ahead of it.”

On Sunday, Australia and Denmark were the latest countries to confirm Omicron cases. Australian officials said the variant was detected among two travelers who were fully vaccinated and had no symptoms. In Denmark, the variant was found among two travelers who recently arrived from South Africa.

Dutch health officials said on Sunday that at least 13 cases of the Omicron variant were detected in 61 passengers who had positive COVID-19 tests. They were among more than 500 people who arrived on two flights from South Africa to The Netherlands and have been quarantined.

Two cases were identified in the U.K. on Saturday among two people who traveled to southern Africa. Germany also confirmed two cases on Saturday among travelers who entered the country at the Munich airport. Italy reported that a case had been detected in a passenger who arrived from Mozambique.

In response, the European Union is restricting travel to and from seven countries in southern Africa — Botswana, Eswatini, Lesotho, Mozambique, Namibia, South Africa, and Zimbabwe. The U.S. and South Korea have announced travel restrictions to those countries, as well as Malawi.

In Israel, which has confirmed one Omicron case and has several suspected cases, officials announced it is barring foreign travelers from all countries for 14 days. Japan has followed suit, Saying that starting Tuesday, it is barring all foreign visitors from around the world.

Omicron cases have also been reported in Belgium, Botswana, Hong Kong, and South Africa. As of Monday morning, 145 Omicron cases had been reported to GISAID, a global database for the genomic sequencing of viruses.

Scientists have urged caution, saying that little is known about the variant so far. Some variants of concern, such as the Delta variant, have been more contagious and spread globally, but others have had limited effects. Existing vaccines likely offer some protection, according to Fauci.

He emphasized that people shouldn’t panic but that they should continue to take precautions such as wearing masks, following physical distancing guidelines and avoiding large indoor gatherings.

“It is absolutely essential that unvaccinated people get vaccinated and that vaccinated people get boosters,” he told TODAY. “We know now clearly that when you get a booster shot…you dramatically increase the level of protection.”