Portuguese health authorities on Monday confirmed 13 cases of the omicron variant among professional soccer players.

The Ricardo Jorge National Health Institute said the players who tested positive are all members of the Lisbon-based Belenenses SAD soccer club and that one of them had recently traveled to southern Africa, where the omicron variant was first identified last week.

The institute is investigating whether this is one of the first reported instances of local transmission of the new coronavirus variant outside of southern Africa, where most of the cases have been recorded so far.

All 13 players have been placed in quarantine and those who have been in contact with them were ordered to isolate, regardless of their vaccination status or their exposure to possible contagion. The players and their close contacts will be regularly tested for COVID-19, the institute said.

-ABC News' Aicha El Hammar Castano

Moderna's chief medical officer, Dr. Paul Burton, said the omicron variant probably emerged around mid-October in southern Africa.

"How transmissible is it? We think it's probably quite transmissible. But how severe is the disease it causes? We don't know the answer to that question yet," Burton told ABC News' Amy Robach in an interview Monday on "Good Morning America."

"While we think that vaccine effectiveness may come down based on the mutations seen in this virus ... we should be able to get antibodies up" with the booster shot, Burton said.

"We'll know from tests in the next couple of weeks how effective the vaccines are against this variant," he added.

Dr. Anthony Fauci, the U.S. government's top infectious diseases expert, is urging Americans who have been vaccinated against COVID-19 and are eligible to get a booster shot to do so now, in anticipation of the omicron variant spreading "widely."

So far, there are no known confirmed cases of the new variant in the United States, according to Fauci, who is the chief medical adviser to U.S. President Joe Biden.

"But obviously, we're on high alert," Fauci told ABC News' George Stephanopoulos in an interview Monday on "Good Morning America."

"It's inevitable that, sooner or later, it's going to spread widely because it has at least the molecular characteristics of being highly transmissible," he added, "even though there are a lot of things about it that we do not know but will be able to ascertain in the next week or two."

Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, said omicron clearly has a "transmissibility advantage," based on what scientists have seen in southern Africa, where the variant was first identified last week.

"But the extent of that, again, still needs to be worked out," he noted. "We'll know soon."

Fauci said the severity of illness that the omicron variant can inflict remains unclear, despite early reports that some patients had mild symptoms.

Although there is still so much unknown about the new variant, Fauci said it's clear that vaccinated individuals, particularly those who have received booster doses, fare better against COVID-19 than their unvaccinated counterparts.

"So we don't know exactly what's going on with this new variant," he said, "but I would assume -- and I think it's a reasonable assumption -- that when you get vaccinated and boosted and your [antibody] level goes way up, you're going to have some degree of protection, at least against severe disease."

Both Pfizer-BioNTech and Moderna booster shots have been authorized for all adults in the United States. Anyone over the age of 18 can get a booster dose at least six months after they received their second dose of the Pfizer-BioNTech or Moderna vaccines, or at least two months after they got their single dose of the Johnson & Johnson vaccine.

"I would strongly suggest you get boosted now and not wait for the next iteration of [the vaccine], which we may not even need," Fauci said. "The pharmaceutical companies are preparing to make a specific booster for [omicron], but we may not need that."

The World Health Organization has assessed the overall global risk related to a newly discovered variant of the novel coronavirus as "very high."

In a technical brief published Sunday, the WHO explained that omicron, or B.1.1.529, "is a highly divergent variant with a high number of mutations," some of which it said "are concerning and may be associated with immune escape potential and higher transmissibility."

"Given mutations that may confer immune escape potential and possibly transmissibility advantage, the likelihood of potential further spread of Omicron at the global level is high," the WHO concluded in a risk assessment. "Depending on these characteristics, there could be future surges of COVID-19, which could have severe consequences, depending on a number of factors including where surges may take place."

The variant was first identified in southern Africa last week and has quickly spread to several countries across the globe, sparking new travel restrictions and shaking financial markets. On Friday, the WHO officially named the variant omicron and designated it as a "variant of concern." Both the WHO and the U.S. Centers for Disease Control and Prevention say that variants of concern have shown to spread more easily than others and cause more severe disease.

While omicron has not yet been detected in the United States, Dr. Anthony Fauci, the government's top infectious diseases expert, told ABC News on Sunday that the variant will "inevitably" arrive.

"The question is," he added, "will we be prepared for it?"