(CNN) The new Omicron variant might prompt a return to stricter Covid-19 measures if not enough people get vaccinated or boosted, health experts say.
Omicron -- or the B.1.1.529 strain of novel coronavirus -- has already spurred international travel bans. The World Health Organization officially called it a "variant of concern" on Friday.
The variant has not yet been detected in the US, the Centers for Disease Control and Prevention said in a Saturday update.
"But it's fairly likely we'll see cases," said Dr. Francis Collins, director of the National Institutes of Health.
"It ought to redouble our efforts to use the tools that we have, which are vaccinations and boosters -- and to be sure we're getting those to the rest of the world, too," Collins told CNN on Sunday.
"It also means we need to pay attention to those mitigation strategies that people are just really sick of, like wearing masks when you're indoors with other people who might not be vaccinated, and keeping that social distance," he said.
"I know, America -- you're really tired of hearing those things. But the virus is not tired of us. And it's shapeshifting itself."
As coronavirus keeps spreading, new mutations -- and new variants -- are expected.
"We have seen a lot of variants pop up over the last five, six months, and most of them have not amounted to much. This looks different," said Dr. Ashish Jha, dean of the Brown University School of Public Health.
The Omicron variant has an unusually high number of mutations, with more than 30 mutations in the spike protein alone, South African genomic scientists said last week.
Spike proteins are the structures used by a virus to get into the cells it attacks.
And "10 or more" of the mutations are on the receptor binding domain, which "binds to the cells in your nasal pharynx and in your lung," Dr. Anthony Fauci told NBC on Sunday.
"In other words, the profile of the mutations strongly suggest that it's going to have an advantage in transmissibility," the director of the National Institute of Allergy and Infectious Diseases said.
When experts looked at other variants, Jha said, it usually took several months for those strains to be dominant -- in other words, the most common strain of the virus spreading in one area.
"This one has become dominant very quickly in South Africa in the regions where it's been found -- within a matter of days to weeks, as opposed to months," Jha said.
"Now, the number of cases in South Africa is quite low, so it can be for other reasons as well -- not just because it's more transmissible. But the speed with which it has taken off is really unlike anything we've seen before."
Collins said it's not yet clear whether the Omicron variant is more contagious than the Delta variant.
"It certainly shows the signs of being able to spread quickly," he said. "What we don't know is whether it can compete with Delta."
It's also too early to tell whether the Omicron variant causes more severe illness, Collins said.
In short: "There's still much we do not know about Omicron," epidemiologist and infectious disease expert Dr. Celine Gounder said.
"We're still learning," Gounder said Sunday. "As we were saying very early in the pandemic, pandemics are not about panicking. They're about policies, protocols and practice. And in this case, that means doing the work of characterizing the virus."
A growing number of countries have confirmed cases of the Omicron variant. Canada joined that list Sunday, when health officials confirmed two cases in Ottawa, Ontario.
Both people had recently traveled from Nigeria and are isolating, Canadian health officials said.
If the Omicron variant isn't already in the US, it's bound to be "soon," said Dr. William Schaffner, a professor at Vanderbilt University Medical Center.
"It's going to make its way around the world. It looks like that way," Schaffner said Saturday.
And that could prompt a return to stricter Covid-19 mitigation efforts.
"I think we may, indeed, be in for a phase of many more masks, much more social distancing, and more restrictions and obligations for vaccination going forward," Schaffner said.
News of the variant spread as Americans packed airports at almost pre-pandemic levels for Thanksgiving and at the start of the holiday season: The Transportation Security Administration said it screened about 2.3 million people at airports across the country Wednesday, making it the busiest day at security checkpoints since March 2020.
About 59% of Americans are fully vaccinated against Covid-19, and about 19% of those fully vaccinated have received a booster dose, according to CDC data.
While vaccine makers test their shots against the new Omicron variant, health experts said it's important for anyone eligible to get a Covid-19 vaccine or booster to do so now.
"The main things people can do to protect themselves right now is if you're not vaccinated, get vaccinated," Gounder said. "If this does turn out to be an immune-evading variant, there may also well be benefit in getting an extra dose or booster dose of vaccine."
Fauci offered similar advice: "If ever there was a reason for the people who were vaccinated to get boosted, and for those who were unvaccinated to get vaccinated, it's now," he told NBC.
"If you're six months or more from the second dose of an mRNA (vaccine) -- either the Pfizer or the Moderna -- get boosted. If you're two months or more following the single dose of J&J, get boosted."
Fauci said he believes the current vaccines -- and especially booster doses -- will help at least somewhat against the Omicron variant.
"When you get boosted, the level of your antibody goes way, way above what the level at its peak was after the second dose," Fauci told NBC.
"And that's the reason why we feel even with variants like Omicron that if you get boosted, you're going to get a level of antibody that's high enough that it is likely you'll be able to get at least some degree -- and maybe a lot -- of protection against this."
Moderna said it's testing the ability of its Covid-19 vaccine to neutralize Omicron, and data is expected in the coming weeks.
The company said it's also testing a larger-dose booster and an Omicron-specific booster in case the current vaccine and booster don't sufficiently work against the new variant.
If scientists determine an Omicron-specific vaccine dose is needed, "we think within weeks to maybe two to three months, we would be able to have a Omicron-specific vaccine booster available for testing," Moderna Chief Medical Officer Dr. Paul Burton said Sunday.
BioNTech, the German company that partnered with Pfizer to make a Covid-19 vaccine, is also investigating Omicron's impact on their vaccine, with data expected in the coming weeks.
Johnson & Johnson said it's also testing the effectiveness of its vaccine against Omicron.
Moderna said the Omicron variant could be a challenge.
"The combination of mutations represents a significant potential risk to accelerate the waning of natural and vaccine-induced immunity," the company said Friday.
But Jha said he doesn't think the new variant means "vaccines will be rendered useless."
"I think that's exceedingly unlikely," he said. "The question is, is there a tiny hit to vaccine efficacy, or is there a large hit?"
Before scientists can say how much the current vaccines might work against Omicron, "we have to go through a couple of weeks yet of uncertainty," Burton said.
"There are three questions we really need answers to: How transmissible is this variant? How severe is it? And will the antibodies produced in response to the current vaccines (be) effective? And we won't know that (answer to the) last question for a couple of weeks," Burton told CNN on Sunday.
"But what we do know is that the best protection is to get vaccinated. If you're on the fence, if you haven't been vaccinated, get vaccinated. And now everybody over 18 in this country is at least eligible for boosting. So if you're now eligible for boosting, get boosted as well," he said.
"By doing that, we at least know that you'll get that first line of critical protection. And then of course, there are additional, simpler things you can do -- hand washing, some social distancing, mask wearing as appropriate. These two together, right now -- until we know exactly what's going on -- are going to be critical in our line of defense."
With or without the Omicron strain, the US is still struggling with the Delta variant.
Covid-19 hospitalizations in 16 states increased by more than 50% in the past week compared to the previous week, according to the US Health and Human Services Department.
"I think we need to just get our mind set that the virus is still in control. I don't care about your Covid fatigue," Schaffner said.
"We're going to have to deal with this in an ongoing way very, very seriously. ... Tighten your seat belts."