• Dr. Anthony Fauci told CNBC on Friday the Covid omicron variant is "spreading very efficiently" and leading to what he describes as "a lot of breakthrough infections."
• However, the White House chief medical advisor said he would get on a plane and suggested holiday gatherings among vaccinated people at home are fairly low risk.

White House chief medical advisor Dr. Anthony Fauci said holiday parties with vaccinated family and friends are fairly low risk even as the Covid-19 omicron variant spreads throughout the U.S. and causes "a lot of breakthrough infections."

"If you are vaccinated, your family is vaccinated, you have friends who are vaccinated — and hopefully also boosted — you can still enjoy a social gathering generally in a home," Fauci said in an interview Friday on CNBC's "Squawk Box."

Covid hospitalizations have jumped by about 20% over the past two weeks, averaging about 68,400 over the last seven days, according to U.S. Department of Health and Human Services data. At the peak of the delta wave in September, more than 100,000 Covid patients were in the hospital.

'The risk is never zero'

While delta remains the dominant variant in the U.S., omicron cases are projected to double every couple of days or so. On Tuesday, the Centers for Disease Control and Prevention said omicron made up about 3% of cases sequenced in the U.S. The Covid positivity rate in New York City doubled from 3.9% on Dec. 9 to 7.8% on Dec. 12, officials there reported Thursday.

Daily Covid cases rose 27% since before Thanksgiving to a seven-day average of 122,000, according to a CNBC analysis of data compiled by Johns Hopkins University. Cases remain below recent peak levels when the country was reporting an average of more than 170,000 daily cases in September.

"You've got to be careful when you go into large, public indoor spaces where there are a lot of people there," Fauci said. "That's the reason why you should be wearing a mask under those circumstances."

Fauci said that if Covid cases keep rising, people "may need to be more restrictive" with their holiday plans. "But for right now, people who are vaccinated and boosted should feel reasonably comfortable."

"The risk is never zero. That's for sure, under any circumstances," he said.

Is it safe to fly?

Despite rising omicron cases, Fauci said he would be comfortable flying, with precautions, if it was necessary.

"If I had to I would have no problem getting on an airplane. I'm vaccinated. I'm boosted. I know we have to wear a mask on an airplane," he said, noting that he doesn't currently plan to fly anywhere, since work keeps him in Washington.

Fauci said that masks on planes and in indoor settings such as airports work. "There's no doubt that fabric masks work," he said. "The N95 is the best one. But they are relatively uncomfortable to wear. Not a lot of people wear them. But a regular surgical mask as well as a cloth mask is fine."

The best way to protect against getting Covid is to be vaccinated and boosted, Fauci added.

— CNBC's Nate Rattner contributed to this report.