• Dr. Anthony Fauci, President Joe Biden's top medical advisor, said Thursday that omicron will become the dominant Covid-19 variant in the United States within a few weeks.
• He added: "And with omicron breathing down our back, things could get really bad, particularly for the unvaccinated."
• Though omicron is more contagious than other variants, it is not yet clear whether it causes more mild or severe disease than past strains.

WASHINGTON — Dr. Anthony Fauci, President Joe Biden's top medical advisor, said Thursday that omicron will become the dominant Covid-19 variant in the United States within a few weeks, urging people to get vaccines and booster shots to fight it off.

"It is the most transmissible virus of Covid that we had to deal with those far. It will soon become dominant here. That's one thing we know," Fauci told a virtual U.S. Chamber of Commerce Foundation audience.

Fauci urged people who are not yet inoculated against Covid and those who are eligible for a booster to receive their doses. He described the omicron variant as "a threat in front of us right now."

"When you look at the pace of the infections now, things will get worse as we go into the depth of the winter," he said. "And with omicron breathing down our back, things could get really bad, particularly for the unvaccinated."

Though omicron is more contagious than previous variants, it is not yet clear whether it causes more mild or severe disease than past strains.

Fauci said that those who are vaccinated and have taken their booster shots "will be relatively well protected at least against severe disease."

Earlier this week, the World Health Organization Director-General Tedros Adhanom Ghebreyesus said that at least 77 countries have reported cases of omicron, but that the variant is likely more widespread and has yet to be identified.

Tedros warned that vaccines alone will not protect countries from omicron, emphasizing the importance of masks and social distancing.

Fauci also said the U.S. is experiencing a resurgence of the delta variant.

"Delta is still a serious problem. I mean, we're looking over our shoulder at omicron, which will assume a dominant role very soon, I would imagine within a period of a few weeks as we go into January," Fauci explained.

"When you look at the delta that we're dealing with right now, we're having an increase in every parameter, an increase in cases, an increase in hospitalization and deaths are still at 1,100. So we are in for a difficult situation, particularly among those who are unvaccinated. They are very vulnerable," Fauci said.

CNBC's Spencer Kimball contributed to this report from New York.