Data released by federal health officials shows that the omicron variant of the coronavirus accounts for about 59% of new cases in the U.S. for the week ending Dec. 25, up from 23% the week prior.
The newly released data from the Centers for Disease Control and Prevention (CDC) shows omicron (58.6%) overtaking the delta variant (41.1%) for the first time.
The CDC had said last week that omicron already accounted for a majority of new cases in the country, but on Tuesday it significantly lowered that previous estimate based on additional data it collected.
Still, it noted that omicron is accounting for a growing proportion of cases, and health officials warn that cases will likely continue to rise.
"Every day it goes up and up," Dr. Fauci, President Joe Biden's chief medical adviser, told ABC's "This Week" on Sunday. "The last weekly average was about 150,000 and it likely will go much higher."
According to data from the CDC, on Sunday, the U.S. reported 164,644 new cases, as well as a seven-day moving average of more than 206,577 cases. The U.S. has seen more than 816,000 deaths from the coronavirus, according to data from Johns Hopkins University.
On Monday, speaking to ABC's "Good Morning America," Dr. Fauci said that it's "very difficult to predict" when the omicron wave will peak, in part because the U.S. has "so many unvaccinated people ... who are really quite vulnerable."
"The way it's going up right now, it's going to get worse before it gets better, that's for sure," he said. "We don't expect things are going to turn around in a few days to a week, it likely will take much longer than that. But that's unpredictable."
Fauci said Monday that it's "possible" that cases peak in January, but said that it depends on what happens in the coming weeks.
"South Africa went way, way up and then came back down," Fauci said, but noted that. "The U.K. is still going up. Hopefully they'll turn around because we usually lag somewhat behind them temporarily. In other words, what happens there generally happens here, a couple two, three weeks later."
The rapid spread comes after the first confirmed case of omicron in the U.S. was identified earlier this month. Studies have provided early hints that it is milder than the delta variant.
However, Dr. Fauci warned on Sunday that people should not get complacent based on those findings, adding: "When you have such a high volume of new infections, it might override a real diminution in severity."
"If you have many, many, many more people with a less level of severity, that might kind of neutralize the positive effect of having less severity when you have so many more people," he said on "This Week" on Sunday, adding that health experts are "particularly worried" about people who are unvaccinated. "Those are the most vulnerable ones when you have a virus that is extraordinarily effective in getting to people."
As of Monday, more than 242 million Americans, or 73% of the U.S. population, have received at least one dose of a COVID-19 vaccine, with more than 205 million, or 61.8%, fully vaccinated, according to the CDC. More than 66 million booster doses have been administered.
The Associated Press contributed to this report.