As COVID-19 cases continue to surge nationwide, driven by the highly contagious omicron variant, Dr. Anthony Fauci, the nation's top infectious disease expert, warned that things are "gonna get worse before it gets better."

"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 Centers for Disease Control and Prevention (CDC), as of last week, the U.S. reported more than 51 million cases of COVID-19, as well as a seven-day moving average of more than 176,000 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."

"I think things will improve greatly as we get into January, but that doesn’t help us today and tomorrow," Dr. Fauci said on "This Week" on Sunday.

"Fortunately," Dr. Fauci said Monday, is that "what we've seen thus far is that if you're vaccinated and boosted, and you do get infected with a breakthrough infection, that your infection is either generally asymptomatic or mildly symptomatic."

According to the CDC, as of Thursday, Dec. 23, more than 241 million Americans have received at least one dose of a COVID-19 vaccine, or 72.7% of the population, and more than 204 million Americans, or 61.7%, are fully vaccinated against COVID-19.

That being said, while COVID cases soared, hospitalizations have not risen nearly as much as cases have, and are far below what they were during the delta variant's peak in September. A study from the U.K. suggests that compared to delta, people with omicron are 50-70% less likely to require hospitalization.

However, Dr. Fauci warned that people should not get complacent, 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."

New York City has also seen a major spike in hospitalization, including a fourfold increase in hospital admissions for children 18 and under. Dr. Fauci said that he's spoken to colleagues in South Africa who have seen increased hospitalization among children, though not necessarily more severe dease.

"We have to keep an eye on that," he said Monday. "That's one of the reasons why we say now that we have the capability and the authority to vaccinate children five to 11, that we're encouraging parents, that if you have a child from five to 11, please get that child vaccinated to prevent them from getting anything that even resembles a serious illness."

When asked if the authorization of the Pfizer and Merck COVID-19 antiviral COVID-19 pills were a breakthrough in the battle against the pandemic, Dr. Fauci said that they are "part of the comprehensive approach to this outbreak."

"Vaccines and boosters, masks and now, very importantly, a highly effective therapy is really going to make a major, major difference,” Fauci replied. “We've just got to make sure that there's the production of enough of that product that we can get it widely used for those who need it as quickly as possible."

Fauci said that the production of the pill will "absolutely" be a top priority of the Biden administration going forward, including possible use of the Defense Production Act to bolster manufacturing: "We've got to get that product into the mouths of those who need it."

As the U.S. is set to roll out 500 million free at-home COVID-19 tests, Dr. Fauci said Sunday that the country has "obviously got to do better" about their availability.

"The beginning of the year, there were essentially no rapid point of care home tests available," he said. "Now, there are over nine of them and more coming."

"The production of them has been rapidly upscaled, and yet because of the demand that we have, which in some respects ... is good, that we have a high demand because we should be using testing much more extensively than we have," he continued. "But the situation where you have such a high demand, a conflation of events, omicron stirring people to get appropriately concerned and wanting to get tested as well as the fact of the run on tests during the holiday season, we've obviously got to do better."

"I think things will improve greatly as we get into January," Dr. Fauci said. "But that doesn't help us today and tomorrow."