The acting head of the Food and Drug Administration warned Tuesday that most people in the United States will contract COVID-19, as the country grapples with record levels of infections and hospitalizations.

"I think it's hard to process what's actually happening right now, which is most people are going to get COVID," Dr. Janet Woodcock, the acting commissioner of the FDA, said while testifying before the Senate Committee on Health, Education, Labor and Pensions on Capitol Hill in Washington, D.C. "What we need to do is make sure the hospitals can still function, [and] transportation, you know, other essential services are not disrupted while this happens."

New COVID-19 cases increased 55% worldwide in the week ending Jan. 9, the World Health Organization said on Tuesday.

More than 15 million new cases were reported, up from the about 9.5 million in the previous week, the United Nation's health agency said in its weekly epidemiological update. Omicron is "rapidly replacing" other variants of the virus, according to the update.

"In terms of disease severity, there is growing evidence that the Omicron variant is less severe as compared to other variants," the WHO said.

Last week's increase marked a slowdown from the 71% spike reported the previous week, following a "gradual" rise since October, according to the update.

Reported cases in the Americas increased 78%, down from a 100% increase the week prior. The greatest increase in new cases came in Southeast Asia, where newly reported cases increased 418%, the WHO said.

On average, the U.S. is now reporting more than 750,000 new COVID-19 cases per day, a more than sixfold increase compared to one month ago, according to federal data.

On Monday, the U.S. recorded a single-day high of more than 1.4 million new cases, following soaring demand for tests and a backlog of weekend reporting data.

The surging national case numbers may not be indicative of what is happening in every region of the country and the overall increase in the U.S. does not necessarily mean that some regions will not see a plateau or decline in numbers.

Although the incredibly high case totals are most certainly due, in large part, to the country's latest omicron surge, the data is incredibly volatile on a day-to-day level. Many states no longer report data over the weekends, which can create an illusion of very low totals on weekends and very high daily numbers on Mondays and Tuesdays.

Also, there was a major surge in demand over the holidays, which could artificially increase numbers. However, experts say overall totals are likely undercounted given the increased use of at-home COVID-19 testing.

-ABC News' Arielle Mitropoulos

In Los Angeles, the hard-hit Providence Cedars-Sinai Tarzana Medical Center is "in complete disaster mode," Dr. Eric Snyder, medical director of the emergency department, told ABC News.

The emergency room is overflowing, with 22 patients currently waiting for beds.

"These are patients who’ve been accepted by a doctor, but have no place to go," he said.

Snyder said the hospital is in far worse shape than at any point in the pandemic. Tents have been set up behind the hospital and a doctor is going patient to patient, hoping to send those home who don't need hospital care, he said.

“We are 100% broken," Snyder said.

-ABC News' Matt Gutman

Following heated exchanges between Dr. Anthony Fauci and several Republican senators at a Congressional hearing Tuesday, more than 200 prominent science and public health leaders have now penned an open letter voicing their support of the White House chief medical adviser's service and leadership -- and condemning attacks against him. 

"We deplore the personal attacks on Dr. Fauci," the letter says. "The criticism is inaccurate, unscientific, ill-founded in the facts and, increasingly, motivated by partisan politics. It is a distraction from what should be the national focus -- working together to finally overcome a pandemic that is killing about 500,000 people a year."

Signatories include former Senate majority leader Bill Frist, a Republican; Rich Besser, former acting director for the Centers for Disease Control and Prevention; members of President Joe Biden's transition COVID-19 task force; and several Nobel laureates.

The letter comes two days after a Health, Education, Labor and Pensions committee hearing, during which Fauci publicly accused Republican Sen. Rand Paul of fomenting the violent threats and harassment that he and his family have had to contend with during the pandemic's politicized climate.

-ABC News' Sasha Pezenik