• The U.S. is facing "impending doom" as daily Covid-19 cases begin to rebound once again, CDC Director Dr. Rochelle Walensky said during a press briefing.
• The U.S. is recording a weekly average of 63,239 new Covid-19 cases per day, a 16% increase compared with a week ago, according to an analysis of data compiled by Johns Hopkins University.
• When cases rise as they have over the last week or so, Walensky said they often "surge and surge big" shortly thereafter.

The U.S. is facing "impending doom" as daily Covid-19 cases begin to rebound once again, threatening to send more people to the hospital even as vaccinations accelerate nationwide, the head of the Centers for Disease Control and Prevention said Monday.

"When I first started at CDC about two months ago I made a promise to you: I would tell you the truth even if it was not the news we wanted to hear. Now is one of those times when I have to share the truth, and I have to hope and trust you will listen," CDC Director Dr. Rochelle Walensky said during a press briefing.

"I'm going to pause here, I'm going to lose the script, and I'm going to reflect on the recurring feeling I have of impending doom," Walensky said. "We have so much to look forward to, so much promise and potential of where we are and so much reason for hope, but right now I'm scared."

The U.S. is recording a weekly average of 63,239 new Covid-19 cases per day, a 16% increase compared with a week ago, according to a CNBC analysis of data compiled by Johns Hopkins University. Daily cases are now growing by at least 5% in 30 states and the District of Columbia.

Coronavirus hospitalizations are also climbing. The U.S. is reporting a seven-day average of 4,816 Covid-19 hospital admissions as of Friday, a 4.2% increase compared with the week prior, according to CDC data.

Walensky urged Americans to "just hold on a little longer" and to get vaccinated against the virus once it's their turn. When cases rise as they have over the last week or so, Walensky said, they often "surge and surge big" shortly thereafter.

"I'm speaking today not necessarily as your CDC director and not only as your CDC director, but as a wife, as a mother, as a daughter, to ask you to just please hold on a little while longer," Walensky said.

Leading public health experts have warned since late February that infections could pick back up again amid the rise of virus variants threatening to sweep across the U.S. much like they did in Europe.

One of those variants first identified in the U.K, known as B.1.1.7, has now been detected in every state except Oklahoma, according to the CDC's most recent data. The agency is also carefully watching another variant found in New York City, known as B.1.526, which is also thought to be more transmissible compared with previous strains, Walensky said last week.

The White House's chief medical advisor, Dr. Anthony Fauci, said Sunday, however, that the troublesome virus mutations aren't the only reason cases are on the rise.

More Americans, tired of pandemic restrictions and reassured by the lifesaving vaccines, are traveling for spring break. Some state leaders are pulling back on restrictions, including mask mandates, intended to slow the virus's spread.

"Variants we take seriously and are concerned, but it is not only the variants that are doing that," Fauci told CBS' "Face the Nation" on Sunday.

Vaccine rollout accelerates

The grim warning from Walensky followed what was an otherwise optimistic update on the nation's vaccine rollout.

The U.S. is administering a weekly average of 2.7 million shots per day, which is "significant progress" toward President Joe Biden's fresh goal of administering 200 million shots in his first 100 days in office, said Andy Slavitt, White House senior advisor for Covid response.

"This is good news. We're headed in the right direction, but we can't slow down. Millions remain unvaccinated and at risk," Slavitt said.

More than 72% of Americans ages 65 and older have now received at least one dose of a vaccine, while nearly half of that age group is considered fully vaccinated. More than a third of all adult Americans have now received at least one shot, CDC data shows. 

A new study from the agency on Monday found that the vaccines from Pfizer and Moderna are proving to be highly effective, even with just one dose.

The study, which examined nearly 4,000 health-care personnel, first responders and front-line workers between Dec. 14 and March 1, found that the vaccines were 80% effective in preventing coronavirus infections after just a single dose.

However, federal health officials maintained that two doses are better than one, adding that the vaccines' effectiveness jumped to 90% two weeks after the second shot.