Biden urges states to restore mask mandates as the C.D.C. director warns of an ‘impending doom’ over a possible virus surge.

“I am asking you to just hold on a little longer, to get vaccinated when you can, so that all of those people that we all love will still be here when this pandemic ends,” she said at a White House briefing.

According to a New York Times database, the seven-day average of new virus cases as of Sunday was about 63,000, a level comparable to late October, and up from 54,000 a day two weeks earlier, an increase of more than 16 percent. Similar upticks in the past over the summer and winter led to major surges in the spread of the virus, Dr. Walensky said. Still, new cases and deaths have declined from the early January peak, though the seven-day average of new deaths remains near 1,000 a day.

Federal health officials have expressed concerned about the spread of variants, as the United States remains behind in its attempts to track them, though the C.D.C.’s efforts to locate them has recently improved and will continue to grow. Some scientists predicted weeks ago that the number of infections could curve upward again in late March, at least in part because of the rise of variants of the coronavirus across the country. The variant that walloped Britain, called B.1.1.7, has led to a new wave of cases across most of Europe. B.1.1.7 is also rising exponentially in Florida where it accounts for a greater proportion of total cases than in any other state, according to numbers collected by the C.D.C.

Dr. Walensky also noted an increase in travel. Over the last week, an average of 1.3 million people passed through security checkpoints at U.S. airports each day, according to the Transportation Security Administration. On Sunday alone, more than 1.5 million people went through T.S.A. screenings — a sharp increase from 180,000 on the same date in 2020.

“I think people want to be done with this,” she said, but what’s different this time is “we actually have it in our power to be done, with the scale of the vaccination. And that will be so much slower if we have another surge to deal with as well.”

The wave of new cases comes as the nation rapidly broadens eligibility for vaccines, the average number of daily shots continues to rise, and a new C.D.C. report released Monday confirmed the findings of last year’s clinical trials that vaccines developed by Moderna and Pfizer are highly effective against Covid-19. The report documented that the vaccines work to prevent symptomatic and asymptomatic infections under real-world conditions.

The seven-day average of vaccines administered hit 2.7 million on Sunday, a slight increase over the pace the previous week, according to data reported by the C.D.C. But worrisome hot spots continue popping up.