The highly transmissible Omicron virus variant is sending daily U.S. caseloads soaring to levels higher than last year’s winter pandemic peak.

Hospitalizations are starting to tick up, too, although not at the same rate as cases. It is unclear if they will continue to follow the rise in cases, especially given evidence in South Africa and Europe that Omicron may cause fewer severe cases of Covid.

On Friday, before holiday interruptions to data reporting began to affect the nation’s daily case totals, the seven-day national average of new daily cases surpassed 197,000, a 65 percent jump over the last 14 days, and hospitalizations reached a seven-day average of more than 70,000, an increase of 10 percent. Deaths also increased by 3 percent during that time, to a seven-day average of 1,345, according to a New York Times database.

The national all-time high for average daily cases is 251,232, set in January during a post-holiday surge.

On Sunday, Dr. Anthony S. Fauci, the nation’s top infectious disease expert, said a growing body of evidence suggested that Omicron was causing less serious illness than its predecessors. But he warned against complacency, saying the variant’s lightning-speed spread across the United States would likely lead to a perilous spike in hospitalizations among the unvaccinated and could overwhelm the country’s health systems.

“When you have such a high volume of new infections, it might override a real diminution in severity,” Dr. Fauci said during an interview on ABC’s “This Week.”

In New York, Gov. Kathy Hochul declared a state of emergency earlier this month and put elective surgeries on pause at many hospitals. This week, Gov. Charlie Baker of Massachusetts said he would activate up to 500 members of the National Guard to help in overburdened hospitals. Many other states have done the same.

From Dec. 5, there has been a fourfold increase of Covid hospital admissions among children in New York City, where the Omicron variant is spreading rapidly, the New York State Department of Health said in an advisory on Friday. About half were under the age of 5, and not eligible for vaccination. The city did not provide numbers, but state data showed a few dozen children under 5 were hospitalized across the state as of Thursday.

The jump in pediatric cases is evident in other states as well. The American Academy of Pediatrics reported last week that Covid cases were “extremely high” among those under 18 across the country. Citing data as of Dec. 16, the academy said that cases among those under 18 had risen by 170,000 from the prior week, an increase of nearly 28 percent since early December. Pediatric cases are higher than ever before in the Northeast and Midwest, the data show, and all regions of the country have significantly more such cases since schools reopened for in-person instruction in the fall.

Even with the rising cases, government data show that vaccination is still a strong protector against severe illness. Unvaccinated people are five times more likely to test positive and 14 times more likely to die of Covid than vaccinated patients, according to the Centers for Disease Control and Prevention.

Promising data out of South Africa and other European countries have also shown that Omicron surges have been milder and with fewer hospitalizations.

The new research is heartening, but experts warn that the surge coming to many countries still may flood hospitals.

“Each place has its own demographics and health care system access and, you know, vaccine distribution,” Akiko Iwasaki, an immunologist and researcher at the Yale School of Medicine, said in an interview on Saturday. She added that people in England, Scotland and South Africa could have acquired enough immunity from other infections to be able to deal with this variant, or that there could be intrinsic differences in the pathogenicity of Omicron that results in fewer people needing to be hospitalized.

“We cannot assume the same things will happen to the U.S.,” Dr. Iwasaki said. “That is not a reason to relax our measures here, and we still need to vaccinate those pockets of people who are unvaccinated.”