Dr. Anthony Fauci, the nation's top infectious disease expert, said a growing body of evidence suggests that the omicron variant of the coronavirus is 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.

Speaking on ABC's "This Week" on Sunday, Fauci said recent data out of Scotland, England and South Africa has been filling in the fragmentary portrait of omicron, which has spread across much of the world and overtaken the delta variant in the United States in the month since it was first identified by scientists in South Africa.

"Even though we're pleased by the evidence from multiple countries — it looks like there is a lesser degree of severity — we've got to be careful that we don't get complacent about that," Fauci said, noting that there are still tens of millions of unvaccinated Americans. "Those are the most vulnerable ones when you have a virus that is extraordinarily effective in getting to people and infecting them the way omicron is."

Last week, scientists at the University of Edinburgh in Scotland reported that people infected with omicron were almost 60% less likely to be hospitalized than those infected with delta. Another study from Imperial College London found that individuals infected by omicron were 15 to 20% less likely to go to an emergency room with severe symptoms and 40% less likely to be hospitalized.

Despite such encouraging data, Fauci said the nation's low vaccination rate — only 62% of Americans are fully vaccinated — would likely dilute the benefits of omicron's reduced virulence. "When you have such a high volume of new infections, it might override a real diminution in severity," he said.

Nearly 71,000 Americans are now hospitalized with COVID-19, up 10% from the previous week but still well below previous peaks.

That said, the nation's medical infrastructure is dangerously frayed two years into the pandemic as hospitals contend with staff shortages fueled by burnout and early retirements. Experts also worry about a coming wave of omicron infections that could sideline an untold number of nurses and doctors.

Despite an alarming spike of cases in the United States — the seven-day average of new daily cases has surpassed 197,000, a 65% jump over the past 14 days — 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 CDC.

Omicron cases overtake delta

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% jump over the past 14 days, and hospitalizations reached a seven-day average of more than 70,000, an increase of 10%. Deaths also increased by 3% 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.

In New York, Gov. Kathy Hochul declared a state of emergency earlier this month and put elective surgeries on pause at many hospitals. Last 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 Friday. About half were younger than 5 and not eligible for vaccination. The city did not provide numbers, but state data showed a few dozen children younger than 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 younger than 18 across the country. Citing data as of Dec. 16, the academy said that cases among those younger than 18 had risen by 170,000 from the prior week, an increase of nearly 28% 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 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.," 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."