The number of hospitalized young children infected with the coronavirus rose precipitously last week to the highest levels since the beginning of the pandemic, according to data released on Friday by the Centers for Disease Control and Prevention.

The increase was observed in children who were 4 and younger, who are not eligible for vaccination, and the data included children who were admitted to hospitals for reasons other than Covid.

The rise may be partly explained by the surge of Omicron cases, which affects all populations, and the spread of other respiratory infections.

But the data do not show a similar steep rise in coronavirus infections among hospitalized children of other ages, and federal health officials were considering the possibility that Omicron may not be as mild in young children as it is older children.

Children infected with the variant are still at much less risk of becoming severely ill compared with adults, and even young children seem less likely to need ventilators than those admitted during previous surges, doctors said.

“We have not yet seen a signal that there is any increased severity in this age demographic,” Dr. Rochelle Walensky, the C.D.C.’s director, told reporters at a news briefing on Friday.

More than four in 100,000 children ages 4 and younger admitted to hospitals were infected with the coronavirus as of Jan. 1 — double the rate reported a month ago and about three times the rate this time last year.

By contrast, the rate of hospitalized 5- to 11-year-olds with Covid was 0.6 per 100,000, roughly the same figure reported over past many months.

Dr. Walensky noted that only 16 percent of children from 5 to 11 had been fully vaccinated, and she urged everyone who was eligible for vaccines and boosters to receive them as soon as possible.

“Sadly, we are seeing the rates of hospitalizations increasing for children 0 to 4, who are not yet currently eligible for Covid-19 vaccination,” she said. “It’s critically important that we surround them with people who are vaccinated to provide them protection.”

The rise has been noticeable at a number of regional medical centers. The hospitalizations of young children now are “blowing away our previous Delta wave at the end of the summer, early fall, which had been our highest prior to that,” said Dr. Danielle Zerr, a pediatric infectious diseases expert at Seattle Children’s Hospital.

Experts are typically cautious about interpreting an increase in pediatric hospitalizations as a sign that a variant is particularly severe in children relative to adults. There were similar fears about the Delta and Beta variants, but the rise in pediatric hospitalizations then turned out to be more a consequence of the contagiousness of the variants.

This time, too, at least part of the increase in cases is a reflection of Omicron’s surge across all age groups. The nation is now recording roughly 600,000 cases on average per day, about one in five of them in children.

“The more kids that get infected, the more you’re going to have kids who are going to be sick enough to be hospitalized,” said Dr. Yvonne Maldonado, chair of the committee on infectious diseases at the American Academy of Pediatrics and a physician at Stanford University.

At Seattle Children’s Hospital, for example, about 21 percent of children are testing positive for the coronavirus, compared with the average of about 1 percent and a high during the Delta wave of about 3 percent.

“That is just a game changer,” Dr. Zerr said of the more recent figures.

Doctors may be quicker to admit a young child than an adult with similar symptoms, and that may account for some of the rising rates in young children. But some experts said the increase this time might be too steep to be explained only by the usual factors.

One alternative hypothesis for the rise may be that young children are particularly vulnerable to infections in the upper airway — exactly where Omicron is thought to be more concentrated in comparison with other variants.

“They’re smaller, their airways are smaller,” Dr. Kristin Oliver, a pediatrician at Mount Sinai Hospital in New York, said of young children.

“It does seem reasonable in a disease that if it looks like it’s affecting the upper airway more, that they would be more impacted,” she added. “They are more at risk for that — for longer, prolonged cases, as well as the hospitalization that can come along with a more severe case.”

That may explain why more hospitalized children aged 4 and younger have tested positive for the coronavirus throughout the pandemic than those 5 and older. It’s also why young children are more vulnerable to other pathogens, like respiratory syncytial virus, and to having the seal-like cough associated with croup.

For parents of young children, the numbers add another layer of worry as they wait for vaccines to become available.

Alicia Henriquez, a public-school teacher in Chicago, has three children, two of them younger than 5. Her younger children — Maxi, 4 and Sofia, 2 — are both infected with the coronavirus, but Maxi has no symptoms at all, and Sofia has only a cough.

Still, Ms. Henriquez said she was monitoring their symptoms carefully. “You don’t know if your child is going to end up in the hospital or not, so I think you still have to be careful,” she said.

The C.D.C.’s new data were collected by Covid-Net, the agency’s hospitalization surveillance network, which includes 14 sites and covers about 10 percent of the U.S. population. The rates are likely to be underestimates because of the lack of availability of tests, according to the agency.

Many children who become severely ill have other conditions or have weak immune systems. “Those kids are definitely at high risk right now,” Dr. Maldonado said. “We’re seeing more of them now than we were before.”

Dr. Julie Binder, a gastroenterologist in Philadelphia, has two daughters under 5. Her older daughter, Annie, 4, has an undiagnosed medical condition that results in “some very bizarre reactions” to viruses, Dr. Binder said. After a viral infection landed Annie in the hospital two years ago, she had complications for months.

Dr. Binder and her husband have been adamant about maintaining as normal a life for Annie as possible, even during the pandemic. But they have kept her home from day care the past couple of weeks to shield her from the post-holiday spike in infections.

“Hearing this information, I’m certainly glad that I did,” Dr. Binder said of the data released on Friday. “I would have felt much more comfortable through this wave right now if she had been vaccinated.”

A coronavirus vaccine is not yet available in the United States to children under 5, and is unlikely to be for a few more months. But many older children are also still unvaccinated.

Fewer than 25 percent of children from 5 to 11, and just over 60 percent of adolescents from 12 to 17, have received at least one dose of a coronavirus vaccine.

“If you’re really worried about your child getting sick, you should be vaccinating your child,” Dr. Maldonado said. “It’s the easiest thing we can do right now to keep our kids healthy.”

As of Oct. 31, about one in three children hospitalized with Covid was obese. Still, about half had no other known medical conditions, according to data collected by the C.D.C.