SEATTLE — The omicron-fueled surge that is sending COVID-19 cases rocketing in the U.S. is putting children in the hospital in close to record numbers, and experts lament that most of the youngsters are not vaccinated.

“It’s just so heartbreaking,” said Dr. Paul Offit, an infectious-disease expert at Children’s Hospital of Philadelphia. “It was hard enough last year, but now you know that you have a way to prevent all this.”

During the week of Dec. 21-27, an average of 334 children 17 and under were admitted per day to hospitals with the coronavirus, a 58 percent increase from the week before, according to the Centers for Disease Control and Prevention.

The previous peak over the course of the pandemic was in early September, when child hospitalizations averaged 342 per day, the CDC said.

On a more hopeful note, children continue to represent a small percentage of those being hospitalized with COVID-19: An average of over 9,400 people of all ages were admitted per day during the same week in December. And many doctors say the youngsters coming in now seem less sick than those seen during the delta surge over the summer.

Two months after vaccinations were approved for 5- to 11-year-olds, about 14 percent are fully protected, CDC data shows. The rate is higher for 12- to 17-year-olds, at about 53 percent.

The issue is timing in many cases, said Dr. Albert Ko, professor of epidemiology and infectious diseases at the Yale School of Public Health. Younger children were not approved for the vaccine until November, and many are only now coming up on their second dose, he said.

Offit said none of the vaccine-eligible children receiving care at his hospital about a week ago had been vaccinated, even though two-thirds had underlying conditions that put them at risk – either chronic lung disease or, more commonly, obesity. Only one was under the vaccination age of 5.

The scenes are heart-rending.

“They’re struggling to breathe, coughing, coughing, coughing,” Offit said. “A handful were sent to the ICU to be sedated. We put the attachment down their throat that’s attached to a ventilator, and the parents are crying.”

None of the parents or siblings was vaccinated either, he said.

The next four to six weeks are going to be rough, he said: “This is a virus that thrives in the winter.”

Aria Shapiro, 6, spent her 12th day Thursday at Phoenix Children’s Hospital. She tested positive for COVID-19 after getting her first dose of the vaccine Dec. 17.

Aria, who is considered “medically fragile” because she has epilepsy, suffered prolonged seizures in the hospital, and a breathing tube had to be put down her throat at one point, though she has since improved.

“We lived our life in for two years to prevent her from getting COVID, finally went for the vax, and the one thing that we didn’t want to happen happened,” said her mother, Sarah Shapiro. “It wasn’t enough time for her body to build antibodies. She did end up getting COVID.”

Overall, new COVID-19 cases in Americans of all ages have skyrocketed to the highest levels on record: an average of 300,000 per day, or 2 1/2 times the figure just two weeks ago. The highly contagious omicron accounted for 59 percent of new cases last week, according to the CDC.

Still, there are early indications that the variant causes milder illness than previous versions, and that the combination of the vaccine and the booster seems to protect people from its worst effects.

In California, 80 COVID-19-infected children were admitted to the hospital during the week of Dec. 20-26, compared with 50 in the last week of November, health officials said.

Seattle Children’s also reported a bump in the number of children admitted over the past week. And while they are less seriously ill than those hospitalized over the summer, Dr. John McGuire cautioned that it is early in the omicron wave, and the full effects will become apparent over the next several weeks.

New York health authorities have also sounded the alarm.

The number of children admitted to the hospital per week in New York City with COVID-19 went from 22 to 109 between Dec. 5 and Dec. 24. Across all of New York state, it went from 70 to 184. Overall, almost 5,000 people in New York were in the hospital with COVID-19.

“A fourfold increase makes everybody jump with concern, but it’s a small percentage,” Ko said of the New York City figures. “Children have a low risk of being hospitalized, but those who do are unvaccinated.”

Dr. Al Sacchetti, chief of emergency services at Our Lady of Lourdes Medical Center in Camden, New Jersey, likewise said vaccinated children are handling the omicron outbreak extremely well.

“It makes a big difference in how these kids tolerate the disease, particularly if the child’s got some medical issues,” he said.

COVID-19 deaths have proved rare among children over the course of the pandemic. As of last week, 721 in the U.S. had died of the disease, according to data reported to the American Academy of Pediatrics. The overall U.S. death toll is more than 800,000.

Almost 199,000 child COVID-19 cases were reported during the week of Dec. 16-23, the pediatrics group said. That was about 20 percent of the more than 950,000 total cases recorded that week.

While many of these children will recover at home, they may have contact with others who are at much greater risk, said Dr. Jason Terk, a pediatrician in North Texas. He cared for a 10-year-old boy with COVID-19 who managed the disease well, but his father got sick and died, he said.

“The death of a parent is devastating, but the toxic stress for a young person in this situation is difficult to measure,” he said.