SEATTLE - The omicron-fueled surge that is sending COVID-19 cases rocketing in the U.S. is putting children in the hospital in 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. 22-28, an average of 378 children 17 and under were admitted per day to hospitals with the coronavirus, a 66% increase from the week before, the Centers for Disease Control and Prevention reported Thursday.

The previous high 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 nearly 10,200 people of all ages were admitted per day during the same week in December. And many doctors say the youngsters seem less sick than those who came in during the delta surge over the summer.

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

A study released Thursday by the CDC confirmed that serious side effects from the Pfizer vaccine in children ages 5 to 11 are rare. The findings were based on approximately 8 million doses dispensed to youngsters in that age group.

Dr. Albert Ko, professor of epidemiology and infectious diseases at the Yale School of Public Health, noted that the low vaccination rate is, in part, a matter of timing: Younger children were not approved for the vaccine until November, and many are only now coming up on their second dose.

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.”

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 2½ times the figure just two weeks ago. The highly contagious omicron accounted for 59% 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.