SEATTLE, United States (AP) — The omicron-fuelled surge that is sending COVID-19 cases rocketing in the US 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 December 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 hospitalisations averaged 342 per day, the CDC said.

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 five.

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.

In California, 80 COVID-19-infected children were admitted to the hospital during the week of December 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 hospitalised 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 December 5 and December 24.

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