Hospitalizations of U.S. children under five with COVID-19 soared in recent weeks to their highest level since the pandemic began, according to government data released Friday on the only age group not yet eligible for the vaccine.

The worrisome trend in children too young to be vaccinated underscores the need for older kids and adults to get their shots to help protect those around them, said Doctor Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention (CDC).

Since mid-December, with the highly contagious Omicron variant spreading furiously around the country, the hospitalization rate in these youngest kids has surged to more than four in 100,000 children, up from 2.5 per 100,000.

The rate among children ages 5 to 17 is about 1 per 100,000, according to the CDC data, which is drawn from over 250 hospitals in 14 states.

Overall, "pediatric hospitalizations are at their highest rate compared to any prior point in the pandemic," Walensky said.

She noted that just over 50 percent of children ages 12 to 18, and only 16 percent of those 5 to 11, are fully vaccinated.

The overall hospitalization rate among children and teens is still lower than that of any other age group. And they account for less than 5 percent of average new daily hospital admissions, according to the CDC.

The severity of illness among children during the Omicron wave seems lower than it was with the Delta variant, said Doctor John McGuire, Seattle Children's Hospital critical care chief.

"Most of the COVID positive kids in the hospital are actually not here for COVID-19 disease," McGuire said in an email. "They are here for other issues but happen to have tested positive."

The nation's top infectious-disease expert, Doctor Anthony Fauci, said earlier this week that Omicron appears to cause less-severe disease across the board but that the sheer number of infections, because of its extreme contagiousness, will mean that many more children will get infected, and a certain share of them will wind up in the hospital.

Fauci also said many children hospitalized with COVID-19 have other health conditions that make them more susceptible to complications from the virus. That includes obesity, diabetes and lung disease.

Fauci and Walensky have emphasized that one of the best ways to protect the youngest children is to vaccinate everyone else.