The number of U.S. children under 5 hospitalized with COVID-19 has surged in recent weeks to the highest level since the pandemic began, according to government data released Friday for the only age group not yet eligible for the vaccine.

Dr. Rochelle Varensky, director of the Centers for Disease Control and Prevention, said the worrying trend of children too young to be vaccinated underscores the need for older children and adults to get vaccinated to protect those around them.

Since mid-December, hospitalization rates for these youngest children have soared from 2.5 per 100,000 to more than 4 per 100,000 as the highly contagious variant of the omicron spreads wildly across the country .

By comparison, the current rate for children ages 5 to 17 is about 1 in 100,000, according to the Centers for Disease Control and Prevention.

Valensky said in a statement that while hospitalization rates for children remain the lowest of any age group, “the rate of pediatric hospitalizations is the highest compared to any time before the pandemic.”

She told the briefing that the figures included children hospitalized with COVID-19 and children admitted for other reasons who were found to be infected.

She noted that just over 50% of children aged 12 to 18 were fully vaccinated, and only 16% of children aged 5 to 11 were fully vaccinated.

As of Tuesday, the average daily number of children and teens hospitalized with COVID-19 was 766, double the number reported two weeks earlier.

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At a White House briefing this week, Dr. Anthony Fauci, the nation’s top infectious disease expert, said many children hospitalized with COVID-19 have other health conditions that make them more vulnerable to complications from the virus. This includes obesity, diabetes and lung disease.

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

The data suggest that booster shots offer the best protection against omicron, and the CDC this week recommended booster shots for children as young as 12. Among already eligible older adults, only 34 percent received the injection.

The surge in hospitalizations has only heightened parental concerns about how to keep infants and young children safe.

Emily Hojara and Eli Zilke of Sawyer, Michigan are taking extra precautions to protect their daughter Flora, who will turn 2 in May. They limit her contact with other children and do not allow visitors into the house, not even grandparents, unless they are masked.

“It’s a struggle, and now with this new variant, I think it’s holding us back,” Hohara said. The new hospitalization data “just reminds you that anxiety is very close,” she said.

“It’s scary that she can’t get vaccinated,” Hojara said of her daughter.

Dr. Jennifer Kusma, a pediatrician at Lurie Children’s Hospital in Chicago, said she sees an increasing number of children being hospitalized for omicron use, and while most children aren’t seriously ill, she understands parents’ concerns.

“As a pediatrician, I really wish we had this vaccine for these young kids,” Kusma said. But she added that the seemingly long wait should reassure parents that vaccine testing will not be rushed.

Many had hoped the new year would bring a vaccine to young children, but Pfizer announced last month that two doses of the vaccine did not provide the hoped-for protection for teens aged 2 to 4.

Pfizer’s study has been updated to provide a third dose for everyone under the age of 5, with data expected in early spring.