The number of hospitalized young children infected with the coronavirus rose precipitously last week to the highest levels since the beginning of the pandemic, according to data released on Friday by the Centers for Disease Control and Prevention.
The increase was observed in children who were 4 and younger, who are not eligible for vaccination, and the data included children who were admitted to hospitals for reasons other than Covid.
The rise may be partly explained by the surge of Omicron cases, which affects all populations, and the spread of other respiratory infections.
But the data do not show a similar steep rise in coronavirus infections among hospitalized children of other ages, and federal health officials were considering the possibility that Omicron may not be as mild in young children as it is older children.
Children infected with the variant are still at much less risk of becoming severely ill compared with adults, and even young children seem less likely to need ventilators than those admitted during previous surges, doctors said.
“We have not yet seen a signal that there is any increased severity in this age demographic,” Dr. Rochelle Walensky, the C.D.C.’s director, told reporters at a news briefing on Friday.
More than four in 100,000 children ages 4 and younger admitted to hospitals were infected with the coronavirus as of Jan. 1 — double the rate reported a month ago and about three times the rate this time last year.
By contrast, the rate of hospitalized 5- to 11-year-olds with Covid was 0.6 per 100,000, roughly the same figure reported over past many months.
Dr. Walensky noted that only 16 percent of children from 5 to 11 had been fully vaccinated, and she urged everyone who was eligible for vaccines and boosters to receive them as soon as possible.