The researchers also tallied Covid-19 hospitalizations among children ages 12 to 17 from March 1, 2020, to April 24, 2021. The data came from Covid-Net, a population-based surveillance system in 14 states, covering about 10 percent of Americans.

The number of adolescents hospitalized with Covid-19 declined in January and February of this year, but rose again in March and April. From Jan. 1, 2021, to March 31, 204 adolescents were likely hospitalized primarily for Covid-19. Most of the children had at least one underlying medical condition, such as obesity, asthma or a neurological disorder.

The rate may have increased this spring because of the more contagious variants of the coronavirus in circulation, as well as school reopenings that brought children together indoors, and looser adherence to precautions like wearing masks and social distancing, the researchers said.

None of the children died, but about one-third were admitted to the intensive care unit, and 5 percent required invasive mechanical ventilation. Roughly two-thirds of the hospitalized adolescents were Black or Hispanic, reflecting the greater risk posed by the virus to these populations.

The researchers compared the numbers for Covid-19 with hospitalizations for flu in the same age group during the 2017-18, 2018-19 and 2019-20 flu seasons. From Oct. 1, 2020, to April 24, 2021, hospitalization rates for Covid-19 among adolescents were 2.5 to 3 times the rate for seasonal flu in previous years.

The data lend urgency to the drive to get more teenagers vaccinated, said Dr. Walensky, who added that she was “deeply concerned” by the numbers.

The Food and Drug Administration approved the Pfizer-BioNTech coronavirus vaccine for children ages 12 to 15 on May 12. The vaccine was approved for anyone older in December.