“Vaccination with a primary series among this age group has lagged behind other age groups, leaving them vulnerable to serious illness,” Dr. Rochelle Walensky, director of the C.D.C., said.

“With over 18 million doses administered in this age group, we know that these vaccines are safe, and we must continue to increase the number of children who are protected.”

The booster shot would be the third dose available for most children and the fourth dose for some immunocompromised children.

A booster dose will offer children an extra layer of protection at a time when infections and hospitalizations are once again rising nationally, scientific advisers to the agency concluded at a meeting on Thursday.

“It is sobering that we have experienced over a million deaths in the U.S. as a consequence of Covid infection,” Dr. Grace Lee, a pediatrician at Stanford University and chair of the Advisory Committee on Immunization Practices, which sets recommendations for the use of vaccines, told other panel members.

In November, the committee recommended booster shots for adults and in January did so for children 12 and older.

Pfizer and BioNTech reported in April that in children aged 5 to 11, a third dose generated antibodies against both the Omicron variant and the original version of the coronavirus. In the trial, the children received 10 micrograms of vaccine — one-third of the dose given to adolescents and adults — in each shot.

As with the first two doses, the booster appeared safe, the companies reported. The most commonly reported side effects were pain, redness and swelling at the injection site as well as aches, chills and fever.

Based on these data, the Food and Drug Administration authorized the boosters for children aged 5 to 11 on Tuesday.

About 70 percent of children in this age group have evidence of prior infection and may have some immunity as a result, Dr. Sara Oliver, a C.D.C. scientist, noted at the science advisers meeting.

But some evidence suggests that compared with vaccination, protection following an Omicron infection may be weaker and may not last as long.

“Prior infection can result in protection against reinfection for a time period, but it’s not 100 percent and likely decreases over time,” Dr. Oliver said.

The rise in pediatric hospitalizations during the Omicron surge suggests that immunity gained from infection is “not sufficient to provide a broad population-level protection,” she added.

The committee members debated whether to recommend that all children aged 5 to 11 “should” receive a booster or only that they “may” do so if their parents or health care providers deem it to be necessary.

Ultimately, the experts voted for the stronger recommendation, after several committee members argued that there was enough evidence suggesting that a booster dose was broadly beneficial in all age groups. Dr. Walensky endorsed the recommendation later in the day.

The extra dose may boost immunity to the current Omicron variants in young children. Studies have shown that two doses of the vaccine offer virtually no barrier against infection with the Omicron variant in children aged 5 to 11, although protection against severe disease remains strong.

In adolescents aged 12 to 17, two doses offered little protection even against hospitalization, but a booster significantly improved effectiveness of the vaccines.

Many parents have hesitated to immunize their children, in part because they are at much lower risk of severe disease than adults. Fewer than one-third of 5- to 11-year-olds in the United States have received two doses. The rates were lower among children from communities of color and low-income families and those living in rural areas.

But record numbers of children were hospitalized during the Omicron surge this winter. Nearly 4,000 children aged 5 to 11 have been diagnosed with a Covid-related condition called multisystem inflammatory syndrome during the pandemic. And some studies find that even children who have a mild illness may experience symptoms for months.

Covid-19 has been responsible for more deaths in children aged 5 to 11 than many other vaccine-preventible diseases, noted Dr. Matthew Daley, a senior investigator at Kaiser Permanente Colorado who heads the C.D.C.’s Covid-19 vaccine working group.

“Most parents accept vaccination for hepatitis A, meningococcal, varicella, rubella and rotavirus, even though deaths from these diseases are relatively rare,” he said.

At the science advisers meeting, Dr. Doran Fink, a deputy director for the F.D.A.’s vaccine division, acknowledged the “continued intense interest in the availability of Covid vaccines” for children younger than age 5.

He said that agency scientists were working to quickly review data on the Moderna vaccine’s effects in the youngest children and were awaiting an application from Pfizer and BioNTech for their vaccine’s use in this age group.

Correction: 

May 19, 2022

An earlier version of this article incorrectly referred to the numbers of children aged 5 to 11 with multisystem inflammatory syndrome. About 4,000 have been diagnosed, not died, with the syndrome.