WASHINGTON — Federal regulators evaluated for the first time on Friday the safety and efficacy of a coronavirus vaccine for children 5 to 11, saying that the benefits of staving off Covid-19 with the Pfizer-BioNTech vaccine generally outweighed the risks of the most worrisome possible side effects in that age group.

The analysis came on the same day that the Food and Drug Administration posted data from Pfizer showing that the vaccine had a 90.7 percent efficacy rate in preventing symptomatic Covid-19 in a clinical trial of 5- to 11-year-olds.

The findings could add momentum for F.D.A. authorization of the pediatric dose on an emergency basis, perhaps as early as next week, opening up a long-awaited new phase of the nation’s vaccination campaign. The agency’s independent vaccine expert committee is set to vote Tuesday on whether to recommend authorization.

In a briefing document posted on the F.D.A. website, the agency said it had balanced the dangers of hospitalization, death or other serious consequences from Covid-19 against the risk of myocarditis. A rare condition involving inflammation of the heart muscle, myocarditis has been linked to the Pfizer-BioNTech and Moderna vaccines, especially among young men.

“The overall analysis predicted that the numbers of clinically significant Covid-19-related outcomes prevented would clearly outweigh the numbers of vaccine-associated excess myocarditis cases,” regulators wrote.

As is customary before a vote of the F.D.A.’s advisory committee, the regulators took no stance on whether the new use of a vaccine should be authorized.

If the F.D.A. rules in favor of authorization and the Centers for Disease Control and Prevention and its own panel of vaccine experts agree, the 28 million children in that age group could become eligible for shots in the first week of November.

“There’s a lot of data to be encouraged by,” said Dr. Kathryn M. Edwards, a professor of pediatrics in the division of infectious diseases at Vanderbilt University School of Medicine. She said the results exceeded the protection offered by the best flu vaccine and could eventually lead to the easing of restrictions intended to prevent elementary school children from contracting the virus.

The Biden administration has been eagerly promoting the prospect of a pediatric shot, and many parents are anxiously awaiting the development. Covid-19 cases among those younger than 18 peaked in the first week of September, when nearly a quarter of a million cases were reported.

But they have remained high, with more than 130,000 cases reported in the second week of October, the American Academy of Pediatrics said, accounting for a quarter of all cases nationwide. Fewer than two percent of those cases resulted in hospitalization, but that figure was based on data from only half the states, the organization said.

Pfizer and BioNTech announced a month ago that their vaccine worked well for young children, but no comprehensive clinical data was released publicly until Friday. Children in the trial received two doses of 10 micrograms — one-third of the adult dose — three weeks apart. Researchers said that the dosage was safe, and that trial participants had seen only mild side effects.

Of 2,268 children in the trial, twice as many were given the vaccine as received a placebo. Sixteen children who received the placebo got Covid-19, compared with three who received the vaccine. All of the Covid cases occurred in July or later, as the highly transmissible Delta variant was spreading in the United States and globally, according to the company. The F.D.A. said it could not determine whether the cases were caused by the Delta variant.

Pfizer said it had follow-up safety data for an additional 2,250 children, but only for a median of two-and-a-half weeks after they received a second dose.

There were no cases of severe Covid among trial participants, and no cases of two rare heart conditions that have been linked to the vaccine in teenagers and adults, especially young males. But the trial was likely not big enough to detect whether those particular conditions — myocarditis and pericarditis — might appear with any frequency in younger children.

Minor side effects in the study group generally occurred more frequently after the second dose, the F.D.A. reported; the most common were pain at the injection site, fatigue and headache.

The F.D.A. modeled different scenarios with varying levels of viral spread to determine the number of symptomatic Covid cases, including severe ones, that would be prevented by the vaccine. They balanced that against the risks from myocarditis and pericarditis cases associated with vaccination.

Even given a low rate of Covid infections, they said, the benefits of vaccination could outweigh the risks because the virus could lead to more serious health consequences than any side effects.

Researchers looked at immune responses, comparing them with levels in adults who had received the vaccine. Pfizer then deduced that the protection afforded by the lower dose in children could be as substantial as that afforded by the higher dose in adults. That approach is particularly important in small trials.

After the second shot, the children had levels of neutralizing antibodies that were at least equal to those of 16-to-25-year-old volunteers in another Pfizer-BioNTech trial. Although antibody levels are just one measure of the immune system’s response, experts have said such a finding would indicate that one-third of an adult dose was the proper dosage for young children.

Studies have shown that the risk of developing myocarditis or pericarditis from Covid-19 is higher than from vaccination. Still, some countries have recommended a single dose of the Pfizer-BioNTech vaccine for children 12 and older, offering less protection but possibly with a lower risk of side effects.

Experts will almost certainly raise concerns about those side effects at the F.D.A. advisory committee meeting, according to Dr. H. Cody Meissner, a member of the panel and the chief of the Division of Pediatric Infectious Diseases at Tufts Children’s Hospital in Boston.

“I certainly hope that we’re in a position to to recommend this because people are certainly waiting for it, they’re anxious to have it,” he said. “But our responsibility is to balance both risk and benefit.”

Vaccine experts have said that Pfizer’s lower dosing could mitigate risks. “We always like to use the least amount of vaccine that’s needed to generate an immune response,” Dr. Edwards, the Vanderbilt expert, said, “because it’s generally associated with fewer adverse events.”

Dr. Brian Feingold, an expert on heart inflammation in children at the UPMC Children’s Hospital of Pittsburgh, said that, “If you just focus on myocarditis or other potentially rare things, you’re probably still going to be left wanting more information.”

But “we cannot consider a risk in isolation,” he added. “Risks from Covid need to be considered when you make a decision about whether this vaccine has greater net benefit.”

Dr. Edwards said that researchers and regulators were in a quandary. “The more people that you that you enroll, the more time that it takes,” she said. “A lot of times, in order to really look at the data for very rare events, you’re not going to be able to do that in studies until the vaccine is really used more broadly.”

The Centers for Disease Control and Prevention, which sets vaccine policy for the government, has scheduled a meeting of its advisory committee for Nov. 2 and 3 to consider the issue. Federal officials have said they intend to ship 15 million doses to the states immediately if regulatory and health officials authorize the move. The needles that administer the vaccine and the vials that hold it will need to be smaller for children who are still largely in elementary school.

About 17 million adolescents aged 12 to 15 became eligible for the Pfizer-BioNTech vaccine in May. Vaccine manufacturers continue to study the possibility of a vaccine for those aged six months to 5 years old.

Only about one in three parents of 5- to 11-year-olds planned to get their children inoculated “right away” once a vaccine is authorized, according to polling by the Kaiser Family Foundation conducted last month. Another third said they wanted to “wait and see” how the vaccine affected children.

But that same polling showed that reluctance among parents of teenagers had dropped in the months since vaccines became available to that age group.

Aina J. Khan contributed reporting from London, and Carl Zimmer from New Haven, Conn.