The Food and Drug Administration on Monday authorized use of the Pfizer-BioNTech Covid-19 vaccine for 12- to 15-year-olds in the United States, a crucial step in the nation’s steady recovery from the pandemic and a boon to millions of American families eager for a return to normalcy.

The authorization caps weeks of anticipation among parents, who have been grappling with how to conduct their lives when only the adults in a household are immunized. It removes an obstacle to school reopenings by reducing the threat of transmission in classrooms, and affords more of the nearly 17 million children in this age group opportunities to attend summer camps, sleepovers and Little League games.

“This is great news,” said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York. “It feels like we’ve been waiting a long time to start protecting children in this age group.” The Pfizer-BioNTech vaccine is already available to anyone over 16.

The F.D.A.’s go-ahead is not the final hurdle. An advisory committee of the Centers for Disease Control and Prevention is expected to meet shortly to review the data and make recommendations for the vaccine’s use in 12- to 15-year-olds.

If the committee endorses the vaccine for that age group, as expected, immunizations in theory can begin immediately. Clinical trials have shown that these children may safely receive the dose already available for adults.

President Biden has said that about 20,000 pharmacies are ready to administer the vaccine to adolescents, and health officials in some states are already drawing up immunization campaigns targeted to youngsters.

In a clinical trial, Pfizer and BioNTech enrolled 2,260 participants ages 12 and 15 and gave them either two doses of the vaccine or a placebo three weeks apart. The researchers recorded 18 cases of symptomatic coronavirus infection in the placebo group, and none among the children who received the vaccine, indicating that it was highly effective at preventing symptomatic illness.

The vaccine also appeared to be safe for these children, with side effects comparable to those seen in trial participants who are 16 to 25 years old. Fevers were slightly more common among inoculated 12- to 15-year-olds; about 20 percent of them had fevers, compared with 17 percent in the older age group.

The trend toward more fevers at younger ages was consistent with observations in an earlier trial, said Dr. Bill Gruber, a senior vice president at Pfizer and a pediatrician.

The trial results were a “trifecta” of good news, Dr. Gruber added: “We have safety, we got the immune response we wanted — it was actually better than what we saw in the 16- to 25-year-old population — and we had outright demonstration of efficacy.”

The company is still gathering information on potential asymptomatic infections by continuing to test the trial participants for the coronavirus every two weeks and checking them for antibodies produced in response to a natural infection, according to Dr. Gruber.

The authorization arrives at an opportune time. Roughly one-third of eighth graders, usually 13 and 14 years old, are still learning fully remotely, and widespread vaccination may help speed a return to classrooms.

Coronavirus outbreaks have been a concern particularly for students participating in team sports, and immunizations should ease the concerns of many parents whose children have been unable to participate in football, basketball and other team sports involving close contact.

In summer, families often catch up the vaccinations required for children to return to schools in the fall, and so pediatricians and family doctors may be best positioned to immunize young teenagers as they come in for annual physical examinations.

The Pfizer-BioNTech vaccine can be stored for only five days in standard refrigerators, but the companies are planning to ship smaller packs for doctors’ offices. It is also developing a formulation that can be refrigerated for up to 10 weeks.

The push to immunize children may run into the same problems with hesitancy that have plagued attempts to inoculate adults. In one recent poll, 30 percent of parents said they would have their children vaccinated right away, while 26 percent said they planned to wait to see how the vaccine was working.

Most of the other parents said they would definitely not have their children vaccinated, or would do so only if schools required it. All 50 states require certain vaccines for children who attend school, but those mandates apply only to vaccines that have been fully approved by the F.D.A. The Pfizer-BioNTech vaccine has been authorized only for emergency use.

The companies have applied to the F.D.A. for full approval, but that process is expected to take several months. Even after approval, students may still opt out by citing medical reasons or religious beliefs.

Still, scientists agreed that the vaccine appeared to meet all expectations regarding safety and efficacy. Dr. Megan Ranney, an emergency room physician and professor at Brown University in Providence, R.I., said she had “zero safety concerns” about the Pfizer-BioNTech vaccine, noting that hundreds of millions of people worldwide had received it.

Her 12-year-old daughter is eager to be vaccinated, and her 9-year-old son will be immunized as soon as he is eligible, she said. Dr. Ranney has not allowed her children to sleep at friends’ houses since the pandemic began. The vaccine should allow them to safely resume social activities, she said.

“The risk of your child catching Covid and getting really sick is low, but it’s not zero,” she said. “And the risk of them getting sick or hospitalized or worse with Covid or with the post-Covid multi-inflammatory syndrome is higher than the risk of something bad from this vaccine.”

Vaccinating children shields others in the community from the virus, she noted, including people who are not protected by the vaccine, such as organ transplant recipients, cancer patients and those with impaired immune responses.

“It also protects all of us from the virus continuing to spread and mutating further,” Dr. Ranney said. “That’s the thing that I’m most scared of right now.”

Vaccinating children is crucial to building up population levels of immunity and curtailing the spread of the coronavirus. Though children spread the virus less efficiently than adults do, they make up about 23 percent of the population.

Experts have said that the country is unlikely to reach the “herd immunity” threshold — the point at which virus transmission essentially stalls — but vaccinating children will be important for getting as close as possible.

Ty Dropic, 14, one of the trial participants, urged others his age to be vaccinated so they could build up widespread immunity and protect themselves. He had no side effects, leading him to suspect that he got the placebo. If that turns out to be the case, he plans to be immunized as soon as possible.

“I know it can be kind of scary, but it’s really not as bad as it seems,” he said. “If you do get Covid, it’ll be a lot worse than getting stuck with a needle for, like, two seconds.”

Ty’s three siblings, ages 8, 10 and 16, are also enrolled in vaccine trials for their age groups. Their mother, Dr. Amanda Dropic, a pediatrician in northern Kentucky, said that in her practice, most parents were eager to have their children vaccinated so they could regain some semblance of normalcy.

“The anxiety and depression that we’re seeing with kids, the social delays, has been tremendous,” she said.

Dr. Dropic said her children understood the risks and were willing to volunteer because they saw it as a civic duty. Every medicine available today came to be because “somebody was willing to go first,” she added.

Pfizer and BioNTech began testing the vaccine in children ages 5 to 11 in March, and last month extended the trial to even younger children, ages 2 to 5. The companies next plan to test children who are 6 months to 2 years old.

Assuming trial results are encouraging, the companies expect to apply to the F.D.A. in September for emergency authorization to administer the vaccine to children ages 2 to 11.

Results from trials of Moderna’s vaccine in 12- to 17-year-olds are expected in the next few weeks. Findings from another trial of the company’s vaccine in children 6 months to 12 years old should be available in the second half of this year.

AstraZeneca is testing its vaccine in children 6 months and older. Johnson & Johnson plans to wait for results from trials in participants older than 12 before testing its vaccine in younger children.