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The Food and Drug Administration cleared the first coronavirus vaccine for emergency use in children as young as 12 on Monday, expanding access to the Pfizer-BioNTech shot to adolescents ahead of the next school year and marking another milestone in the nation’s battle with the virus.

The decision that the two-shot regimen is safe and effective for younger adolescents had been highly anticipated by many parents and pediatricians, particularly with the growing gap between what vaccinated and unvaccinated people may do safely. Evidence suggests that schools can function at low risk with prevention measures, such as masks and social distancing. But vaccines are poised to increase confidence in resuming in-person activities and are regarded as pivotal to returning to normalcy.

“Adolescents, especially, have suffered tremendously from the covid pandemic. Even though they’re less likely than adults to be hospitalized or have severe illness, their lives really have been curtailed in many parts of the country,” said Kawsar R. Talaat, an assistant professor of international health at the Johns Hopkins Bloomberg School of Public Health. “A vaccine gives them an extra layer of protection and allows them to go back to being kids.”

Expert advisers to the Centers for Disease Control and Prevention are scheduled to meet Wednesday to recommend how the vaccine should be used in that age group, and the vaccine can be administered as soon as the CDC director signs off on the recommendation.

In a news briefing Monday evening after the announcement, FDA officials said the Pfizer authorization for 12- to 15-year-olds was a straightforward decision because the data showed that the vaccine was safe and that the response to the vaccine was even better than among the 18- to 25-year-olds who got the shots.

Children rarely suffer serious bouts of covid-19, the illness caused by the coronavirus. But there is no way to predict the few who will become dangerously sick or develop a rare, dangerous inflammatory syndrome. Out of more than 581,000 covid-19 deaths in the United States, about 300 have been people under 18 — a tiny fraction of the total. But that exceeds the number of children who die in a bad flu season.

Children appear to be less efficient at spreading the virus, although their role in transmission is still not fully understood — another reason for pediatric vaccinations.

Clinicians also worry that with a new virus with many unknowns, the possibility exists for long-term impacts of infection, even from the mild or asymptomatic courses of illness common among children.

The Pfizer-BioNTech vaccine, already authorized for adolescents 16 and older, was the first to be tested in younger adolescents. The FDA’s decision will provide a potential path for other vaccine-makers to follow, most of which have launched or plan to initiate trials of their vaccines in teenagers and younger children.

The agency based its authorization on a trial of nearly 2,300 adolescents between 12 and 15 years old, half of whom received the same two-shot regimen shown effective and safe in adults. Researchers took blood samples and measured antibody levels triggered by the shots and found stronger immune responses in the teens than those found in young adults. There were 16 cases of covid-19 in the trial, all of them among adolescents who received a placebo, suggesting the regimen offered similar protection to younger recipients as it does to adults.

Robert W. Frenck Jr., the researcher who led the adolescent trial at Cincinnati Children’s Hospital Medical Center, said the study was designed to test whether it triggered immune responses, not whether it prevented disease. But because of the number of children who became ill in the placebo arm of the trial, it also became evident the vaccine offered robust protection.

“That really points out how much covid there is in the adolescent community,” Frenck said.

The data has not been published or peer-reviewed, but Kathryn M. Edwards, a pediatric infectious-disease specialist at Vanderbilt University Medical Center, said the results announced by Pfizer were “pretty exciting — it looked very effective and the immune responses were really good.”

Edwards said she is comfortable the benefits of vaccinations are clear among teens, noting that while children, in general, are at lower risk of severe covid-19 than adults, older adolescents seem to be more like adults in their risk for covid-19 than the very youngest children.

Audrey Baker, 15, and Sam Baker, 12, rolled up their sleeves for shots in the Pfizer-BioNTech trial at Cincinnati Children’s Hospital Medical Center. Audrey said she had no hesitation about signing up, and misses little things about how life used to be — eating out in restaurants and seeing family.

“I just trusted the science,” Audrey said. “I knew it was tested in adults. I was really just joining, hoping that maybe I could get vaccinated and help out science.”

Sam said he was more hesitant, in part because participating meant many follow-up lab tests. But he decided to do it and thinks he may have gotten the vaccine in the trial because he developed a headache and fever after his second dose.

Their mother, Rachel Baker, said she felt relief because of Sam’s symptoms.

“The biggest benefit has been that I feel a weight off my shoulders,” Rachel said. “We haven’t changed how we do anything. … We’re still masking, we’re still social distancing, but we’re a bit calmer about it all.”

H. Cody Meissner, a pediatrician at Tufts Medical Center and a member of an external advisory committee to the FDA, said he thinks a pediatric vaccine is needed. But he said he would like to see more safety data because the messenger RNA technology at the core of vaccines from Pfizer-BioNTech and the biotechnology company Moderna does not have a long, established safety record, and its first large-scale use began in December.

Meissner abstained from the December vote that overwhelmingly recommended authorization of the Pfizer-BioNTech vaccine for people 16 and older, because he thought the vaccine should be authorized in people 18 and older.

“For those who are eager to get it, it’s important for them to understand that this is very rarely a severe disease in young adolescents, number one, and this is an entirely new vaccine,” Meissner said. “I just don’t want people to get too swept up in fear of hospitalization and death from covid-19 for the first few decades of life.”

But many other physicians take comfort knowing that 250 million shots of messenger RNA vaccine have been given in the United States alone. Serious side effects, such as a risk of anaphylaxis, are extremely rare. Because the trial in teens was an “immune bridging” trial designed to test whether the vaccine triggered immune responses similar to those in adults, researchers did not need to recruit tens of thousands of people to see if those who received a vaccine were protected against illness. The immune bridging technique is commonly used to expand access to vaccines that have been proved effective and safe to adolescents or other populations.

The expansion of eligibility to children will probably ignite debates in families about when to get vaccinated, and among policymakers about whether it should be required.

Dorit Reiss, a law professor focused on vaccine policy at the University of California Hastings College of Law, said she thinks it is unlikely children will be mandated to receive a coronavirus shot until the vaccines win full approval and not just emergency use authorization.

She predicted that acceptance of the vaccine will evolve as more children are vaccinated and depend on the state of the pandemic. She noted that when vaccines are introduced, the rollout often starts slowly before accelerating.

“Nervousness about a new vaccine is normal, especially when it’s for kids,” Reiss said. “Parents that are nervous now might feel different in a few months, once their friends’ kids have gotten vaccinated. And the views of the kids are also going to matter — if teens are going to think this is going to make their lives easier.”

Opening up vaccinations to children may sharpen a debate unfolding globally about the equity of vaccine access. Talaat said that while she can’t wait for her kids to have access to a vaccine, she is troubled by the global inequities as high-risk front-line workers or older people still don’t have access to vaccines in countries where the coronavirus is out of control.

Moderna announced Thursday that an initial analysis of its teen trial found its vaccine was 96 percent effective among participants who received at least one dose. Moderna is in discussions with regulators about the data. Pfizer-BioNTech and Moderna are testing their vaccines in children as young as infants. Johnson & Johnson is planning pediatric trials of its single-shot vaccine.

Trials in younger children are expected to take longer, because researchers must step down gradually in age and determine a safe and effective dose. William Gruber, senior vice president of vaccine clinical research and development at Pfizer, said data from tests in children as young as 2 years old may be available by September or October, with data on children as young as 6 months possible by the end of the year.

Within each age category, a separate risk-benefit assessment may take place. In the youngest children, given the low risk from the coronavirus, side effects may figure more prominently into the analysis, for example. Researchers may end up choosing a lower dose of vaccine. The understanding of children’s role in transmission may also evolve and help guide vaccine use and public policy.

“We are proceeding carefully, cautiously,” Edwards said. “We’re using the same rigid guidelines we use in all vaccines, and we take this very seriously. I think as time goes on and more information becomes available, some of the questions may be easier to address.”