The U.S. Food and Drug Administration on Friday authorized use of the Pfizer-BioNTech vaccine in children 5 to 11, the first it has cleared for that age group. The shots, given in two doses three weeks apart, contain one-third the dosage of the vaccine authorized for people 12 years and older.

The FDA said clinical trials of Pfizer’s vaccine showed that it was 90.7% effective at preventing Covid-19 in children 5 to 11 years and that it was generally safe.

The FDA’s authorization was the first of two steps needed before the shots can become widely available to children.

The FDA, which regulates a range of medical products, must authorize or approve a vaccine before it can be used outside clinical trials. Once a vaccine is authorized, the CDC sets policies about whether people should get it, which groups and on which schedule.

The CDC considers input from the Advisory Committee on Immunization Practices, a panel of outside doctors and public-health experts. That committee is scheduled to meet to review the evidence for use of Pfizer’s vaccine in children.

If the committee recommends the vaccine and the CDC signs off on it, shots could become available to children shortly afterward.

The FDA didn’t impose any restrictions in its authorization of the Pfizer vaccine for children 5 to 11 years. It is possible the CDC’s vaccine advisory committee could make a different recommendation, by stating that children with chronic medical conditions putting them at higher risk for more-severe Covid-19 should get vaccinated, while issuing a milder recommendation that healthy children may get vaccinated.

The risk of severe Covid-19 is much lower in children than in adults. Yet many have been hospitalized, and at least 691 children under 18 years have died from Covid-19 in the U.S., including 146 deaths in the 5- to 11-year age group, according to the FDA.

Vaccination might also help prevent children from transmitting the virus, according to many health experts.

Last week, an FDA advisory committee recommended the agency authorize the Pfizer vaccine for 5-to-11-year-olds. Yet some committee members suggested the CDC could further refine the recommendation by giving priority to vaccination for children with underlying medical conditions that put them at higher risk of severe Covid-19.

Some committee members suggested it was less urgent to vaccinate healthy children or those who previously had a coronavirus infection, especially in light of the potential risk of rare vaccine-associated heart-inflammation conditions.

Pfizer’s vaccine, along with the Covid-19 vaccine from Moderna Inc., have been associated with an elevated risk of heart-inflammation conditions including myocarditis, particularly in males under 30.

The FDA said the vaccine’s overall benefits in preventing Covid-19 disease and hospitalizations would outweigh the risk of the heart conditions in children 5 to 11 years.