The F.D.A. is assessing whether the Moderna vaccine can cause heart problems in adolescents.

The Food and Drug Administration is reviewing reports suggesting the coronavirus vaccine made by Moderna can cause heart problems in some adolescents, the company said on Sunday.

Moderna requested authorization from the F.D.A. for use of its vaccine in children ages 12 to 17 years in June. The adolescents would receive 100 micrograms of the vaccine — the same dose given to adults 18 and above. But the agency has not yet made a ruling on the application, prompting speculation about reasons for the delay.

In a statement on Sunday, Moderna said the F.D.A. “requires additional time to evaluate recent international analyses of the risk of myocarditis after vaccination.”

The European Medicines Agency approved the vaccine for use in adolescents in July. But since then, several European countries have paused the vaccine’s use in people 30 and younger, citing concerns about myocarditis — an inflammation of the heart muscle.

Moderna said more than 1.5 million adolescents worldwide have received its coronavirus vaccine, and the data thus far do not suggest an increased risk of myocarditis. But studies from Israel and the United States have linked both the Pfizer-BioNTech and Moderna vaccines to rare and transient cases of myocarditis, with a higher risk from the Moderna vaccine.

The F.D.A. notified Moderna on Friday that it would need more time to assess the vaccine’s safety and may not deliver a decision until January 2022, the company said in a statement on Sunday. The agency took roughly a month to authorize the Pfizer-BioNTech vaccine for children ages 12 to 15 years. That vaccine has been available to adolescents in the United States and Europe since May.

Even with the heightened risk, myocarditis as a result of the vaccine is rare, mild, and resolves quickly, noted Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and a member of the F.D.A.’s vaccine advisory committee.

Covid-19 is much more likely to cause myocarditis, Dr. Offit noted, because the virus can infect and damage the lining of the heart. “That would be the decision point I would make for my child,” he said.

In studies from Israel and the United States, the incidence of heart problems among people who had received Pfizer-BioNTech or Moderna vaccine is highest in males aged 16 to 29 years. The risk appears to decline in children 12 to 15, and is expected to be even lower in younger children, Dr. Offit said.

The F.D.A. in July asked Pfizer-BioNTech and Moderna to enroll more children in their clinical trials in order to detect less common side effects. Last week, after reviewing data from a clinical trial of the Pfizer-BioNTech vaccine in children aged 5 through 11 years, the F.D.A. authorized the vaccine for that age group.

Results from Pfizer’s vaccine trial in children under 5 are not expected till the fourth quarter of this year at the earliest. Last week, Moderna said its vaccine produced a potent immune response in children ages 6 through 11 who received half the adult dose. The company plans to request authorization from the F.D.A. for the vaccine’s use in this age group.

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