Moderna’s coronavirus vaccine for adolescents has yet to be authorized by federal health officials in part because they are investigating emerging reports that the shots may be associated with a higher risk of a heart condition in younger adults than previously believed, according to two people familiar with the review who emphasized the side effect still probably remains very uncommon.

The investigation, which involves the Food and Drug Administration and the Centers for Disease Control and Prevention, is focusing on Canadian data that suggests the Moderna vaccine may carry a higher risk of myocarditis for young people than the Pfizer-BioNTech vaccine, especially for males below the age of 30 or so. The authorities also are scrutinizing data from the United States to try to determine whether there is evidence of an increased risk from Moderna in the U.S. population for the condition, which causes inflammation of the heart.

The two people who described the investigation spoke on the condition of anonymity to discuss an ongoing review because they were not authorized to discuss it.

Moderna asked the FDA on June 10 to expand the emergency use of its coronavirus vaccine to adolescents between the ages of 12 and 17, but the agency has not yet granted the request. Pfizer’s vaccine was authorized for 12- to 15-year-olds May 10, about a month after the company applied.

One of the people familiar with the review emphasized it is still too early to reach a conclusion about whether Moderna’s vaccine carries a higher risk than the Pfizer shot. The person said the agencies must do additional work before deciding whether to issue any kind of new or revised warning or recommendation about the situation and said the additional data should be available in coming days.

In June, the FDA added a warning label for the Pfizer and Moderna shots — both known as mRNA vaccines — about increased risk of myocarditis, but emphasized that the virus itself presents a much greater threat of heart complications than vaccine-related complications.

“We have not come to a conclusion on this,” one of the people familiar with the investigation said. “The data are not slam bang.”

The United Kingdom on Tuesday approved Moderna’s vaccine for children between 12 and 17.

Moderna did not immediately respond to a request for comment.

FDA spokeswoman Stephanie Caccomo said that although the agency will not comment on internal meetings or discussions, “we can say that FDA is absolutely committed to reviewing data as it becomes available to us. We have previously communicated about myocarditis and COVID-19 vaccines and if new information changes the risk/benefit profile, we will update the public accordingly.”

In a statement, the CDC said its vaccine advisers, the Advisory Committee on Immunization Practices, “has and continues to review reports of myocarditis and pericarditis following COVID-19 mRNA vaccination. CDC, FDA, and our vaccine safety partners are actively monitoring these reports, including reviewing data and medical records, to learn more and understand any relationship to COVID-19 mRNA vaccination.”

One individual said the Canadian data, which was provided by that country’s government, indicates there might be a 2½ times higher incidence of myocarditis in those who get Moderna compared with the Pfizer-BioNTech vaccine.

The myocarditis side effect is extremely rare and even if it is more likely in people receiving the Moderna vaccine, it probably is still uncommon. Officials want to be careful not to cause alarm among the public, especially when officials are trying to persuade more people to be vaccinated amid a surge of cases fueled by the fast-moving delta variant.

On May 10, the Food and Drug Administration authorized the Pfizer coronavirus vaccine for 12-to-15-year-olds. Here's what parents should know. (Joy Yi/The Washington Post)

Vaccine safety experts in May said they were studying a small number of cases of heart muscle inflammation that have been reported in multiple countries among young people who had recently received their second dose of one of the coronavirus vaccines. The CDC said most cases were mild and urged that people continue getting the shots.

In late June, federal health officials said there was a “likely association” between the Moderna and Pfizer-BioNTech vaccines and an increased risk of the rare heart condition in adolescents and young adults, the strongest assertion so far regarding a potential link.

But the CDC’s vaccine advisers, at a June 23 meeting, said that getting covid-19, the illness caused by the virus, puts someone at far greater risk of heart inflammation and other serious medical problems than the risk of developing myocarditis from vaccination.

At that meeting, a chart showed that through early June, there was a higher rate of myocarditis among people who received Moderna than Pfizer-BioNTech — about 20 per 1 million doses for Moderna and eight for Pfizer-BioNTech. But the public statement about the vaccines did not emphasize the difference. And the numbers were small, making it hard to draw conclusions.

The CDC and the Department of Health and Human Services, together with 15 of the country’s leading medical and public health organizations — including the American Academy of Pediatrics, the American College of Obstetricians and Gynecologists, the American Medical Association and the American Nurses Association — issued a joint statement after the June meeting saying that they “strongly encourage everyone 12 and older” to get the shots because the benefits far outweigh potential harms.

The CDC and FDA, through their safety monitoring, have confirmed 762 reports of myocarditis or pericarditis, according to the most recent data on the CDC website. Most cases, the agency has said, have occurred following the second shots of the mRNA vaccines, the agency said.

About 202 million doses of Pfizer-BioNTech vaccine and 142 million Moderna shots have been administered in the United States, according to the CDC. About 14 million Johnson & Johnson shots have been administered.

Lena H. Sun contributed to this report.