The FDA is probing whether the Moderna vaccine can cause a rare side effect in teens

The U.S. Food and Drug Administration will need more time to decide whether to approve Moderna's COVID-19 vaccine for use in children ages 12 to 17, the company announced Sunday.

The extended timeline is so the FDA can look into reports of a rare side effect — myocarditis, or the inflammation of the heart muscle — in those who've gotten the shot. Moderna said the FDA informed the company of the delay on Friday.

"The safety of vaccine recipients is of paramount importance to Moderna. The Company is fully committed to working closely with the FDA to support their review and is grateful to the FDA for their diligence," Moderna said in a statement.

The government's review of Moderna's application for an emergency use authorization of its coronavirus vaccine in adolescents may not be completed before January, the company said. Moderna requested the FDA's authorization for use in adolescents in June.

The FDA is reviewing "recent international analyses" of the possible side effect, according to Moderna.

A recent unpublished study by Sweden's Public Health Agency showed a slightly increased risk of inflammation of the heart muscle and prompted four countries to stop giving the shot to young men and boys. The company's vaccine had previously been approved by The European Medicines Agency for use in children between ages 12 and 17.

Moderna said it did not yet have access to the recent international analyses. The company estimated that more than 1.5 million adolescents had received its vaccine and said there didn't appear to be an increased risk of myocarditis in those younger than 18.

Both the Modern and Pfizer-BioNTech COVID-19 vaccines have been shown to produce myocarditis as a rare side effect. However, viral infections, including COVID-19, also can cause myocarditis. Most children and adolescents who experience myocarditis recover.

The FDA recently authorized the use of Pfizer-BioNTech's COVID-19 vaccine in children ages 5 to 11 on an emergency basis. Experts on the advisory panel said the benefits of protecting children against COVID-19 outweigh the possible increased risk of the rare side effect.

Moderna has already been granted emergency use authorization for its COVID-19 vaccine in adults.

The company said Sunday that it would delay applying for emergency authorization for its vaccine to be given to children ages 6 to 11 until the FDA completes its current review.

Copyright 2021 NPR. To see more, visit https://www.npr.org.