(Bloomberg) -- Moderna Inc. said it would expand an ongoing trial of its coronavirus vaccine in kids under 12 years old to gather more safety data, amid worries that messenger RNA shots may trigger rare heart side effects.

“The objective is to enroll a larger safety database which increases the likelihood of detecting rarer events,” a Moderna spokeswoman said in a statement.

Trial timelines are regularly re-evaluated based on regulatory agency discussions and requests, she said, but Moderna currently expects to have data that would support authorization in late 2021 or early 2022.

The company is actively discussing a proposal for a bigger trial with the U.S. Food and Drug Administration, she said.

The move comes in the wake of reports of heart and heart-lining inflammation in young adults with both the Moderna and Pfizer Inc.-BioNTech SE mRNA-based vaccines.

Moderna shares declined 2.5% to $340.17 at 1:41 p.m. Monday in New York. Shares of Pfizer and BioNTech were each up about 0.3%.

The New York Times reported Monday that both Moderna and Pfizer were expanding the size of their pediatric Covid vaccine trials at the request of the FDA.

In a statement, Pfizer declined to comment on whether its trial had gotten bigger. The FDA didn’t immediately respond to a request for comment.

Moderna’s child trial originally planned to enroll 6,750 kids from ages 6 months old through 11 years old. The company said that it couldn’t confirm the scope of the expansion.

Moderna’s vaccine is currently authorized for adults ages 18 and older, and it is awaiting an FDA decision on an application to expand that to 12 and up. Pfizer’s vaccine is already authorized for 12 and up.

In June, advisers to the CDC met to discuss a potential link between mRNA-based Covid vaccines and cases of both heart inflammation, called myocarditis, and inflammation of the membrane around the heart, or pericarditis. A total of 1,226 cases have been reported through June 11, according to the CDC, most of which were among young men and adolescents.

Following the meeting, U.S. public-health leaders sought to reassure Americans that Covid-19 shots are safe and to get vaccinated. They suggested the risk potentially posed by the shots is extremely low, and that it is much more likely that the coronavirus itself would pose a serious health threat.

©2021 Bloomberg L.P.