The Food and Drug Administration (FDA) reported Wednesday that it expanded existing warnings on the two main COVID-19 vaccines about a rare cardiac side effect seen mostly in young men.

Myocarditis, a type of heart inflammation that is usually mild, emerged as a complication after the first vaccines became widely available in 2021. The prescribing information for both Pfizer and Moderna already warns doctors about the issue.

In April, the FDA sent letters to both drugmakers asking them to update and expand their warnings to add more detail about the problem and cover a broader group of patients. While the FDA can require label changes, the process is typically more of a negotiation with the companies.

Specifically, the new warning indicates a risk of myocarditis of 8 cases per 1 million people vaccinated against COVID-19 in 2023-2024, between ages 6 months and 64 years. The label also indicates that the problem has been more common among males ages 12 to 24. The previous label indicated that the problem occurred mainly in 12- to 17-year-olds.

The FDA's labeling change appears to conflict with some previous findings by scientists elsewhere in the US government.