The U.S. Food and Drug Administration (FDA) has ordered Pfizer and Moderna to expand their warning labels on COVID-19 vaccines.
The updated warnings highlight a rare risk of heart inflammation in teen boys and young men, CBS News reported.
The warning applies to males ages 16 to 25 and is based on new data from FDA safety monitoring and a 2023 study.
This includes both Pfizer's Comirnaty and Moderna's Spikevax vaccines.
The updated label will note that "the highest estimated incidence of myocarditis and/or pericarditis was in males 16 through 25 years of age."
Myocarditis is inflammation of the heart muscle, while pericarditis is inflammation of the lining around the heart.
For every million doses in 16- to 25-year-old males, there were 38 cases of these heart conditions, the warning adds. For all people under 65, the rate was about 8 cases per million doses, CBS News reported.
The U.S. Centers for Disease Control and Prevention (CDC) says most of these cases happen soon after the shot and tend to improve quickly.
The FDA’s new warning also says that heart MRI scans in affected people usually showed signs of "improvement over time in most people."
"It is not known if these heart MRI findings might predict long-term heart effects of myocarditis," the FDA said. "Studies are underway to find out if there are long-term heart effects in people who have had myocarditis after receiving an mRNA COVID-19 vaccine."
Both vaccine makers have 30 days to respond to the FDA's letter.
Neither company has made a public statement yet, CBS News said.
The FDA made the letters public just before a U.S. Senate hearing on "how health officials downplayed and hid myocarditis and other adverse events associated with the COVID-19 vaccines."
FDA spokesperson Andrew Nixon said in a statement that transparency is key.
"Americans deserve radical transparency around the safety and efficacy of COVID vaccines and the FDA is delivering on their promise to do just that," the statement said. "Moderna and Pfizer should take steps to ensure that individuals are aware of COVID vaccine-related adverse events resulting in myocarditis and pericarditis."