The Centers for Disease Control is investigating a small number of young adults and adolescents who may have experienced heart problems following a COVID-19 vaccine, though the agency stressed that it is unclear the vaccine is responsible.
There have been "relatively few" reports of myocarditis and "most cases appear to be mild," but the COVID-19 Vaccine Safety Technical Work Group said it felt the potential issue should be communicated to providers.
The CDC noted that mRNA vaccines, which are made by Moderna and Pfizer-BioNTech, are potentially causing the problem.
Reports of myocarditis have been mostly in adolescents and young adults, are more common in males than females, typically occur after the second dose, and show up about 4 days after vaccination.
The COVID-19 Vaccine Safety Technical Work Group discussed the issue during a meeting on May 17 and the news was first reported by the New York Times on Saturday.
The FDA approved the use of Pfizer's vaccine for children as young as 12 earlier this month following a successful trial in which more than 2,000 U.S. adolescent volunteers were given the shot.
Moderna is also seeking approval of its vaccine for adolescents, saying that its vaccine has proven to be safe and effective in 12- to 17-year-olds.
There are about 1.5 million cases of myocarditis every year, which amounts to 10 to 20 cases per 100,000 persons, according to a study published last year in the medical journal StatPearls.
Viral infections are the most common cause of the disease, but bacteria, cancer, and exposure to environmental toxins can also lead to Myocarditis.
The Department of Defense said last month that it is looking into 14 cases of possible myocarditis in people who were vaccinated by the military.
CDC Director Rochelle Walensky said they have found no link between the vaccines and heart inflammation or myocarditis.
"We have not seen a signal, and we’ve actually looked intentionally for the signal in the over 200 million doses we’ve given," Dr. Walensky said at a White House briefing on April 27.
More than 162 million Americans have received at least one dose of a COVID-19 vaccine. About 4.6 million Americans between the ages of 12 and 18 have received one dose, according to the CDC.
The CDC noted that the rates of Myocarditis in young adults following the COVID-19 vaccine "have not differed from expected baseline rates."
Some doctors and health experts have also argued that COVID-19 can cause Myocarditis and other heart damage, but this link is still being studied.
The Associated Press contributed to this report.