What Scientists Have Learned About Rare Myocarditis After mRNA COVID-19 Vaccination

Rare cases of myocarditis, an inflammation of the heart muscle, have been observed following some COVID-19 vaccinations. The issue has received particular attention because the cases have occurred most often among adolescent and young adult males. U.S. health agencies recognize the association and continue to monitor vaccine safety.

A recent study from Stanford Medicine has provided additional insight into the biological processes that may be involved. The findings are scientifically interesting, but they should be understood as research into a rare adverse event rather than evidence that vaccination commonly causes heart problems.

What Is Myocarditis?

Myocarditis occurs when the heart muscle becomes inflamed. It can have many possible causes, including infections and other immune-related conditions.

Following COVID-19 vaccination, reported cases have been uncommon. According to the CDC, they have occurred most frequently in adolescent and young adult males, particularly within a week after a second mRNA vaccine dose. Cases have also been reported in females, older adults, and after other doses.

The FDA has also updated the safety information for mRNA COVID-19 vaccines. Its 2025 labeling update reported an estimated incidence of approximately 8 cases of myocarditis and/or pericarditis per million doses among people ages 6 months through 64 years after the 2023–2024 vaccine formula. Among males ages 12 through 24, the estimated rate was approximately 27 cases per million doses.

What Did the Stanford Researchers Discover?

Researchers at Stanford Medicine investigated why vaccination appears to trigger myocarditis in a small number of people.

Their work identified two immune-signaling proteins, CXCL10 and interferon-gamma (IFN-gamma), that appear to play important roles in the inflammatory process observed in their experiments. The researchers studied immune cells, heart-cell models, and animal models to examine how these signals could contribute to cardiac inflammation.