Researchers at Stanford Medicine may have found a biological reason why a small number of people develop myocarditis after receiving an mRNA COVID-19 vaccine. While this side effect remains rare, scientists are continuing to investigate why it occurs in certain individuals. Health experts still emphasize that mRNA vaccines are safe and effective for the vast majority of people.

Most reported cases of vaccine-associated myocarditis have been mild to moderate, and the majority of patients recover fully. Researchers also note that COVID-19 infection itself can pose a greater risk of heart inflammation than vaccination.

The study compared immune responses in people who developed myocarditis after vaccination with those who did not. The team identified two immune signaling molecules—CXCL10 and interferon-gamma—as possible contributors to the inflammatory process.

According to the findings, some immune cells produced unusually high levels of CXCL10. This molecule then interacted with T cells, increasing interferon-gamma activity and creating stronger inflammatory signals that may contribute to heart inflammation in rare cases.

Laboratory and animal experiments showed promising results. When researchers blocked these pathways, inflammation decreased while the broader immune response remained intact. This suggests a potential path for developing future treatments or preventive strategies.

Scientists stress that the findings are part of ongoing research designed to improve vaccine safety and better understand rare side effects. The study is not intended to challenge the overall benefits of vaccination but rather to explain why uncommon reactions may occur in a small number of people.