A study sheds new light on a rare side effect: how mRNA vaccines for COVID-19 can, in some young men and adolescents, trigger heart inflammation, and suggests a possible way to reduce its likelihood. The researchers used advanced laboratory techniques and analyzed published data from vaccinated people to map a two-step immune response. First, the vaccine activates a specific immune cell, which then prompts another type of immune cell to ramp up. The resulting inflammatory activity can directly injure heart muscle cells and provoke further damage.
The COVID-19 vaccines have been given billions of times and are considered extremely safe, according to Joseph Wu, MD, PhD, director of the Stanford Cardiovascular Institute. Vaccine-associated myocarditis occurs in about 1 in 140,000 people after the first dose and rises to about 1 in 32,000 after the second dose. Interestingly, the highest risk appears in males age 30 and younger, at about 1 in 16,750 vaccinees. Most cases recover fully, often with normal heart function returning quickly, Wu notes. He clarifies that this is not a traditional heart attack caused by blocked arteries; rather, inflammation is the primary issue, and many patients simply require observation to ensure recovery.
Still, severe inflammation can lead to significant heart injury, hospitalization, ICU care for the critically ill, and, rarely, death. Wu emphasizes that COVID-19 infection itself is a stronger risk factor for myocarditis than vaccination, with a case of COVID-19 being roughly ten times more likely to cause myocarditis than an mRNA vaccine.
The study, published December 10 in Science Translational Medicine, is co-authored by Masataka Nishiga, MD, PhD, now a faculty member at The Ohio State University, with Xu Cao, PhD as lead author. The team sought to answer: why does vaccination trigger myocarditis in some, and how might this be mitigated?
Key suspects identified
By comparing blood samples from vaccinated individuals, some of whom developed myocarditis, the researchers found elevated levels of two proteins: CXCL10 and IFN-γ. They propose these two act as a coordinated “tag team” driving myocarditis, as they belong to a class of signaling proteins called cytokines that coordinate immune cell communication.
To explore how these signals might drive heart inflammation, the team created human macrophages (immune cells that act as first responders) in the lab and exposed them to mRNA vaccines. The macrophages produced high levels of CXCL10 and, when later exposed to T cells (sentinels that recognize pathogens but can amplify immune responses), the T cells ramped up IFN-γ production. This suggests macrophages are the main source of CXCL10 and T cells the main source of IFN-γ in response to the vaccine.
Could this duo directly injure the heart? In experiments with young male mice, researchers observed higher levels of cardiac troponin, a marker of heart muscle injury, after vaccination. The heart tissue showed infiltration by macrophages and neutrophils, another frontline immune cell type that can contribute to tissue damage when overactivated.
Importantly, blocking CXCL10 and IFN-γ reduced heart injury in these models while preserving much of the vaccine-induced immune response. In human-made cardiac tissues, bathwater enriched with CXCL10 and IFN-γ caused stress markers to rise, and inhibitors of these cytokines mitigated these effects, helping preserve beating and other indicators of healthy heart function.
A potential preventive angle
Wu wondered whether a dietary compound might help prevent such damage. He previously studied genistein, a soy-derived compound with mild estrogen-like and anti-inflammatory properties. In prior work, genistein appeared to counter certain vascular and tissue damages. In the current series of experiments, pre-treating cells, cardiac tissues, and mice with genistein largely blocked the harmful effects driven by CXCL10 and IFN-γ. The genistein used in these studies was purer and more concentrated than typical over-the-counter supplements.
These findings raise the possibility that genistein, or similar anti-inflammatory strategies, could help reduce myocarditis risk from mRNA vaccines, and perhaps influence inflammatory responses in other organs such as the lungs, liver, and kidneys. It’s worth noting that IFN-γ signaling plays a fundamental role in defending against foreign nucleic acids, so a balance is necessary: while these cytokines help fight infection, they can become harmful in excess.
Broader implications and questions
The authors caution that while these results illuminate a mechanism behind vaccine-associated myocarditis in this preclinical work, broader vaccines could induce myocarditis through similar inflammatory pathways, though with varying patterns and symptom profiles. The findings invite ongoing discussion about risk-benefit considerations, individual susceptibility, and whether targeted anti-inflammatory interventions could reduce rare adverse events without dampening protection against disease.
If you’re curious about how this translates to real-world decisions, consider: Do these findings meaningfully shift the overall risk profile of mRNA vaccines for most people? How might future vaccine design or adjunct therapies reduce rare inflammatory responses while preserving strong immunity? Share your thoughts in the comments—do you think targeting CXCL10 and IFN-γ could become a standard part of vaccine safety in the future, or are these results too early to influence policy?
Reference: Cao X, Manhas A, Chen YI, et al. Inhibition of CXCL10 and IFN-γ ameliorates myocarditis in preclinical models of SARS-CoV-2 mRNA vaccination. Sci Transl Med. 2025;17(828):eadq0143. doi:10.1126/scitranslmed.adq0143
