From Fox News: One of the most widely known risks linked to the COVID-19 vaccine is myocarditis, especially in young males — and now a new Stanford study has shed some light on why this rare effect can occur.
Myocarditis, which is inflammation of the heart, occurs in about one in 140,000 people who receive the first dose of the vaccine and one in 32,000 after the second dose, according to a Stanford press release. Among males 30 and younger, that rises to one in 16,750.
Symptoms of the condition include chest pain, shortness of breath, fever and palpitations, which can occur just one to three days after vaccination. Another marker is heightened levels of cardiac troponin, which indicates that the heart muscle has been damaged.
In most cases, people who experience myocarditis recover quickly and restore full heart function, according to study author Joseph Wu, MD, PhD, the director of the Stanford Cardiovascular Institute and a professor of medicine and radiology.
Stanford released their findings in a press release on December 10. The press release states:
Using advanced but now common lab technologies, along with published data from vaccinated individuals, the researchers identified a two-step sequence in which these vaccines activate a certain type of immune cell, in turn riling up another type of immune cell. The resulting inflammatory activity directly injures heart muscle cells, while triggering further inflammatory damage.
Below is another excerpt from Stanford:
mRNA vaccines are viewed as a breakthrough because they can be produced quickly enough to keep up with sudden microbial strain changes and they can be rapidly adapted to fight widely divergent types of pathogens. But, as with all vaccines, not everyone who gets the shot experiences a purely benign reaction.
One rare but real risk of the mRNA-based COVID-19 vaccines is myocarditis, or inflammation of heart tissue. Symptoms — chest pain, shortness of breath, fever and palpitations — appear in the absence of any viral infection. And they happen quickly: within one to three days after a shot. Most of those affected have high blood levels of a substance called cardiac troponin, a well-established clinical indicator of heart-muscle injury. (Cardiac troponin is normally found exclusively in the heart muscle. When found circulating in blood, it indicates damage to heart muscle cells.)
Vaccine-associated myocarditis occurs in about one in every 140,000 vaccinees after a first dose and rises to one in 32,000 after a second dose. For reasons that aren’t clear, incidence peaks among male vaccinees age 30 or below, at one in 16,750 vaccines.
CLICK HERE to read the full report from Stanford.
A new Stanford Medicine study shows why mRNA-based COVID-19 vaccines can cause myocarditis.https://t.co/wy6QO55HSu
— Stanford Medicine (@StanfordMed) December 12, 2025
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