Multiple studies have shown that in rare cases, teen and young adult males have developed myocarditis after getting ​​the mRNA COVID-19 vaccine — and now new research published in the journal Science Immunology sheds some light on potential causes.

Scientists from Yale University School of Medicine in New Haven, Connecticut, conducted tests on a group of 23 patients with vaccine-associated myocarditis (an inflammation of the heart muscle) and/or pericarditis (a swelling and irritation of the membrane surrounding the heart).

The scientists found that the condition was caused not by antibodies produced by the vaccine, but by the body’s natural immune response to the vaccine.


“The [researchers] examined a number of different mechanisms in terms of immune responses and were able to rule out adaptive immune responses — so, no specific antibody-mediated responses targeting the virus or the heart itself,” said Dr. Donald M. Lloyd-Jones, immediate past president of the American Heart Association and chair of the department of preventive medicine at Northwestern University’s Feinberg School of Medicine in Chicago.

After reviewing the study findings, Lloyd-Jones told Fox News Digital that researchers identified a “nonspecific immune response in which the heart is an innocent bystander, with nonspecific inflammation and some fibrosis (scar tissue) formation.”

The 23 patients included in the study ranged in age from 13 to 21, with an average age of 16. Males made up 87% of the group, according to the journal Science Immunology.

All the patients were “generally healthy” before receiving the vaccine, per a discussion of the study’s findings in the journal. Most of them started experiencing symptoms within four days after receiving the second dose of the Pfizer-BioNTech COVID-19 vaccine.

Researchers looked into several possible causes of the myocarditis, including an immune response to antibodies triggered by the vaccine, an allergic reaction to the vaccine and antibodies that specifically target the heart, noted the journal Science.

After ruling out those possibilities, they concluded that the vaccine triggered an exaggerated immune response in which the heart was inadvertently affected.

In the Yale study, the participants experienced many of the symptoms commonly associated with myocarditis, including chest pain, heart palpitations, shortness of breath, fever, fatigue, headaches, nausea, muscle aches and excessive sweating.

Most cases of vaccine-related myocarditis resolve quickly with treatment. 

Common treatments include non-steroidal anti-inflammatories (ibuprofen) and blood pressure medications, according to Beaumont Health’s website.

In rare cases, patients may require IV medications or machines to assist the heart.

“Even if this inflammation occurs, it seems to be quite self-limited and does not seem to have long-term consequences for the heart,” said Lloyd-Jones.

Anyone experiencing symptoms should see a doctor for testing and medication as needed, experts recommend.

A majority of the cases of myocarditis and pericarditis in teen and young adult males occur within a week after receiving the second dose of an mRNA COVID-19 vaccine, according to the Centers for Disease Control and Prevention (CDC).

It can, however, occur after the first dose or after a booster dose.

Myocarditis remains a very rare complication of the mRNA vaccines, affecting about one in every 50,000 people, Lloyd-Jones said.


“The chances of having serious complications (heart, lung, brain and other) remain far greater from getting the COVID-19 virus itself if you are unvaccinated, than from this rare complication of the vaccine,” Lloyd-Jones told Fox News Digital.

“People should be aware of the risks and weigh those against the far greater risks of complications from actually getting the COVID-19 virus and make an informed decision, using information from trustworthy sources,” he added.

As far as which young men will develop the condition post-vaccine, Lloyd-Jones said it seems to be an “idiosyncratic reaction” that cannot be predicted.

“It only seems to occur early after vaccination, within days to a couple of weeks,” he added. “It has not been seen later to my knowledge.”

The study authors pointed out some limitations, primarily the small size of the patient group.

“Although our cohort of LNP-mRNA vaccine-associated myopericarditis is one of the largest studied to date, and our hypothesis is consistent with published reports from other patients, the number of participants remains limited to make broad conclusions,” the researchers wrote in the findings. 

There was also some variance in age, vaccine dose and length of time after vaccination, they noted.


Additionally, it is unclear how genetics may come into play with myocarditis risk, the authors said.

The study also lacked any biopsied samples of heart tissue.

Fox News Digital has reached out to the Yale researchers seeking comment.


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