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SciCheck’s COVID-19/Vaccination Project

Posts Exaggerate Significance of Swiss Study on Heart Risk and COVID-19 Vaccination

This article is available in both English and Español

SciCheck Digest

A Swiss study found that after a COVID-19 booster, less than 3% of people briefly had a slightly elevated blood level of a protein that can be a marker of heart injury. No one in the study had any serious heart damage, and other experts say the findings are unlikely to be clinically significant. Viral posts, however, are spinning the results to falsely claim that the study shows the vaccine’s risks are “off the scale.”

Full Story

Multiple studies have shown that mRNA COVID-19 vaccines are safe and effective in protecting against severe disease and death. While myocarditis and pericarditis, or inflammation of the heart muscle and its surrounding tissue, have been identified as rare serious side effects of the vaccines, the benefits of mRNA COVID-19 vaccination still outweigh the risks across all age groups.

Myocarditis is a potentially serious condition that can be triggered by a viral infection, including an infection with the coronavirus, or SARS-CoV-2. As we’ve written, studies have shown that even though both the vaccine and COVID-19 can cause myocarditis, the overall risks associated with COVID-19 are higher. Vaccine-associated myocarditis is rare, occurring most frequently in young males after a second dose, and is usually mild and resolves quickly.

review of studies on the topic published in May in the American Heart Association’s journal Circulation Research found that in people 12 years and older, the frequency of myocarditis after two doses of the Pfizer/BioNTech and Moderna vaccines is estimated at 3.5 cases per 100,000 people, and 1.9 per 100,000 people for those 16 years and older.

But viral social media posts, including one by Florida Surgeon General Joseph Ladapo, misrepresent a recent study published by Swiss researchers to say it proves mRNA vaccines are too risky.

“Two USC basketball players experience cardiac arrest in the past year and both almost certainly were forced or misled into taking a vaccine never proven to meaningfully benefit young, healthy people, but definitely proven to cause cardiac injury,” Ladapo posted on X, formerly known as Twitter, on July 27. “Florida data and a new Swiss study show it,” he added, linking to the study, and referring to a flawed Florida analysis that we’ve written about before. 

There is no evidence to suggest that the cardiac arrests suffered by University of Southern California basketball players Vince Iwuchukwu in July 2022 and Bronny James, LeBron James’ son, in July 2023 were due to the vaccines.

Update, Aug. 29: Bronny James has been diagnosed with an “anatomically and functionally significant Congenital Heart Defect,” which caused his sudden cardiac arrest, a spokeswoman said in an Aug. 25 statement on behalf of the James family. “We are very confident in Bronny’s full recovery and return to basketball in the very near future,” the statement said.

John Campbell, a nurse educator in the U.K. who often spreads misinformation on his YouTube channel, said the risk shown by the Swiss study was “off the scale.” “The only way you would take this kind of risk in health care is if the alternative was certain death,” he said in a video that has over a million views. 

An Instagram user posted a snapshot of a headline from the conservative news site the Gateway Pundit that reads: “KILL SHOT: Recent Peer-Reviewed Report Finds 1 in 35 People Who Took Moderna COVID Shot Had Signs of Heart Damage.” 

But these claims are distorting a study published by researchers at the University Hospital of Basel in the European Journal of Heart Failure in July. It found mild and transient levels of a protein that can be a marker of heart injury, but no cases of myocarditis or other serious cardiac events, among 777 hospital workers who received a booster of the Moderna mRNA COVID-19 vaccine. 

“No patient had electrocardiographic changes, and none developed major adverse cardiac events within 30 days,” the abstract reads, referring to changes in an electrocardiogram. “No definitive case of myocarditis was found,” the study continues, although there were two probable cases.

The study measured the volunteers’ levels of cardiac troponin three days after they received a Moderna booster. Cardiac troponin is a protein that exists inside heart muscle cells. Normally, only tiny amounts of it circulate in the bloodstream. But when cardiac muscle cells are injured, more troponin is released into the blood.

The University of Basel researchers wanted to find out if myocardial injury was more common after vaccination than reported — if they proactively looked for indicators of possible injury, instead of relying on passive surveillance that mostly detects myocarditis cases that require hospitalization. To do so they measured high-sensitivity cardiac troponin T, using a test that can detect very low levels of troponin that go undetected in regular tests.

The results did show a small, temporary, above-average presence of troponin in 22 of the 777 hospital employees, or about 1 in 35 people. These people did not report more symptoms than those who did not have elevated troponin levels. The median age of the participants in which troponin was detected was 46 years, and 20 of the 22 cases occurred in women. Troponin levels decreased in all but one patient in a follow-up visit.

“These increases in cardiac troponin are not clinically significant and would not be associated with any quantifiable effect on the heart using imaging,” Dr. Nicholas Mills, professor of cardiology at the University of Edinburgh, who studies troponin, told us in an email. 

“The principle conclusion — that if you look really hard for minor amounts of injury after vaccination you can find it — is likely true,” Dr. James de Lemos, a cardiologist at UT Southwestern Medical Center in Dallas, told Lead Stories. But, he said, the results were “overplayed” and the levels of troponin found were “trivial.”

“These small troponin elevations are not likely events of clinical significance,” he said.

Mills explained that troponin elevation is not always an indication of consequential damage — it also occurs in healthy people after exercise, “where it is not thought to have any pathophysiological consequences,” he said, pointing us to a 2008 study published in the Journal of the American College of Cardiology. He also said the fact that the study showed higher troponin elevation among women, contrary to the existing evidence on vaccine-associated myocarditis, “is counter-intuitive and suggests troponin elevations were less likely to be a consequence of subclinical myocarditis.” 

Mills, who has collaborated with the University of Basel researchers but had no participation in this study, said the results “merit further study.” But the study had an important limitation, he said, since it didn’t measure troponin levels prior to vaccination, and the researchers’ attempts to address this limitation weren’t adequate.

“It is therefore not possible to determine whether troponin elevation was a consequence of vaccination or occurred at this frequency in this population prior to vaccination,” he said.

What the Swiss Study Actually Says

According to the authors, their findings confirm their hypothesis. “mRNA-1273 booster vaccination-associated elevation of markers of myocardial injury occurred in about one out of 35 persons (2.8%), a greater incidence than estimated in meta-analyses of hospitalized cases with myocarditis (estimated incidence 0.0035%) after the second vaccination,” the study reads.

IllustrActions / stock.adobe.com

Yet, that doesn’t mean the vaccines are harmful or should be avoided.

“[A]ll cases were mild with only a transient and short period of myocardial injury,” the study reads. “COVID-19 associates with a substantially higher risk for myocarditis [than] mRNA vaccination, and myocarditis related to COVID-19 infection has shown a higher mortality than myocarditis related to mRNA vaccination,” the study adds.

Dr. Christian Mueller, director of the Cardiovascular Research Institute Basel and the senior author of the study, told us in an email that even though his study found “myocardial cells were severely injured,” the results have to be considered taking into account “all the benefits of vaccination.” To do so, “we would need to know the prevalence and extent of myocardial injury after COVID-19 infection with the strain circulating at that time and the (possible) reduction in the likelihood that people get infected and/or get ill from infection,” he said. 

Furthermore, the authors explain that even though the long-term consequences of the low and temporary troponin elevations they found are unknown, “good long-term outcomes can be expected.”

As we said, Mills and de Lemos said the troponin levels weren’t likely to be clinically significant.

In an interview published in November 2022 on the University of Basel’s website, Mueller explained that the marker they used is “extremely sensitive” and emphasized that the damage to the cardiac muscle is mild. People should not be skeptical about mRNA vaccination based on these results, he added. “mRNA vaccination technology is a fantastic development … The vaccines saved millions of lives,” he said. But he said the findings could help improve vaccination in the future.

A spokesperson for Moderna told us the company has a “robust pharmacovigilance function” and makes sure all adverse events are reported to regulators. 

“mRNA-1273 has been administered to hundreds of millions of people worldwide and has been shown to reduce severe illness, hospitalizations, and deaths caused by COVID-19. Regulatory agencies around the world have stated that the benefits of COVID-19 mRNA vaccines significantly outweigh the risk across all age groups,” Luke Mircea-Willats, senior director of media relations and communications, told us in an email. 

Editor’s note: SciCheck’s articles providing accurate health information and correcting health misinformation are made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over FactCheck.org’s editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation.


Safety of COVID-19 Vaccines. CDC. Updated 7 Mar 2023. 

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Troponin Test. Cleveland Clinic. 17 Mar 2022. 

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Payne, Ed. “Fact Check: Video, Swiss Study Do NOT Prove 1 In 35 Given COVID-19 Vaccine Booster Develop Heart Injury.” Lead Stories. 31 Jul 2023. 

Middleton, Natalie, et al. “Cardiac troponin T release is stimulated by endurance exercise in healthy humans.” Journal of the American College of Cardiology. 25 Nov 2008.

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Jacobs, Angelika. “Temporary mild damage to heart muscle cells after Covid-19 booster vaccination.” University of Basel. 9 Nov 2022. 

Mircea-Willats, Luke. Senior director ofmedia relations and communications at Moderna. Email sent to FactCheck.org. 5 Aug 2023.