Fact check: Claim misinterprets BMJ study's findings on COVID-19 vaccine and myocarditis

The claim: Study shows COVID-19 vaccine causes 5 times more myocarditis than the virus

A Feb. 5 Instagram post (direct link, archived link) includes a chart detailing myocarditis cases in Denmark, Finland, Norway and Sweden from 2018 to 2022.

"Breaking: New paper published in BMJ shows Covid vaccine causes 5x more myocarditis than Covid itself – Vax:530 vs Covid:109," reads the post, which is a screenshot of a tweet. "For young people the risk is 10x higher – Vax:202 vs Covid:19."

The post generated over 700 likes in less than month, and the original tweet accumulated over 13,000 likes.

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Our rating: False

Medical experts say this claim misrepresents the study's findings and makes a key statistical error. The claim is based on the raw number of myocarditis cases rather than factoring in how many received the vaccine and how many contracted the disease. The study doesn't examine the size of those groups, and without that information any comparisons between the two are meaningless, experts say.

Post misrepresents study findings, co-author says

Myocarditis, an inflammation of the heart's muscle, is most commonly caused by a virus or the body’s immune response to that virus, according to Dr. Richard Martinello, an infectious disease specialist at Yale University. Both the COVID-19 virus and the vaccines protecting against it have been associated with myocarditis, according to the Centers for Disease Control and Prevention.

The study referenced in the tweet was published in the medical journal BMJ on Feb. 1 and compared clinical outcomes of myocarditis associated with COVID-19, the COVID-19 vaccine and conventional myocarditis in patients between 2018 and 2022 using registry data from Norway, Sweden, Denmark and Finland.

The researchers examined 7,292 patients admitted to the hospital with new onset myocarditis. Of those, 530 cases were associated with the COVID-19 vaccine, 109 cases were associated with COVID-19 and 6,653 cases were associated with conventional myocarditis, as seen in the chart highlighted in the post.

The researchers found that myocarditis cases after the mRNA vaccine were less serious than those associated with COVID-19 and conventional myocarditis.

The post's claim that the COVID-19 vaccines cause five times more cases of myocarditis than the virus itself comes from comparing the 530 vaccine-associated myocarditis patients to the 109 vaccine-associated myocarditis patients identified in the study.

But these figures cannot be used to determine accurate rates of myocarditis related to the vaccines and the virus, according to Anders Peter Hviid, who co-authored the study.

“To compare risks, you need denominators of population at risk," Hviid said. "Our study does not compare these risks, and we do not provide the denominators."

William Schaffner, a professor of medicine in the division of infectious diseases at the Vanderbilt University School of Medicine, agreed that the claim misinterprets the study.

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"The study cannot determine the frequency of myocarditis because you don't know the denominator," Schaffner said. "You don't know how many people had COVID-19. You don't know how many people had vaccination. That was not their intention. They didn't look at frequency. What they looked at was severity."

The post also claims the study found the risk of myocarditis in young people who received the vaccine was "ten times higher" than those who contracted COVID-19. But this claim makes the same mistake of comparing raw numbers rather than rates within each population.

"They never consider any denominators to say the rate of myocarditis in the population," Dr. Jeffrey Morris, the director of the division of biostatistics at the University of Pennsylvania, told USA TODAY.

Studies show higher risk of myocarditis after COVID-19 infection

To bolster his case, the Twitter user pointed USA TODAY to another study conducted in Israel that found no increased incidence of pericarditis or myocarditis in adult patients recovering from COVID-19 infection.

However, Morris said the sample size of the study was too small, as there were only nine cases of myocarditis in nearly 200,000 COVID-19-infected adults and 27 cases of myocarditis in the control group of nearly 600,000 patients.

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"What he's ignoring, which many people do that want to make this narrative that 'Oh COVID-19 doesn't lead to high risk of myocarditis,' they're ignoring the other papers," Morris said. "They cherry pick this paper, and they ignore the others."

Another study, using a greater sample size of data from the U.K., found that the risk of myocarditis was greater after COVID-19 infection than after COVID-19 vaccination. Likewise, a 2022 meta-analysis found the risk of myocarditis was more than seven times higher in people infected with COVID-19 compared to those vaccinated.

USA TODAY reached out to the social media users who shared the claim for comment.

Lead Stories also debunked the claim.

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This article originally appeared on USA TODAY: Fact check: Claim misinterprets study's findings on myocarditis