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How worried should we be about myocarditis?

A faulty study that overstated the risks has been withdrawn — but only after it was shared widely on social media.

This article was originally published in the Montreal Gazette

One of the most frequent questions I get about vaccines is about the risk of myocarditis. Ever since the association was first raised over the summer, people have been concerned about the risk of myocarditis, inflammation of the heart muscle, especially in young men. Unfortunately, some people have been using that risk to scare people off the vaccines and have been using inflated numbers to make the case, with a recent study from the Ottawa Heart Institute as a prime example.

The study was particularly timely. Since the summer reports from Israel and the U.S. raised questions about the risk of myocarditis in young people. In general, though, the risk seemed relatively low, even when you restricted the analysis to young men who seemed to be the main at-risk group. In the U.S., Centres for Disease Control data from the Vaccine Safety Datalink recorded 3.2 cases of myocarditis per 100,000 in young men 12 to 39 years old. A recent study in the New England Journal of Medicine using Israeli data estimated 2.7 cases per 100,000 people, though they did not break down the risk by age and sex.

In short, the risks with the vaccine were not zero but were very small and far less than the risks associated with getting COVID-19. However, Canadian data on the issue was lacking and thus an analysis from a Canadian hospital centre would be useful. But the study from the Ottawa Heart Institute suggested that the risks of myocarditis were much higher than expected, though still relatively low. They found 32 cases of myocarditis in their hospital centre over a two-month period, which translated to 10 cases per 100,000 doses, which would be substantially higher than the U.S. data and Israeli data that have reported fewer than five cases per 100,000 people vaccinated.

When the study was posted online, the cause for the discrepancy was quickly discovered. The researchers had calculated that 32,379 vaccine doses had been administered during the two-month study period. However, this number seemed implausibly low to many who read the study and a quick check found that the correct number was over 800,000 vaccine doses. This meant that the real risk was closer to three cases per 100,000 people, which is consistent with the existing data.

The problem with this study is not that someone committed a mathematical error. We humans are imperfect creatures and there is no shame in admitting that you made a mistake if you then take the steps to fix it, as was done here. In fact, virtually all studies have errors in them if you go through them carefully. But most of these mistakes are picked up during the review phase and are corrected before the study sees the light of day.

Except that, increasingly, studies are being posted to pre-print servers and are not reviewed externally before being made available to the public. And this is problematic when some members of the public are looking for any excuse to cast vaccines in a negative light and scare people about their side effects. Even though this study has now been withdrawn so that the numbers can be corrected, it has already been shared widely on social media and will likely live forever on the internet.

Some people have already used this incorrect data to scare parents into reconsidering vaccination as we start vaccinating younger patients. But COVID-19 is by far the greater threat with the Morbidity and Mortality Weekly Report estimating COVID caused 122 excess cases of myocarditis per 100,000 infections, which is far more than any estimate post-vaccination. It is also worth noting that the cases of myocarditis post vaccination were generally mild, resolved quickly, and did not seem to cause any major heart damage.

Most people agree that fear of myocarditis should not dissuade anyone from being vaccinated. The risk is very, very low and COVID-19 is much, much worse.


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