A Canadian study of Covid-19 vaccine side effects contains a major mistake

Anti-vaccine passport protestors in The Hague.
Anti-vaccine passport protestors in The Hague.
Image: Reuters/Eva Plevier
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A team of Canadian researchers unwittingly spewed gasoline on the dumpster fire of anti-vax propaganda earlier this month when they published a non-peer-reviewed study on Covid-19 vaccine side effects.

The problem? A major statistical error.

In the paper, CBC news reports, the scientists claimed that the incidence of heart inflammation seen in patients as a side effect of Moderna and Pfizer-BioNTech vaccination was 1 in 1,000. In reality, that number was closer to 1 in 25,000, which is more aligned with previously reported figures.

How did the supposed rates of myocarditis and pericarditis get inflated?

The faulty number was apparently the result of incomplete data and rushed work. The study authors, from the University of Ottawa Heart Institute, collected data on diagnoses of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of heart’s outer lining) that were identified as likely side effects of mRNA vaccines administered in June and July. They found 32 cases of the side effects, which they initially said occurred among 32,000 administered vaccine doses in Ottawa. In fact, Ottawa Public Health had provided over 800,000 doses in that period.

That higher, accurate number was publicly available. When explaining how the mistake was made, Peter Liu, a co-author and the scientific director of the Ottawa Heart Institute, told the CBC that the scientists were working “on the run.”

Soon after the paper appeared on medRxiv on Sept. 16, its authors heard from peers who pointed out their mistake. The scientists officially withdrew the paper on Sept 24.

Unfortunately, the alarming false statistic was already running free in the world, being shared online, particularly by anti-vaxxers and dubious influencers with large followings. It has been tweeted more than 15,000 times.

The risk of developing myocarditis from a Covid-19 vaccine is very low

Scientists are still studying the slight risk of myocarditis as a side effect of Covid-19 vaccines, and emphasize that the odds of getting the heart condition as part of a Covid-19 infection is far greater than the chance you’ll get it after being inoculated.

On its website, the Centers for Disease Control and Prevention (CDC) explains that heart inflammation has been reported as a rare side effect of the Pfizer and Modern shots “especially in male adolescents and young adults,” and more often after the second dose. When inflammation occurs, it usually happens within several days of vaccination. “Most patients with myocarditis or pericarditis who received care responded well to medicine and rest and felt better quickly,” according to the CDC.

So, the benefits of being fully vaccinated outweigh the slight risk of experiencing heart inflammation afterward. However, the mildly elevated risk of myocarditis is also one reason healthy adults under age 65 do not need Covid-19 boosters, according to the US Food and Drug Administration.

The Covid-19 pandemic has sparked a surge in the number of preprints published and shared

The mishap in Ottawa highlights the dangers of sharing scientific studies that have not been peer-reviewed—known as “preprints”—in general, but especially during a pandemic when public officials are battling an invigorated anti-science movement and conspiracy theories.

Proponents of sharing preprints on open-access websites say that the practice allows scientists around the world to share knowledge quickly during a crisis. There is no evidence, they argue, that preprint science is of lower quality than the research published by established peer-review journals.

Others believe that preprints are here to stay, so the question scientists need to address is how to deal with them. Amid a surge of preprint submissions during Covid-19, the platforms that publish the papers have begun screening pitches more carefully.

Fortunately, the Ottawa Heart Institute study didn’t attract mainstream press coverage, another common complication of the preprint boom. But that didn’t stop it from going viral. Now, anyone who encounters the 1 in 1,000 number online will also need to stumble into tweets and articles about the study’s correction to know that the frightening figure is totally inaccurate—unless they are the type of news readers who routinely check for original sources and examine them carefully.

For its part, the institute has apologized for the glaring mistake. “We are sorry this error led to misinformation about the incidence of post-vaccine myocarditis,” the Ottawa Heart Institute said in a statement about the flawed preprint. “COVID-19 vaccines are safe and have been proven effective against the disease. We invite anyone who has not yet received the shot to please get vaccinated.”