Fact Check-Pre-print study that claimed 1 in 1,000 risk of myocarditis following COVID-19 vaccine was withdrawn due to miscalculation

Correction Oct. 4: corrects spelling of denominator in paragraph 13, and in the same paragraph corrects “25 orders of magnitude off” to “25 times smaller”. Changes ‘magnitude’ to ‘factor’ in verdict.

A pre-print study which claimed that there is a 1 in 1,000 risk of contracting a heart inflammation condition known as myocarditis has since been retracted due to a calculation.

The study which was conducted by researchers at The University of Ottawa Heart Institute, was featured in numerous blogs and social media posts as proof that the COVID-19 vaccine is unsafe for use.

“New Study Shows 1 in 1000 Develop Heart Inflammation After Covid Vaccination; Myocarditis and Other Related Heart Conditions Have Increased Death Rate Within 5 Years,” the headline of an article published on September 21 on The Gateway Pundit reads ( ).

Examples of the claim shared on social media are viewable ( here ), ( here ), and ( here ).

Other links to blogs referencing the retracted study can be found ( ) and ( ).

The study was first published on September 16 on the platform MedRxiv, a website that publishes studies that have yet to be peer-reviewed ( here ).

Studies go through a process known as peer-review where experts in a specific field analyze the piece to ensure its accuracy and to assess that it is of high academic standard ( here ), ( ).

The pre-print paper that has since been retracted is viewable in full ( ).

The study had calculated an incidence rate of myocarditis in the Ottawa region post-vaccination by dividing the number of occurrences of the heart inflammation condition over a two-month period (June and July 2021) in Ottawa (32) by the total number of vaccinations in the area (reported as 32,379 in the pre-print).

The incidence rate of myocarditis, using these figures, equates to 10 for every 10,000 doses of the vaccine.

The pre-print paper had used an incorrect figure for the number of doses administered in Ottawa over that two-month period, however.

Between the week beginning May 30 and the week starting July 25, there had been 845,930 vaccines administered in the Ottawa region, according to data published by Ottawa Public Health, which is far greater than the figure used to calculate the incidence rate (32,379) ( here ).

The denominator (total vaccines administered over a two-month period in Ottawa) used to calculate the incidence rate of myocarditis in the pre-print study was approximately 25 times smaller than the correct figure.

The study was then withdrawn on September 24 and in a statement the researchers said: "Our reported incidence appeared vastly inflated by an incorrectly small denominator (ie number of doses administered over the time period of the study). We reviewed the data available at Open Ottawa and found that there had indeed been a major underestimation, with the actual number of administered doses being more than 800,000 (much higher than quoted in the paper)” ( ).

“In order to avoid misleading either colleagues or the general public and press, we the authors unanimously wish to withdraw this paper on the grounds of incorrect incidence data,” they added.

The University of Ottawa Heart Institute released a statement saying that they were “sorry this error led to misinformation about the incidence of post-vaccine myocarditis” ( ).

Meanwhile, numerous blog posts that reported on the study did not issue an update on these blogs regarding the retraction, nor did they mention that this study was a pre-print that had not been reviewed.

In response to the retracted pre-print, a spokesperson for the Centers for Disease Prevention and Control (CDC) sent Reuters a study released on September 3 which monitored adverse events following mRNA vaccines between December 14, 2020, and June 26, 2021, with data from Vaccine Safety Datalink. ( here ).

“Analyses of all ages combined did not detect a significant association between myocarditis/pericarditis and mRNA vaccines,” the report noted, although adding that there was evidence of “an association between mRNA vaccines and myocarditis/pericarditis in younger individuals”.

The latest CDC guidance states that young men are at greater risk of getting myocarditis after the second dose of an mRNA vaccine, although the benefits of the vaccine still outweigh risks ( here ).


Missing Context. A Canadian study had found that the incidence rate of myocarditis is 1 in 1,000 following an mRNA vaccine. The study has since been retracted due to an error making the above calculation, where the denominator was incorrect by a factor of roughly 25.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here.