Myocarditis risk from mRNA COVID vaccines re-evaluated in Canadian study (Research Now Withdrawn)

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Research Now Withdrawn

During the process of open peer review on medRxiv we quickly received a number of messages from reviewers concerned that there was a problem with our reported incidence of myocarditis post mRNA vaccination. 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. We thank the many peer reviewers who went out of their way to contact us and point out our error. We apologize to anyone who may have been upset or disturbed by our report. In summary, the authors have withdrawn this manuscript because of a major error pertaining to the quoted incidence data. Therefore, the authors do not wish this work to be cited as a reference for the project. If you have any questions, please contact the corresponding author.

Editors Note: While the data about total doses were publicly available, the researchers failed to accurately measure the number of vaccinations given during that two-month period, and the figure was astronomically higher than they had estimated. In June and July, Ottawa Public Health provided over 800,000 shots, not 32,379, as the initial paper suggested. Therefore, the true rate of side effects is closer to 1 in 25,000 - not 1 in 1,000.

Original news story based on the now withdrawn paper. "mRNA COVID-19 Vaccination and Development of CMR-confirmed Myopericarditis"

Scientists from the University of Ottawa, Canada, have recently estimated the prevalence of myocarditis/pericarditis in individuals recently immunized with mRNA-based coronavirus disease 2019 (COVID-19) vaccines. The analysis reveals a prevalence of 10 myopericarditis cases for every 10,000 vaccine doses. The study is currently available on the medRxiv* preprint server.

Study: mRNA COVID-19 Vaccination and Development of CMR-confirmed Myopericarditis. Image Credit: Kateryna Kon / Shutterstock
Study: mRNA COVID-19 Vaccination and Development of CMR-confirmed Myopericarditis. Image Credit: Kateryna Kon / Shutterstock

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.


The recent outbreak of COVID-19 caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has put a severe burden on the healthcare system of many countries, with more than 228 million infections and 4.69 million deaths reported. COVID-19 vaccines were developed in record time to control and reduce the spread of the pandemic. As of September 20, 2021, more than 5.9 billion vaccine doses have been administered globally, equivalent to 76 doses per 100 individuals.

Among currently available COVID-19 vaccines, mRNA-based vaccines have shown high efficacy against SARS-CoV-2 infection and severe disease in clinical trials and real-world setups. However, according to the World Health Organization (WHO) vaccine safety committee report, a small proportion of individuals, especially men, may develop myocarditis (heart muscle inflammation) or pericarditis (pericardium inflammation) shortly after receiving mRNA COVID-19 vaccines.

In the United States, the prevalence of myopericarditis in vaccine recipients aged 16 to 24 years has been estimated to be more than expected.

The current study has been conducted to identify myocarditis/pericarditis cases among recently vaccinated individuals in Ottawa.   

Study design

The scientists analyzed electronic health record to identify all patients who were admitted to a hospital in Ottawa between June and July 2021 with a suspected diagnosis of vaccine-induced myocarditis/pericarditis.

They diagnosed the disease based on clinical presentation, electrocardiogram (ECG)/echocardiogram (echo) findings, and serial troponin, and further confirmed the diagnosis using cardiovascular magnetic resonance imaging (CMR).  

To estimate the prevalence of myocarditis/pericarditis, they collected information from the Public Health Agency Ottawa about total doses of mRNA vaccine administered in the Ottawa region during the study period.

Important observations

A total of 32 patients with vaccine-related myocarditis/pericarditis were identified during the study period. The average age group of the patients was 33 years, and the majority were men. Symptoms developed after the second vaccine dose in 27 patients and after the first vaccine dose in only 5 patients. The most commonly reported symptoms were chest pain, breathlessness, fever, and chills.   

The average duration between the last vaccination and symptom onset was 1.5 days. Regarding types of vaccine, Pfizer/Moderna mRNA-based vaccines were identified as the most common combination, followed by Moderna/Moderna. A combination of AstraZeneca/Moderna was identified in only two patients.

At the time of admission, 22 patients showed abnormal ECG findings, including diffuse ST segment elevation with PR depression or T wave inversion. However, in 10 patients, ECG findings were normal.

Prevalence of myocarditis/pericarditis

During the study period, a total of 32,379 doses were administered locally (15,997 Moderna, 16,382 Pfizer). Based on the total vaccine doses, the estimated prevalence of myocarditis/pericarditis was 10 cases for every 10,000 vaccine doses. The estimations made separately for each vaccine type revealed that the prevalence of myocarditis/pericarditis among Pfizer and Moderna vaccine recipients was 4 and 16 cases per 10,000 immunizations, respectively.

In-hospital clinical course of Myocarditis/pericarditis

Of 32 patients, 30 were admitted to the hospital for clinical observation. Two were monitored as outpatients. The average duration of hospital stay was 4 days. The majority of patients were treated empirically with aspirin and colchicine, and none of them required steroids or other types of immunosuppressants. In all patients, symptoms subsided soon after the standard treatment. No adverse cardiac events and arrhythmias were observed during the hospital stay.

Based on the CMR findings, 18 patients were identified with myocarditis, 10 with myopericarditis, and 2 with pericarditis. Although the average left ventricular ejection fraction was 57% in most patients, nine showed mild left ventricular dysfunction with an ejection fraction below the normal threshold of 55%. Non-syncopal non-sustained ventricular tachycardia was observed in only one patient.

Study significance

The study reveals that the incidence of myocarditis is relatively infrequent among recipients of mRNA-based COVID-19 vaccines. However, younger males are more susceptible to develop the condition than females.

As mentioned by the scientists, patients with a documented history of vaccine-induced myocarditis should be carefully selected for booster vaccine doses.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Sanchari Sinha Dutta

Written by

Dr. Sanchari Sinha Dutta

Dr. Sanchari Sinha Dutta is a science communicator who believes in spreading the power of science in every corner of the world. She has a Bachelor of Science (B.Sc.) degree and a Master's of Science (M.Sc.) in biology and human physiology. Following her Master's degree, Sanchari went on to study a Ph.D. in human physiology. She has authored more than 10 original research articles, all of which have been published in world renowned international journals.


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  1. Doug Foley Doug Foley Canada says:

    1 in a 1000, too risky for me. Back to looking at the J&J I guess, in the meantime where is the Canadian 'Vegan' vaccine.

  2. Rania Makar Rania Makar Canada says:

    Why is the study focused on average 27y old , though the prominence is in young teenagers boys

  3. Holger Lundstrom Holger Lundstrom Austria says:

    Sorry, but 1 in 1,000 is NOT infrequent. Especially if it is more common with the second dose.

  4. Ed Jasper Ed Jasper United States says:

    1 out of every 1000 is NOT infrequent. STOP LYING. These are the most dangerous and fatal "vaccines" in history.  Did you know that the life expectancy for those who get diagnosed with Myocarditis is less than 10 years?

  5. Leo Notify Leo Notify Canada says:

    I'm amused that none of the prior comments agree with the article. When the government says that the vaccines are safe despite the very high incidence of adverse events, compared to previous vaccines, then it would be stupid to believe them. That is why they have to come up with vaccine mandates: (1) to blame the unvaccinated for the failure of the vaccines, and (2) to pressure people to get vaccinated.

  6. Mark Emanuel Mark Emanuel Canada says:

    Compare it to even a mild case of COVID, and it makes these vaccines at least 500 times better than the alternative.

    Also, with a double Pfizer shot, that is a 0.04% chance. It is about the same risk factor as DYING during a colonoscopy.

    • John Smith John Smith Canada says:

      First, many COVID cases are asymptomatic.
      Second, in Quebec, the proportion of *confirmed* cases among persons 10-19 that end up in the ICU is 0.5 per 1000 (28 admitted to ICU out of 52000 confirmed cases).  If the risk of mycoarditis is 1 in 1000, that is much worse than *having* COVID, let alone the risk of getting COVID.

  7. Theo Peterbroers Theo Peterbroers Netherlands says:

    I think this study has the number of vaccinations wrong. Their source 'COVID-19 Vaccinations by Day by Vaccine Type – Vaccinated in Ottawa' gives 833,951 mRNA inoculations for that period. Which is quite a different number; 352,687 Moderna and 481,264 Pfizer.

  8. Andre La Prairie Andre La Prairie Canada says:
    "The researchers mistakenly failed to record the accurate number of vaccinations given out during that two-month period, despite the data on total doses being publicly available, and the figure turned out to be astronomically higher than what was presented in the study.
    Instead of 32,379 mRNA vaccine doses administered in June and July, as the study suggests, there were actually more than 800,000 shots given out at that time, according to Ottawa Public Health. "

  9. Sam Anton Sam Anton U.A.E. says:

    On the comment in CBC Canada, for the first time ever, I pasted a link from this article as an example of a scientific article (author, intro, background, sampling, results, then conclusion)

    That was on Adam Miller's (very scientific) article of bashing this research that was bash-authoted on Swp 25 2021.
    I got a response (comment deactivated, account deactivated)

    My comment was: I publicly compared the Dr writing an article against Miller's (scientific article) and asked any college grad to read both and rule out which style is scientific.

  10. george george george george Greece says:

    Ok so the real number is 0.008% as opposed to 0.1%, still not small enough.

  11. Paul Connelly Paul Connelly Canada says:

    Population of greater Ottawa is 1.4 mil. Did 800,000 cue up for jabs in the two months under study? Pressure from somewhere.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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