Myocarditis risk from mRNA COVID vaccines re-evaluated in Canadian study (Research Now Withdrawn)

Research Now Withdrawn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 In summary, the authors have withdrawn this manuscript because of a major error pertaining to the quoted incidence data.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Original news story based on the now withdrawn paper.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


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:

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.


Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Dutta, Sanchari Sinha. (2021, September 26). Myocarditis risk from mRNA COVID vaccines re-evaluated in Canadian study (Research Now Withdrawn). News-Medical. Retrieved on January 27, 2023 from

  • MLA

    Dutta, Sanchari Sinha. "Myocarditis risk from mRNA COVID vaccines re-evaluated in Canadian study (Research Now Withdrawn)". News-Medical. 27 January 2023. <>.

  • Chicago

    Dutta, Sanchari Sinha. "Myocarditis risk from mRNA COVID vaccines re-evaluated in Canadian study (Research Now Withdrawn)". News-Medical. (accessed January 27, 2023).

  • Harvard

    Dutta, Sanchari Sinha. 2021. Myocarditis risk from mRNA COVID vaccines re-evaluated in Canadian study (Research Now Withdrawn). News-Medical, viewed 27 January 2023,