Myocarditis and Pericarditis Following COVID19 mRNA Vaccines

1 October 2021, Public Health Ontario


Surveillance of myocarditis/pericarditis following COVID-19 vaccination from passive and active vaccine safety surveillance systems in Canada demonstrates a higher number of cases in younger people (i.e., less than 40 years of age) than would normally be expected in the general population. Weekly updates on national vaccine safety data can be found on the Public Health Agency of Canada’s (PHAC) website. 3. Health Canada recently updated the product monographs for both Moderna Spikevax® and PfizerBioNTech Comirnaty® COVID-19 vaccines to include information about these risks.4 Health Canada and PHAC continue to monitor reports of myocarditis/pericarditis and the evolving information regarding the association with mRNA vaccines.

Public Health Ontario’s (PHO) enhanced epidemiological summary of myocarditis/pericarditis following COVID-19 mRNA vaccines in Ontario from December 13, 2020 to August 7, 2021 similarly found that the highest reporting rate of myocarditis/pericarditis was observed in males aged 18-24 years following the second dose of mRNA vaccines.5 Additionally, the reporting rate of myocarditis/pericarditis was higher for those receiving the Moderna Spikevax® as the second dose of the series (regardless of the product received for the first dose). 5 For males aged 18-24 years old, the reporting rate for second dose Moderna Spikevax® was 263.2 per million doses compared to 37.4 per million doses in this group with Pfizer-BioNTech Comirnaty® second dose.5

In the United States, analysis from the Vaccine Safety Datalink (VSD) has shown an elevated rate ratio of confirmed myocarditis/pericarditis cases among 12 to 39 year olds, after both Pfizer-BioNTech Comirnaty® and Moderna Spikevax® COVID-19 vaccines, during days 0-21 and especially, days 0-7 following vaccination, most notably after the second dose (adjusted rate ratio, 23.84, 95% confidence interval, 8.49 to 83.64).6 In Israel, Pfizer-BioNTech Comirnaty® was found to be associated with an excess myocarditis risk of 1 to 5 events per 100,000 persons (risk ratio, 3.24; 95% confidence interval, 1.55 to 12.44).7

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