ENHANCED EPIDEMIOLOGICAL SUMMARY Myocarditis and Pericarditis Following Vaccination with COVID-19 mRNA Vaccines in Ontario: December 13, 2020 to September 4, 2021

Between the start of the COVID-19 immunization program and September 4, 2021, there have been 284 reports of myocarditis/pericarditis in Ontario that met the Brighton Collaboration case definition levels of diagnostic certainty 1, 2 or 3 for myocarditis or pericarditis. 10,11 Among the 284 reports, 77.5% occurred in males and 69.7% occurred following second dose.

The reporting rate of myocarditis/pericarditis was higher following the second dose of mRNA vaccine than after the first dose, particularly for those receiving the Moderna vaccine as the second dose of the series, regardless of the product received for the first dose (Tables 2a and 2b).

The highest reporting rate of myocarditis/pericarditis was observed in males aged 18-24 years following second dose; in this age group the reporting rate following Pfizer-BioNTech vaccine as second dose was 43.4 per million doses and was 283.4 per million following the Moderna vaccine as second dose.

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