1. The incidence of myocarditis following administration of mRNA SARS-CoV-2 vaccine was higher than the baseline incidence.

2. Rates of myocarditis were higher amongst males, individuals aged 12-29 years old, patients receiving the Moderna mRNA-1273 vaccine, and following the second dose.

Evidence Rating Level: 2 (Good)

Study Rundown: Postvaccination myocarditis has been identified as an adverse event associated with the mRNA SARS-CoV-2 vaccines, produced by Pfizer-BioNTech (BNT162b2) and Moderna (mRNA-1273), particularly in adolescents and younger adults. However, the evidence stems mainly from case reports and case series, and there is also insufficient data on myocarditis occurrence after the third vaccine dose. Therefore, this observed-to-expected analysis compared the cases of post-SARS-CoV-2 vaccination myocarditis, against a calculated number of cases under the null hypothesis, if there were no association between myocarditis and vaccination. This calculation was performed using background rates of myocarditis ER visits and admissions from 2019, in the same population. The primary outcome was an emergency room visit or hospital admission for myocarditis, which may have resulted in missing milder cases that were diagnosed in an outpatient context. The study found an observed to expected case ratio of 7.03. The incidence was higher for individuals aged 12-29 years compared to older adults, for males compared to females, and for the second compared to third dose. The highest ratio was observed for males 18-29 years old following their second dose, with an mRNA-1273 vaccine. Overall, this study demonstrated that the incidence of myocarditis after vaccination with mRNA SARS-CoV-2 vaccines were higher than expected, and were highest in males, in those aged 12-29 years, after the mRNA-1273 vaccine, and after the second vaccine dose.

Click here to read the study in CMAJ

Relevant Reading: Clinical characteristics and prognostic factors of myocarditis associated with the mRNA COVID-19 vaccine

In-Depth [retrospective cohort]: The study population consisted of individuals 12 years and older, receiving a BNT162b2 or mRNA-1273 vaccine between December 2020 and March 2022, in British Columbia, Canada. These dates included individuals receiving their 1st, 2nd, and 3rd doses of vaccine. Patients with a history of myocarditis or myopericarditis within 1 year prior to a vaccine dose were excluded. In total, there were 10,255,385 mRNA vaccine doses administered in the study period, with 6,989,921 BNT162b2 vaccines and 3,265,464 mRNA-1273 vaccines. There were 99 myocarditis cases observed within 7 days of vaccine (7 cases expected), and 141 cases within 21 days of vaccine (20 cases expected). This resulted in an incidence of 0.97 myocarditis cases per 100,000 vaccine doses within 7 days, and 1.37 cases per 100,000 vaccine doses within 21 days. The ratio of observed to expected cases was 7.03. Rates of myocarditis at 7 days was higher for males than females (1.64, 95% CI 1.30-2.04 vs 0.35, 95% CI 0.21-0.55), and highest among those aged 12-17 (2.64, 95% CI 1.54-4.22) and 18-29 (2.63, 95% CI 1.94-3.50) years. As well, rates were higher for the mRNA-1273 vaccine compared to the BNT162b2 vaccine (1.44, 95% CI 1.06-1.91 vs 0.74, 95% CI 0.56-0.98), with the largest difference after the second dose in males 18-29 years old (22.1, 95% CI 14.1-32.8 vs 5.1, 95% CI 2.7-8.7). Furthermore, the rate was higher after the second dose compared to the third dose (1.90, 95% CI 1.50-2.39 vs 0.76, 95% CI 0.45-1.20).

Image: PD

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