Aim of the study: Post-mRNA coronavirus diseases 2019 (COVID-19) vaccines myocarditis emerged as a rare adverse effect, particularly in adolescents and young adults, and was labeled as such for both vaccines in the summer of 2021. This study aims to summarize the timeline and process of signal detection, substantiation, and quantification of myocarditis cases related to mRNA vaccines in France.

Methods: The intensive monitoring plan for COVID-19 vaccine safety was based on case-by-case analysis of all cases collected in the French spontaneous reporting database (base nationale de pharmacovigilance, BNPV). Cases were evaluated by drug safety medical professionals and discussed at a national level for signal detection purposes. Reported cases were compared to the number of vaccine-exposed persons up to September 30, 2021. Reporting rates (Rr) of myocarditis per 100,000 injections were calculated and stratified according to age, gender, and injection rank of BNT162b2 and mRNA-1273 vaccines. Poisson distribution was used to compute Rrs 95% Confidence Interval (95% CI). Results.- The case-by-case analysis detected a possible cluster of myocarditis in April 2021 (5 cases, 4 after the 2nd injection). In June 2021, the signal was substantiated with 12 cases (9 related to BNT162b2, and 3 to mRNA-1273). As of September 2021, almost 73 million BNT162b2 and 10 million mRNA-1273 doses had been injected. The Rr per 100,000 injections was 0.5 (0.5-0.6) for BNT162b2 and 1.1 (95% CI 0.9-1.3) for mRNA-1273. The difference among vaccines was more pronounced after the second injection, particularly in men aged 18-24 years (4.3 [3.4-5.5] for BNT162b2 vs. 13.9 [9.2-20.1] for mRNA-1273) and aged 25-29 years (1.9 [1.2-2.9] vs. 7.0 [3.4-12.9]).

Conclusion: The study highlighted the role of the spontaneous reporting system in the detection, assessment, and quantification of myocarditis related to m-RNA vaccines. It suggested from September 2021 that mRNA-1273 was reasonably related to a higher risk of myocarditis than BNT162b2 in people under 30, particularly after the second injection.