Myocarditis after mRNA COVID-19 vaccines: A comparative analysis using French Spontaneous Reporting Data

ABSTRACT

Introduction: Post-mRNA vaccine myocarditis emerged as a rare adverse effect quickly after adolescents and young adult populations started to be vaccinated, and it was labelled as such for both vaccines. This work aimed to study the Reporting rates (Rr) of myocarditis after mRNA-1273 and BNT162b2 injections. Material and methods: mRNA vaccine exposures were retrieved from the French SI-VAC database, while cases of myocarditis were retrieved from the French spontaneous reporting database (Base Nationale de Pharmacovigilance, BNPV) up to 30 September 2021. All cases were routinely evaluated by drug safety medical professionals and repeated at national level in the context of an intensive pharmacovigilance monitoring. Reporting rates (Rr) per 100.000 injections were calculated according to age, gender and injection rank;Poisson distribution was used to compute Rrs 95% Confidence Interval (95% CI). Results: Over the study period, almost 73 million BNT162b2 and 10 million mRNA-1273 doses were injected, and 376 and 106 confirmed cases of myocarditis were reported, respectively. These occurred in the week following the injection for 76.4% of cases reported after mRNA-1273 (n = 81) vs. 58.4% after BNT162b2 (n = 219). Rr per 100.000 injections was 1.1 (95% CI 0.9 to 1.3) for mRNA-1273 and 0.5 (0.5 to 0.6) for BNT162b2. After the second injection, the difference among vaccines was more pronounced in men aged 18-24 years (13.9 [9.2 to 20.1] for mRNA-1273 vs. 4.3 [3.4 to 5.5] for BNT162b2) and aged 25-29 years (7.0 [3.4 to 12.9] vs. 1.9 [1.2 to 2.9]);in younger people, the low number of cases did not allow any meaningful conclusion. In women, Rrs were globally lower, but differences between vaccines were close to those found for men. Discussion/Conclusion: The present study suggests that mRNA-1273 is related with a higher risk of myocarditis than BNT162b2 in people aged under 30, especially after the second injection.