Comparative assessment of myocarditis and pericarditis reporting rates related to mRNA COVID-19 vaccines in Europe and the United States.

The novel mRNA vaccines proved to be safe and effective in averting severe COVID-19. Vaccine-related complications recorded by pharmacovigilance systems, such as 'EudraVigilance' in Europe and 'VAERS' in the United States (US), rarely include myocarditis and pericarditis. Given the novelty of the platform and the increasing global-scale vaccine production needs, we assessed their reporting rates comparatively across continents. Data of myocarditis and pericarditis cases post COVID-19 vaccination reported from week 52/2020 (December 21 to 27, 2020) to week 40/2021 (October 4 to 10, 2021) were collected for mRNA vaccines from EudraVigilance and VAERS. The corresponding administered vaccine doses were used as denominators to estimate reporting rates for comparison purposes. Cross-tabulation analysis was employed to compare the reporting rates of mRNA vaccines-associated myocarditis and pericarditis between EudraVigilance and VAERS. Low reporting rates of myocarditis (7.64/million vaccine doses) and pericarditis (5.32/million) were found, with higher rates of both disorders in EudraVigilance compared to VAERS; these differences were more pronounced post-mRNA-1273 (5-6-fold, p=0.000 for myocarditis and p<0.001 for pericarditis) than post-BNT162b2 vaccination (1.5-2-fold, p<0.001 for both conditions). Most myocarditis cases occurred in males <30 years. Pericarditis affected predominantly males <40 and both sexes >40 years. The extremely rare fatalities related to myocarditis (0.102/million) or pericarditis (0.017/million) were also higher in EudraVigilance versus VAERS. Understanding the underlying causes of the observed differences could provide guidance for the enhanced quality of mRNA vaccines that would also foster vaccine acceptance.