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Myocarditis and pericarditis may constitute adverse reactions of mRNA coronavirus disease 2019 (COVID-19) vaccines. This study aimed to document these reactions and to assess the association with patient sex and age. This is as an observational retrospective study using a case-non-case design (also called disproportionality study) on inflammatory heart reactions reported with mRNA COVID-19 vaccines within the World Health Organization (WHO) global safety database (VigiBase), up to June 30, 2021. Results are expressed using reporting odds ratios (RORs) and their 95% confidence interval (95% CI). Of 716,576 reports related to mRNA COVID-19 vaccines, 2,277 were cases of inflammatory heart reactions, including 1241 (55%) myocarditis and 851 (37%) pericarditis. The main age group was 18-29 years (704, 31%), and mostly male patients (1,555, 68%). Pericarditis onset was delayed compared with myocarditis with a median time to onset of 8 (3-21) vs. 3 (2-6) days, respectively (P = 0.001). Regarding myocarditis, an important disproportionate reporting was observed in adolescents (ROR, 22.3, 95% CI 19.2-25.9) and in 18-29 years old (ROR, 6.6, 95% CI 5.9-7.5) compared with older patients, as well as in male patients (ROR, 9.4, 95% CI 8.3-10.6). Reporting rate of myocarditis was increased in young adults and adolescents. Inflammatory heart reactions may rarely occur shortly following mRNA COVID-19 vaccination. Although an important disproportionate reporting of myocarditis was observed among adolescents and young adults, particularly in male patients, reporting rates support a very rare risk, that does not seem to compromise the largely positive benefit-risk balance of these vaccines. Furthermore, this study confirmed the value of disproportionality analyses for estimation of relative risks among subgroups of patients., (© 2021 The Authors. Clinical Pharmacology & Therapeutics published by Wiley Periodicals LLC on behalf of American Society for Clinical Pharmacology and Therapeutics.)