Myocarditis and pericarditis may constitute adverse reactions of mRNA 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 WHO global safety database (VigiBase), up to June 30th 2021. Results are expressed using reporting odds-ratios (ROR) and their 95% confidence interval (95%CI). Of 716,576 reports related to mRNA COVID-19 vaccines, 2277 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 males (1555, 68%). Pericarditis onset was delayed compared to 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 [19.2-25.9]) and in 18-29 years old (ROR, 6.6 [5.9-7.5]) compared to older patients, as well as in males (ROR, 9.4 [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 males, 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.