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.