In this descriptive study of reports of myocarditis to the Vaccine Adverse Event Reporting System (VAERS) that occurred after administration of the mRNA-based COVID-19 vaccine between December 2020 and August 2021. Men comprised 82% Of the cases, the median age was 21 years and the median time to symptom onset was 2 days. There were 826 cases of myocarditis among those under 30 years of age; of these cases, 98% had elevated troponin levels, 72% had abnormal EKG results, and 72% had abnormal cardiac MRI results. 96% of people were hospitalized. Based on surveillance reports in the US, the risk of myocarditis after receiving mRNA-based COVID-19 vaccines increased after the second vaccination dose in adolescent and young men. This risk should be considered in the context of vaccination against COVID-19.