This analysis of passive surveillance reports in the US revealed that an elevated risk of myocarditis after receipt of mRNA-based COVID-19 vaccines was present across multiple age and gender strata, and the risk was found to be the highest following the second vaccination dose in adolescent males and young men.

• This is a descriptive study of 1,626 cases of myocarditis occurring post-vaccination with BNT162b2 (Pfizer-BioNTech) or mRNA-1273 (Moderna) in a national passive reporting system.

• Within 7 days post-COVID-19 vaccination, crude reporting rates for myocarditis were greater than the expected rates across multiple age and gender strata.

• Rates of myocarditis cases were noted to be the highest following the second vaccination dose in adolescent males aged 12 to 15 years (70.7 per million doses of the BNT162b2 vaccine), in adolescent males aged 16 to 17 years (105.9 per million doses of the BNT162b2 vaccine), and in young men aged 18 to 24 years (52.4 and 56.3 per million doses of the BNT162b2 vaccine and the mRNA-1273 vaccine, respectively).

• Of 826 cases of myocarditis among those younger than 30 years of age with detailed clinical information, 98% exhibited increased troponin levels, 72% showed abnormal electrocardiogram results, and 72% exhibited abnormal cardiac magnetic resonance imaging results.

• Hospitalization occurred in nearly 96% of individuals (784/813), and 87% (577/661) of these experienced resolution of presenting symptoms by hospital discharge.

• Nonsteroidal anti-inflammatory drugs was the most common treatment (589/676; 87%).