Results:

During the study period, 192,405,448 individuals aged >12 years received 354,100,845 mRNA-based COVID-19 vaccines. VAERS received 1,991 reports of myocarditis, of which 1,626 met the CDC case definition for probable or confirmed myocarditis. Among individuals meeting the case definition for myocarditis, 1,195 (73%) were <30 years of age, and 1,334 (82%) were male. Of the reports of myocarditis with dose information, 1,265/1,538 (82%) occurred after the second dose. Median time from vaccination to symptom onset was 3 days (interquartile range [IQR], 1-8 days) for cases that occurred after the first vaccination dose and 2 days (IQR, 1-3 days) for cases that occurred after the second dose. Reporting rates of myocarditis were highest in males aged 12-15 years (70.7 per million doses), 16-17 years (105.9 per million doses), and 18-24 years (52.4 per million doses of BNT162b2 and 56.3 per million doses of mRNA-1273), after the second vaccine dose. Among females, rates were highest in individuals aged 16-17 years (11.0 per million doses). For multiple age and sex strata, observed myocarditis rates exceeded expected rates based on historical data.

Among individuals with confirmed myocarditis <30 years of age, 89% presented with chest pain/pressure/discomfort, 29.6% with dyspnea, and 8% with palpitations. Elevated troponin level was present in 97.9% of these individuals, 71.7% had electrocardiographic abnormalities, 11.7% had decreased left ventricular ejection fraction (<50%) on echocardiogram, and 71.5% had cardiac magnetic resonance imaging findings consistent with myocarditis. Commonly used treatments were nonsteroidal anti-inflammatory drugs (87.1%), intravenous immunoglobulin (11.5%), and glucocorticoids (12%). Vasoactive medications were used in 1.8% of cases. Of the 96.4% of patients who were hospitalized, none died, and 98% had been discharged from the hospital at the time of writing.