Background: Understanding the clinical course and short-term outcomes of suspected myocarditis following COVID-19 vaccination has important public health implications in the decision to vaccinate youth. Methods: We retrospectively collected data on patients <21 years-old presenting before 7/4/2021 with suspected myocarditis within 30 days of COVID-19 vaccination. Lake Louise criteria were used for cardiac magnetic resonance imaging (cMRI) findings. Myocarditis cases were classified as confirmed or probable based on the Centers for Disease Control and Prevention definitions. Results: We report on 139 adolescents and young adults with 140 episodes of suspected myocarditis (49 confirmed, 91 probable) at 26 centers. Most patients were male (N=126, 90.6%) and White (N=92, 66.2%); 29 (20.9%) were Hispanic; and median age was 15.8 years (range 12.1-20.3, IQR 14.5-17.0). Suspected myocarditis occurred in 136 patients (97.8%) following mRNA vaccine, with 131 (94.2%) following the Pfizer-BioNTech vaccine; 128 (91.4%) occurred after the 2nd dose. Symptoms started a median of 2 days (range 0-22, IQR 1-3) after vaccination. The most common symptom was chest pain (99.3%). Patients were treated with nonsteroidal anti-inflammatory drugs (81.3%), intravenous immunoglobulin (21.6%), glucocorticoids (21.6%), colchicine (7.9%) or no anti-inflammatory therapies (8.6%). Twenty-six patients (18.7%) were in the ICU, two were treated with inotropic/vasoactive support, and none required ECMO or died. Median hospital stay was 2 days (range 0-10, IQR 2-3). All patients had elevated troponin I (N=111, 8.12 ng/mL, IQR 3.50-15.90) or T (N=28, 0.61 ng/mL, IQR 0.25-1.30); 69.8% had abnormal electrocardiograms and/or arrythmias (7 with non-sustained ventricular tachycardia); and 18.7% had left ventricular ejection fraction (LVEF) <55% on echocardiogram. Of 97 patients who underwent cMRI at median 5 days (range 0-88, IQR 3-17) from symptom onset, 75 (77.3%) had abnormal findings: 74 (76.3%) had late gadolinium enhancement, 54 (55.7%) had myocardial edema, and 49 (50.5%) met Lake Louise criteria. Among 26 patients with LVEF <55% on echocardiogram, all with follow-up had normalized function (N=25). Conclusions:Most cases of suspected COVID-19 vaccine myocarditis occurring in persons <21 years have a mild clinical course with rapid resolution of symptoms. Abnormal findings on cMRI were frequent. Future studies should evaluate risk factors, mechanisms, and long-term outcomes.

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ACP with cTn elevation occurs in DMD boys and may be indicative of cardiomyopathy progression as evidenced by acute left ventricular dysfunction and development or progression of myocardial fibrosis. This clinical presentation is under recognized. These events may represent an important pathophysiological mechanism in cardiomyopathy progression.

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Of 3,383 apparently healthy newborn infants studied in one region of England, 33 showed cardiac arrhythmias or preexcitation on a standard ECG. Twenty-six infants had multiple atrial or ventricular premature beats. One infant had atrial flutter, two had multifocal atrial tachycardia, and two had supraventricular tachycardia. Two infants showed the ECG pattern of the Wolff-Parkinson-White syndrome, but did not demonstrate cardiac arrhythmias. Performance of 24-hour ECG recordings on 15 of the 26 subjects with premature beats showed additional supraventricular or ventricular tachycardia in five cases. In one of two infants with multifocal atrial tachycardia, 24-hour recordings also demonstrated long episodes of rapid supraventricular tachycardia. Follow-up recordings (standard and 24-hour ECG) showed that in the majority of infants arrhythmias or preexcitation could not be demonstrated after 12 weeks of age. One infant with atrial premature beats, one infant with ventricular premature beats and ventricular tachycardia, and one infant with multifocal atrial tachycardia showed persisting arrhythmias. Four infants received antiarrhythmic therapy which probably influenced the natural history of their arrhythmias. Disorders of cardiac rhythm and conduction occur in apparently healthy infants. This study provides a basis for further prospective research into their natural history.