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.

The mRNA vaccines against COVID-19 infection have been effective in reducing the number of symptomatic cases worldwide. With widespread uptake, case series of vaccine-related myocarditis/pericarditis have been reported, particularly in adolescents and young adults. Males tend to be affected with greater frequency, and symptom onset is usually within one week following vaccination. Clinical course appears to be mild in the overwhelming majority of cases. Based on the available evidence, we highlight a clinical framework to guide providers on how to assess, investigate, diagnose and report suspected and confirmed cases. In any patient with highly suggestive symptoms temporally related to COVID-19 mRNA vaccination, standardized work-up includes serum troponin measurement and PCR testing for COVID-19 infection, routine additional labwork, and a 12-lead ECG. Echocardiography is recommended as the imaging modality of choice for patients with unexplained troponin elevation and/or pathologic ECG changes. Cardiovascular specialist consultation and hospitalization should be considered based on the results of standard investigations. Treatment is largely supportive, and myocarditis/pericarditis that is diagnosed by defined clinical criteria should be reported to Public Health authorities in every jurisdiction. Finally, we recommend COVID-19 vaccination in all individuals in accordance with the Health Canada and NACI guidelines. In patients with suspected myocarditis/pericarditis following the 1 st dose of an mRNA vaccine, deferral of 2 nd dose is recommended until additional reports become available. With widespread uptake of COVID-19 mRNA vaccines, reports of vaccine-related myocarditis/pericarditis have emerged. Based on the available evidence, we highlight a clinical framework to guide providers on how to assess, investigate, diagnose and report suspected and confirmed cases. We recommend COVID-19 vaccination in all individuals in accordance with the Health Canada and NACI guidelines. For patients with suspected myocarditis/pericarditis following the 1 st dose of mRNA vaccine, deferral of 2 nd dose is recommended pending further data.