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Background: Since the onset of widespread COVID-19 vaccination, increased incidence of COVID-19 vaccine-associated myocarditis (VA-myocarditis) has been noted, particularly in male adolescents., Methods: Patients <18 years with suspected myocarditis following COVID-19 vaccination within 21 days were enrolled in the PedMYCVAC cohort, a substudy within the prospective multicenter registry for pediatric myocarditis "MYKKE." Clinical data at initial admission, 3- and 9-months follow-up were monitored and compared to pediatric patients with confirmed non-vaccine-associated myocarditis (NVA-myocarditis) adjusting for various baseline characteristics., Results: From July 2021 to December 2022, 56 patients with VA-myocarditis across 15 centers were enrolled (median age 16.3 years, 91% male). Initially, 11 patients (20%) had mildly reduced left ventricular ejection fraction (LVEF; 45%-54%). No incidents of severe heart failure, transplantation or death were observed. Of 49 patients at 3-months follow-up (median (IQR) 94 (63-118) days), residual symptoms were registered in 14 patients (29%), most commonly atypical intermittent chest pain and fatigue. Diagnostic abnormalities remained in 23 patients (47%). Of 21 patients at 9-months follow-up (259 (218-319) days), all were free of symptoms and diagnostic abnormalities remained in 9 patients (43%). These residuals were mostly residual late gadolinium enhancement in magnetic resonance imaging. Patients with NVA-myocarditis (n=108) more often had symptoms of heart failure (P = .003), arrhythmias (P = .031), left ventricular dilatation (P = .045), lower LVEF (P < .001) and major cardiac adverse events (P = .102)., Conclusions: Course of COVID-19 vaccine-associated myocarditis in pediatric patients seems to be mild and differs from non-vaccine-associated myocarditis. Due to a considerable number of residual symptoms and diagnostic abnormalities at follow-up, further studies are needed to define its long-term implications., Competing Interests: Disclosures None reported., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Myocarditis is a major cause of severe heart failure in pediatric patients. In some patients, this may lead to the need of mechanical circulatory support (MCS) or heart transplantation. MYKKE is a multicenter prospective registry of pediatric patients with suspected myocarditis. We aimed to determine characteristics and outcome of patients with severe heart failure requiring MCS and/or heart transplantation within the MYKKE registry in order to get a better understanding of this critically ill subgroup. Between September 2013 and October 2022, patients within the MYKKE registry were analyzed. Requirement of MCS including Extracorporeal Membrane Oxygenation (ECMO) and Ventricular Assist Device (VAD), heart transplantation and/or occurrence of death defined a sub-cohort of special interest. A total amount of 649 patients was enrolled by 26 medical centers. Median age was 14.3 years, 67% were male. Thirteen percent within this population (n=84/649) received MCS (3 % ECMO; 6 % VAD; 4 % both, ECMO and VAD). Thirty-nine patients (6 %) needed a heart transplant and 33 patients (5 %) died. The subgroup of MCS, heart transplant and dead patients (17 %, n=108) presented with a young median age of 2 years, 55 % were male. Left ventricular dysfunction at initial admission was represented by a median LVEF of 21 %. Out of 63 patients requiring VAD, 17 could be successfully weaned (27 %) after a median time of 97 days. Thirty-one patients were MCS-bridged to transplantation (37 %, n=31/84). Within our subgroup 79 % of patients (n = 85/108) underwent endomyocardial biopsy and in 64 cases diagnosis of myocarditis was confirmed (28 % acute myocarditis, 64 % chronic/healing myocarditis, 8 % healed), further diagnoses included dilated cardiomyopathy (n=9) or others (n=12). Myocarditis represents a life-threatening condition in pediatric patients. Especially young patients are affected by a severe clinical course, which requires MCS and transplantation due to end-stage heart failure. Different underlying pathomechanisms may contribute in a possibly age-dependent manner as genetic predispositions and/or immunological factors. [ABSTRACT FROM AUTHOR]