The COVID-19 global pandemic began in December 2019.¹ The introduction of the messenger RNA (mRNA) vaccine against SARS-CoV-2 has resulted in a significant decline in COVID-19–related morbidity and mortality all over the world.^2-4 COVID-19 mRNA vaccines are the current standard approach to contain the pandemic and the Emergency Use Authorizations for these vaccines were recently extended to children aged 6 months and older in the US. Since the emergency use was authorized, the association of the mRNA-based COVID-19 vaccine with myopericarditis, which is a rare but serious adverse event, has been reported.^5,6 Cases of myopericarditis following COVID-19 mRNA vaccination have been reported worldwide, especially in adolescents and young adults. In addition, in June 2021, the Centers for Disease Control and Prevention (CDC) observed a rate of postvaccine myopericarditis that was higher in young males after the second mRNA vaccine doses.⁷ Previous studies reported largely favorable outcomes in adults with myocarditis following COVID-19 mRNA vaccination, demonstrating resolution of clinical symptoms, preservation of cardiac function, and no complications.^8-11 However, data on the clinical features and outcomes of myopericarditis after COVID-19 vaccination in adolescents and young adults are scarce compared with adults and often consist of small case series. Most large-scale studies on COVID-19 vaccine–associated myopericarditis among adolescents and young adults are derived from national population-based studies that contain homogeneous populations or from surveillance networks that rely on passive reporting. In this study, we conducted a systematic review and meta-analysis to investigate the clinical spectrum and outcomes of COVID-19 vaccine–associated myopericarditis in adolescents and young adults from an international population.

Background: There is significant overlap in clinical features between multisystem inflammatory syndrome in children (MIS‐C) and Kawasaki disease (KD). We sought to compare the prevalence, severity, and associated factors for coronary artery (CA) involvement. Methods and Results: From January 1, 2020 through January 31, 2023, 1191 patients with MIS‐C and 554 patients contemporaneously diagnosed with KD were enrolled into the International Kawasaki Disease Registry. Demographic and clinical features, laboratory values, maximum Z score in any CA branch at any time point, and worst left ventricular ejection fraction, were compared between groups. Factors associated with CA aneurysms (maximum Z score in any CA branch +2.5 or greater) were determined separately for each diagnosis using multivariable logistic regression analyses. The prevalence of CA aneurysms was lower for MIS‐C versus KD (16% versus 25%, respectively; P <0.001) and less severe by size category (1.2% with medium/large CA aneurysm versus 9.6%, respectively). Male sex and lower nadir hemoglobin levels were associated with greater odds of CA aneurysms for both groups. Additional associated factors for KD patients included age<6 months, fewer clinical KD criteria (more incomplete presentation), presentation with shock, and greater total days of fever. There were no additional associated factors for patients with MIS‐C. Using exploratory splines, there was a trend of improvement in Z scores within 30 days of illness for both MIS‐C and KD for CA involvement other than large aneurysms. Conclusions: CA involvement for patients with MIS‐C was less prevalent and milder in severity compared with contemporaneous patients with KD, with fewer associated factors, and a high prevalence of regression to a normal luminal dimension.