PROJECT SUMMARY The cardiac inflammatory disorder myocarditis is a rare but important complication following vaccination against COVID-19. Other than younger age and male sex, no risk factors for this adverse outcome have been found. Recognition of host factors that predispose to vaccine-associated myocarditis (VAM) would highlight those individuals for whom the risk of vaccination may be higher than the risk of COVID-19 infection and should therefore avoid or alter vaccination. Furthermore, fundamental insights into the pathogenesis of this complication and the interaction of innate cardiac immune responses with host genetics are lacking. We previously discovered that ~16% of adults and children with idiopathic or viral myocarditis harbor rare putatively damaging variants in a set of cardiac genes. Our findings suggest that genetically-mediated impairments in the structural integrity of cardiomyocytes render them susceptible to injury by cardiotoxic agents and, therefore, such variants may similarly underlie VAM. Leading a multi-institution consortium, we will recruit 50 individuals aged ≥ 5 years with VAM and perform exome sequencing to interrogate for relevant variation in 92 myocarditis-related genes, in comparison to exome sequencing of individuals with a history of uncomplicated COVID-19 vaccination. This directed genomic approach is well-powered to identify host genetic risk factors for VAM and enable precision risk stratification to minimize potential harms in genetically vulnerable individuals. Additionally, we will perform multi-scale immunophenotyping via single cell RNA sequencing (scRNA), cytometry by time of flight (CyTOF), cytokine and autoantibody assays to uncover the pathogenesis of this condition. Comparing immune responses in those with and without susceptible genotypes will improve understanding of VAM susceptibility and new prophylactic/treatment strategies.