Myopericarditis requiring emergency care or hospitalization after COVID-19 mRNA vaccination occurs most frequently in adolescent males. In the acute phase, vaccine-associated heart inflammation is characterized by elevated cardiac biomarkers (troponin, B-type natriuretic peptide), electrocardiographic abnormalities, abnormal cardiac magnetic resonance imaging, elevated interleukins and chemokines, expansion of activated cytotoxic T lymphocytes, and monocyte dysregulation. This adverse event may occur one or two days after the first injection but is far more frequent after the second, suggesting contributions from trained innate immunity and/or cumulative dose effects. A recent mouse study in this journal reported dramatic increases in both cardiac biomarkers two days after the second dose of lipid nanoparticles (LNPs) containing mRNA coding for the Omicron spike, despite absence of histopathological heart damage at 14 days—even after intravenous administration. Here, these findings are discussed in the context of human observations and additional mouse experiments. I propose that endothelial cells (ECs) of the myocardial microvasculature are a preferred off-target for LNPs because of the unique features of myocardial anatomy and perfusion. Endothelial injury via toll-like receptor 4 (TLR4) activation by ionizable lipids and/or endosomal rupture may represent an initiating step (“endothelitis”), followed by recognition of spike-derived peptides presented by ECs to activated monocytes and T lymphocytes. The potential role of the Wuhan spike protein in establishing a trained innate immunity phenotype, and species differences in TLR sensitivity, are considered.