After they received a coronavirus disease 2019 (COVID-19) mRNA vaccination, blood samples were collected from adolescents and young adults who developed myocarditis or who had no vaccine-related complications. The concentrations of anti–severe respiratory syndrome coronavirus disease 2 (SARS-CoV-2) antibodies, SARS-CoV-2 antigens, and cytokines were measured, and hematology and T-cell profiling was performed with the collected blood samples.
