Abstract Background Myocarditis is a very rare event following COVID-19 vaccination, with highest risk observed in males aged 12–24 years. Cardiac troponin I (cTnI) is a cardiac-specific biomarker of myocardial injury but is not etiologically specific and may be elevated in the absence of cardiac pathology, including after vigorous physical activity. This study assessed cTnI levels following administration of mRNA-1273.712 vaccine versus placebo in participants aged 12–30 years. Methods In this