evidence: The present nationwide study involving more than 44 million vaccinated individuals in Korea demonstrated several clinically important findings on acute myocarditis after COVID-19 vaccination. First, VRM was a very rare complication of COVID-19 vaccination (1.08 cases per 100 000 vaccinated persons) and mainly developed in association with mRNA vaccines, especially in young males. Second, the demographic characteristics of COVID-19 VRM differed from those of the previous studies. Third, notably, we demonstrated severe COVID-19 VRM including FM or death was not uncommon (19.8% of total VRM). Sudden cardiac death attributable to COVID-19 VRM demonstrated in this study warrants the careful monitoring or warning of SCD as a potentially fatal complication of COVID-19 vaccination, especially in individuals who are ages under 45 years with mRNA vaccination. Fourth, the incidence of severe cases of COVID-19 VRM significantly decreased in the third vaccination than in the first or second COVID-19 vaccination (Structured Graphical Abstract). [...] Sudden cardiac death was the most serious and worrisome adverse reaction of COVID-19 vaccination in our study. In eight SCD cases, VRM was not suspected as a clinical diagnosis or a cause of death before performing an autopsy. All SCD cases attributable to COVID-19 VRM were aged under 45 years and received mRNA vaccines. Vaccine-related myocarditis was the only possible cause of death in all SCD cases. Therefore, SCD attributable to COVID-19 VRM demonstrated in this study warrants the careful monitoring or warning of SCD as a potentially fatal complication of COVID-19 vaccination, especially in individuals who are ages under 45 years and receiving mRNA vaccination.