evidence: Our population based study provides new prognostic information on myocarditis associated with vaccination. The low cumulative incidence of heart failure or death by 90 days for patients developing myocarditis after vaccination is reassuring. We previously found that the incidence of myocarditis after a second dose of mRNA vaccine was higher than after a positive test result for SARS-CoV-2 infection among younger patients.7 In this study, however, our findings strongly suggested that the clinical outcomes were substantially worse for myocarditis associated with covid-19 disease. Among younger patients with no predisposing comorbidities, we found that the risk of heart failure or death within 90 days of new onset myocarditis was about six times higher for patients with myocarditis associated with covid-19 disease than for those with myocarditis after vaccination. Also, comparing the incidence of myocarditis after vaccination versus after covid-19 disease might not be meaningful when determining recommendations for the use of mRNA vaccines against the SARS-CoV-2 virus. Vaccination with mRNA vaccines has many well described beneficial properties, including protection against severe forms of covid-19 disease18 19 and death.18 20