off label use; interchange of vaccine products; acute myocarditis after third dose sars-cov2 mrna vaccine; this is a spontaneous report received from a contactable reporter(s) (physician) from the regulatory authority-web. regulatory number: it-minisal02-843223. a 15 year-old male patient received bnt162b2 (comirnaty), intramuscular, administered in arm left, administration date 06feb2022 10:00 (lot number: 33295tb) as dose 3 (booster), 30 ug, single for covid-19 immunisation. the patient's relevant medical history and concomitant medications were not reported. vaccination history included: moderna (dose 1), for covid-19 immunisation; moderna (dose 2), for covid-19 immunisation. the following information was reported: off label use (hospitalization), outcome "unknown", described as "off label use"; interchange of vaccine products (hospitalization), outcome "unknown", described as "interchange of vaccine products"; myocarditis (hospitalization, medically significant) with onset 09feb2022, outcome "not recovered", described as "acute myocarditis after third dose sars-cov2 mrna vaccine". the patient underwent the following laboratory tests and procedures: magnetic resonance imaging: unknown results. therapeutic measures were taken as a result of myocarditis. reporter comment: cardiac mri confirmed acute myocarditis with infarct-like presentation, after administration of third dose anti-sars-cov2 mrna vaccine (1 moderna, 2 moderna, 3 pfizer) no follow-up attempts are possible. no further information is expected.; reporter's comments: cardiac mri confirmed acute myocarditis with infarct-like presentation, after administration of third dose anti-sars-cov2 mrna vaccine (1 moderna, 2 moderna, 3 pfizer)