A Study to Learn About The COVID-19 (Study) Vaccine (Called COMIRNATY) in People That Are Less Than 21 Years Old.

This is a low-interventional cohort study to determine cardiac and non-cardiac long-term outcomes of persons \<21 years of age with myocarditis/pericarditis after the administration of COMIRNATY, compared with similarly aged persons with myocarditis/pericarditis associated with COVID-19, including MIS-C. To be classified as having COMIRNATY-associated myocarditis/pericarditis, a person must 1) meet the CDC case definition for probable or confirmed myocarditis/pericarditis, 2) have received any dose of COMIRNATY ≤ 21 days of symptom onset, and 3) have no other plausible alternative etiology at the time of enrollment. To be classified as having myocarditis/pericarditis associated with COVID-19, a person must have 1) either acute severe COVID-19 infection or MIS-C, as defined by the CDC, 2) findings of probable or confirmed myocarditis in the CDC definition, 3) no other plausible alternative etiology. A description of the three cohorts is as follows: Cohort 1: Prospectively ascertained cases of probable or confirmed myocarditis/pericarditis associated with COMIRNATY , i.e., participants enrolled under protocol during hospitalization or \</= 2 weeks of hospital discharge. Cohort 2: Retrospectively ascertained cases of probable or confirmed myocarditis/pericarditis associated with COMIRNATY , i.e., participants enrolled \> 2 weeks after hospital discharge. Participants can be retrospectively ascertained and enrolled at any time from their COMIRNATY-associated myocarditis/pericarditis. Cohort 3: Comparator cohort of COVID-19- related myocarditis/pericarditis , including MIS-C, both retrospectively and prospectively ascertained, and enrolled at any time from their COVID-19 or MIS-C associated myocarditis/pericarditis diagnosis. Cohort 3 participants will not be stratified by MIS-C diagnosis because of the rarity of myocardial involvement in children with acute severe COVID-19 Participants in all cohorts will be those who present to participating medical centers for care. This study is a collaboration between the National Heart, Lung, and Blood Institute (NHLBI)'s Pediatric Heart Network (PHN) and Pfizer. Enrollment will include approximately 200 prospectively and retrospectively ascertained cases of children, adolescents, and young adults \<21 years of age who receive care for myocarditis/pericarditis associated with COMIRNATY (Cohort 1 and 2); and approximately 100 persons \<21 years of age with COVID -19-associated myocarditis/pericarditis, including MIS-C (Cohort 3).