acute myocarditis or myopericarditis; This is a literature report. This is the third of seven reports.  Introduction: On 11Dec2020, the FDA issued an Emergency Use Authorization (EUA) for the Pfizer-BioNTech COVID-19 mRNA vaccine for prevention of COVID-19 for individuals 16 years of age and older.1. On 10May2021, the FDA revised the EUA for this vaccine to include children 12 years and older. The Pfizer vaccine remains the only vaccine with an EUA for 12-to 17-year-old children. This vaccine demonstrated 94-95% efficacy in preventing COVID-19 infection in 16- 55-year-old participants, and 100% efficacy in the 12-15 year old age group. Systemic reactogenicity occurred more commonly in younger patients and after the second dose of vaccine. Post-immunization myocarditis is a known rare adverse event following other vaccinations, particularly following smallpox vaccination. Recently the news media has highlighted reports of myocarditis after COVID-19 mRNA vaccination involving military patients and patients from a foreign country. The foreign cohort identified a male predominance with an incidence of 1/20,000 (men aged 18 to 30 years old). However, a conclusive causal link to vaccination has not been confirmed at this time. Additionally, two recently published case reports describe myocarditis after COVID-19 mRNA vaccination in a 56-year-old man with previous COVID-19 and a 39-year-old man with no history of COVID-19. This report summarizes case histories of 7 healthy male adolescents 14 to 19 years of age who developed acute myocarditis or myopericarditis within 4 days after receiving the second dose of the Pfizer-BioNTech COVID-19 vaccine, none of whom met criteria for MIS-C. All 7 patients were vaccinated in April and May of 2021 and have been reported to VAERS. Patient 3: A 17-year-old previously well male presented with chest pain two days after his second Pfizer- BioNTech COVID 19 vaccine. Chest pain was worse when lying flat and was associated with left arm pain and paresthesias. He had no recent or remote history of viral illness, and no known COVID 19 exposures. ECG showed abnormal T waves with diffuse ST elevation consistent with pericarditis. Notable laboratory studies included elevated troponin I (5.550 ng/mL, normal range, <0.045 ng/mL), NT Pro BNP (376pg/mL, normal range, <100 pg/mL) and CRP (25.3 mg/L, normal range, < 1.0 mg/dL). Echocardiogram showed normal function and coronaries, no effusion, trace mitral and aortic valve insufficiency, and decreased left ventricular basolateral and posterior regional strain. Cardiac MRI showed delayed enhancement at the left ventricular subepicardial basal anterolateral segment and basal to mid ventricular inferolateral segments, consistent with myocardial necrosis. There was evidence of diffuse fibrosis on T1 weighted imaging and myocardial edema on T2 mapping. SARS CoV 2 spike antibody was positive and nucleocapsid antibody was negative. Workup for other infections and a urine drug screen were negative. Troponin peaked at 12.200 ng/ml. His symptoms resolved with ibuprofen 600 mg orally every 6 hours and he was discharged at 48 hours. Based on the characteristics of his chest pain, ECG findings, and prompt response to anti-inflammatory medication, pericardial involvement was suspected. The presence of elevated cardiac markers and inflammation on cardiac MRI prompted the diagnosis of myopericarditis. At one week followup, he remained asymptomatic with normal troponin, CRP, and ECG; the echocardiogram was unchanged. Partial thromboplastin time: 31.4 seconds, Adenovirus diagnostics: Negative, Alanine transaminase: 33 IU/L, Aspartate transaminase: 41 iu/l, Albumin: 4.1 g/dl, Cytomegalovirus  diagnostics: Negative, Urine drug screen: Negative, Enterovirus diagnostics: Negative, Epstein-Barr virus diagnostics: negative, International Normalized Ratio INR: 1.06, Absolute lymphocyte: 2.13, Absolute neutrophil count: 7.46x10 3/mm3, Platelet count: 231x10 3/mm3, Prothrombin time: 14.0 seconds, erythrocyte sedimentation rate: 6, Ferritin: 90ug/L, Lowest troponin prior to discharge: 5.79 ng/ml, Peripheral white blood cell count: 11.8x10 3/mm3. Author report 7 cases of clinical myocarditis or myopericarditis that developed in 14 to 19-year-old males within 4 days of receiving the second dose of the Pfizer-BioNTech COVID-19 vaccine with no evidence of acute SARS-CoV-2 infection and who did not fulfill criteria for MIS-C. Extensive diagnostic evaluation for other myocarditis etiologies was negative, including respiratory pathogens from nasopharyngeal swabs, serum PCR tests, and infectious serologies. Additionally, all cardiac MRIs were diagnostic for myocarditis based on the modified criteria rather than MIS-C characteristics described by (withheld) et al (diffuse myocardial edema without evidence of late gadolinium enhancement). There was some suggestion of abnormal left ventricular myocardial echocardiographic strain corresponding to regions of myocardial necrosis on cardiac MRI. All patients in this series had myocarditis or myopericarditis, which is the term for diagnosis of both myocardial and pericardial inflammation. These terms are often used interchangeably, which can make surveillance of these diseases challenging. Myocarditis and pericarditis are rare diseases. The true baseline incidence of myocarditis is unknown and varies by season, geography, and age: it has been reported to occur in 1.95/100,000 person- years in children <15 years of age in another country and in 2.16 cases per 100,000 service members in a 30-day period. It is more common in males, and among children demonstrates a bimodal incidence pattern, with peaks at <2 years of age and in adolescence. An evaluation for potential viral causes is recommended, although a cause is usually not found. There have been prior reports of myocarditis following smallpox vaccination10. In patients with myocarditis, restriction from competitive sports is recommended for at least 3 months until cleared by a cardiologist in order to avoid sudden cardiac events while the heart muscle recovers. Less is known about the true incidence of pericarditis. Pericarditis can occur in the setting of a variety of infectious and non-infectious illnesses. In a study of patients more than16 years of age in foreign country, the incidence rate of hospitalizations for acute pericarditis was 3.32/100,000 person-years, with males at higher risk than females and in 2007, the incidence of acute pericarditis in one study was 27.2 cases per 100,000 per year. Treatment for myocarditis and pericarditis may vary considerably depending on the patient characteristics, clinical condition, underlying cause, and physician preference. Consistent with a known male preponderance of myocarditis and pericarditis, all seven of our cases were male. The Pfizer-BioNTech clinical trials demonstrated an increased systemic reactogenicity and immunogenicity in younger study participants following mRNA vaccine. For example, 41.5% of adolescents developed chills after dose #2, compared to 35.1% of subjects 18-55 years of age. In terms of immunogenicity, an analysis of SARS-CoV-2 50% neutralizing titers 1 month after dose#2 demonstrated higher geometric mean titer (GMT) in children 12-15 years of age (GMT=1,239.5), compared to subjects 16-25 years of age (GMT=705.1). Adverse events often occurred more frequently after dose #2 and within 2 days following vaccination and included injection site pain, fatigue, myalgia, chills, arthralgia, fever, injection site swelling or redness, nausea, malaise, and lymphadenopathy. It is possible that myocarditis or myopericarditis may be an additional rare adverse event related to systemic reactogenicity, but currently no causal association has been established between this vaccine and myopericarditis. In our case series, 6 patients received non-steroidal anti-inflammatory drug (NSAID) treatment. Four patients received IVIG and oral prednisone; one of these four patients also initially received high-dose methylprednisolone. The recognition of a possible temporal relationship of COVID-19 vaccine and myocarditis is critical, because the correct diagnosis may spare healthy adolescents and young adults presenting with chest pain and ECG ST elevation from undergoing unnecessary invasive medical procedures such as cardiac catheterization. It is unclear if treatment with intravenous immunoglobulin and/or corticosteroids, in the absence of MIS-C criteria, is warranted with all cases of myocarditis that develop temporally after COVID-19 vaccination. Notably, 3 patients recovered with NSAID therapy alone. Myocarditis and myopericarditis after COVID-19 vaccination appear rare. As of 23May2021, the Centers for Disease Control and Prevention (CDC) reports that 1,560,652 people <18 years of age have completed a two-dose series of COVID-19 vaccine17. Of these, 652, 758 adolescents received their second dose more than fourteen days ago17. Currently, the Pfizer-BioNTech COVID-19 vaccine is the only COVID-19 vaccine authorized for children <18 years of age. We urge physicians and healthcare providers to consider myocarditis in the evaluation of adolescents and young adults who develop chest pain after COVID-19 vaccination. All cases of myocarditis in patients with recent COVID-19 vaccination should be reported promptly to VAERS. Our case series has inherent limitations. We compiled cases through personal communications between colleagues rather than using a systematic surveillance system to identify cases. It was not possible to exclude all alternative etiologies including idiopathic and other infectious etiologies, and there was not a systematic diagnostic evaluation for other viral etiologies. Cardiac biopsy was not performed on any patients, because they were all clinically stable during hospitalization. However, no patient had evidence of a preceding or concurrent symptomatic viral illness to implicate as an etiology of myocarditis, and the lack of eosinophilia dissuades a hypersensitivity reaction. The pathophysiology of myocarditis in these patients is indeterminate and we do not know if it is the same or different than classic myopericarditis or myopericarditis following other vaccines, associated with acute COVID-19, or MIS-C. Given the nature of a case series, we cannot determine the incidence rate of myocarditis/myopericarditis following COVID-19 mRNA vaccination. Finally, a negative nucleocapsid antibody does not conclusively rule out the possibility of natural infection. This report summarizes a series of cases of myocarditis and myopericarditis following the Pfizer BioNTech COVID-19 mRNA vaccine in adolescent males. All cases in this report occurred after the second vaccine dose. Fortunately, none of our patients was critically ill and each was discharged home. At present, there is no definite causal relationship between these cases and vaccine administration.  No follow-up attempts are possible; information about lot/batch number cannot be obtained.; Sender's Comments: The event myocarditis is serious and unexpected with this suspect product, Pfizer-BioNTech COVID-19 mRNA vaccine.  This case will be updated when new information becomes available.,Linked Report(s) : PFIZER INC-2021665013 Same reporter, drug, event but different patient