Lymphohistiocytic Myocarditis Possibly Due to Moderna mRNA-1273 Vaccine

OBJECTIVES: Despite the clear benefits of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination in mitigating the impact of the coronavirus disease 2019 pandemic, there are emerging reports of postvaccination myocarditis, the majority of which are diagnosed based on the clinical and radiologic findings without biopsy confirmation. We report a case of biopsy-confirmed lymphohistiocytic myocarditis after Moderna mRNA-1273 vaccination. METHODS: We describe a case of a previously healthy 45-year-old woman who had palpitations, exercise intolerance, and syncope 1 week after her first mRNA-1273 vaccine dose. Laboratory tests and cardiac imaging were compatible with myocarditis. Given her unusual clinical presentation, an endomyocardial biopsy was performed to exclude other potential etiologies. RESULTS: The endomyocardial biopsy specimen showed patchy endocardial and intramyocardial lymphohistiocytic infiltrates with scattered eosinophils and focal myocyte injury. CD3 and CD68 immunostains confirmed the lymphocytic and histiocytic nature of the infiltrate, respectively. A focal histiocytic collection suggestive of an ill-defined granuloma was present. The histologic and immunohistochemical findings of a lymphohistiocytic myocarditis were highly suggestive of a postvaccination hypersensitivity reaction. CONCLUSIONS: Myocarditis following SARS-CoV-2 vaccination is a rare adverse event. The findings of a lymphohistiocytic myocarditis with scattered eosinophils and a possible ill-defined granuloma are highly suggestive of a hypersensitivity reaction. The mechanism by which this inflammation occurs remains uncertain. Despite our findings, the benefits of SARS-CoV-2 vaccination far outweigh the risks.