• Hiroshi Minato, Akane Yoshikawa, Sho Tsuyama, Kazuyoshi Katayanagi, Satoaki Hachiya, Keisuke Ohta, and Yasuhiro Myojo.
• Department of Diagnostic Pathology, Ishikawa Prefectural Central Hospital, Kanazawa City, Ishikawa Prefecture, Japan.
• Medicine (Baltimore). 2024 Feb 2; 103 (5): e37196e37196.

RationaleThe benefits of COVID-19 mRNA vaccination are claimed to be substantial; however, vaccination-related myocarditis and pericarditis have also been observed globally, particularly among young men. In most cases, the symptoms are mild and resolve on their own; however, fatal cases have rarely been described.Patient ConcernsA healthy 40-year-old Japanese man suddenly experienced tachycardia and lost consciousness 2 days after vaccination. Continued resuscitation recovered the spontaneous heartbeat; however, the patient did not regain consciousness and died 9 days later. Electrocardiography after resuscitation showed marked ST-segment depression and incomplete right bundle branch block. Influenza antigen and polymerase chain reaction tests for SARS-CoV-2 were negative.DiagnosesFatal arrhythmia after a second COVID-19 mRNA vaccination.InterventionsWe performed an autopsy and studied the material morphologically and immunohistochemically.OutcomesAt autopsy, several small inflammatory foci with cardiomyocytic necrosis were scattered in the right and left ventricles, with a propensity for the right side. Some inflammatory foci were located near the atrioventricular nodes and His bundles. The infiltrating cells predominantly consisted of CD68-positive histiocytes, with a small number of CD8-positive and CD4-positive T cells. In this case, myocarditis was focal and mild, as is mostly observed following COVID-19 mRNA vaccination. However, the inflammatory foci were close to the conduction system and were considered the cause of fatal arrhythmia.LessonsAlthough the benefits of COVID-19 vaccination appear to outweigh the side effects, it should be noted that fatal arrhythmias may rarely occur, and caution should be taken if individuals, particularly young men, complain of any symptoms after vaccination.Copyright © 2024 the Author(s). Published by Wolters Kluwer Health, Inc.

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