To the Editor Kurihara et al. reported that 82.1% of patients with COVID-19-like symptoms were diagnosed with non-COVID-19, with potentially fatal diseases detected in 14.2% of them. In addition, they concluded that, "Several potentially fatal diseases remain masked among the wave of COVID-19 mimics." (1) We recently experienced a case of COVID-19 vaccine-related probable myocarditis that was masked at the first clinic visit. In this case, COVID-19 infection was suspected and treated because of chest symptoms with a fever. A cardiac examination was not performed. COVID-19 vaccine-caused myocarditis has been reported in young men. (2-4) However, few reports on cases of COVID-19 vaccine-caused myocarditis exist in Japan, especially among young women. We herein report a young woman with chest pain and abnormal electrocardiogram (ECG) changes after receipt of a COVID-19 vaccine manufactured by Moderna. A 19-year-old woman (158 cm, 63 kg) visited our outpatient clinic because of recurrent chest pain with a low-grade fever (37.6 °C) 26 days after receiving her second COVID-19 vaccination. The chest pain lasted for over six hours. She had first complained of a fever (37.6 °C) with chest pain 1 week after receiving her second vaccination and visited another clinic. COVID-19 infection was suspected because of the fever, but aside from a polymerase chain reaction (PCR) test, which was negative, no detailed examination was performed. Acetaminophen was prescribed. Her pain temporarily subsided but then recurred. Her history was deemed non-contributory. She visited our outpatient clinic because of sustained chest pain. An ECG revealed sinus tachycardia and new ST-T segment abnormal changes compared with the ECG findings obtained at an annual health checkup. New ST segment depression in II, III and aVF and inverted T wave in III, aVF and V2-3 were observed, as shown in the Figure . Chest X-ray was normal.