Tachycardia/ Heart rate was dangerously high; This spontaneous case was reported by a consumer and describes the occurrence of TACHYCARDIA (Tachycardia/ Heart rate was dangerously high) in a 37-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 939909, 038B21A and 004C21A) for COVID-19 vaccination.     No Medical History information was reported.   On 09-Apr-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 07-May-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 21-Nov-2021, received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 21-Nov-2021, the patient experienced TACHYCARDIA (Tachycardia/ Heart rate was dangerously high) (seriousness criterion hospitalization). The patient was hospitalized from 22-Nov-2021 to 23-Nov-2021 due to TACHYCARDIA. On 23-Nov-2021, TACHYCARDIA (Tachycardia/ Heart rate was dangerously high) had resolved.        For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter considered TACHYCARDIA (Tachycardia/ Heart rate was dangerously high) to be possibly related.    No concomitant medication was reported. No treatment medication use was reported.  The patient was in hospital for 24 hours. There were tests run on him. It was determined that his symptoms were unrelated to the heart, but could be related to the booster dose. No physical damage on the heart was observed. The patient asked to check for myocarditis, but they could not determine the cause. After he stayed in the hospital for one night, his heart rate went down and it's been stable since then.  Company comment: This case concerns a 37-year-old, female patient with no relevant medical history, who experienced the unexpected event of tachycardia, which required hospitalization. The event occurred on the same day after the third dose of mRNA-1273 and the patient was hospitalized on the next day. As reported, the patient was in hospital for 24 hours and it was determined that his symptoms were unrelated to the heart, but could be related to the booster dose. No physical damage on the heart was observed. The patient asked to check for myocarditis, but they could not determine the cause, as reported. The event resolved after one day and the patient was discharged. The rechallenge was not applicable as events occurred after third dose and no further dosing was reported or planned, based on information provided.The benefit-risk relationship of mRNA-1273 is not affected by this report.; Sender's Comments: This case concerns a 37-year-old, female patient with no relevant medical history, who experienced the unexpected event of tachycardia, which required hospitalization. The event occurred on the same day after the third dose of mRNA-1273 and the patient was hospitalized on the next day. As reported, the patient was in hospital for 24 hours and it was determined that his symptoms were unrelated to the heart, but could be related to the booster dose. No physical damage on the heart was observed. The patient asked to check for myocarditis, but they could not determine the cause, as reported. The event resolved after one day and the patient was discharged. The rechallenge was not applicable as events occurred after third dose and no further dosing was reported or planned, based on information provided.The benefit-risk relationship of mRNA-1273 is not affected by this report.