palpitations; atrial flutter with RVR; This spontaneous case was reported by a physician (subsequently medically confirmed) and describes the occurrence of PALPITATIONS (palpitations) and ATRIAL FLUTTER (atrial flutter with RVR) in a 62-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 046B21A and 037A21B) for COVID-19 vaccination.     The patient's past medical history included Ex-smoker and COVID-19 (was not hospitalized and got recovered at home) in December 2020. Concurrent medical conditions included Hypertension, Hyperlipidemia and Obesity.   On 19-Apr-2021, the patient received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) dosage was changed to 1 dosage form. On an unknown date, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) 1 dosage form. On 17-May-2021, the patient experienced PALPITATIONS (palpitations) (seriousness criterion hospitalization) and ATRIAL FLUTTER (atrial flutter with RVR) (seriousness criterion hospitalization). The patient was hospitalized from 17-May-2021 to 18-May-2021 due to ATRIAL FLUTTER and PALPITATIONS. The patient was treated with DILTIAZEM (CARDIZEM [DILTIAZEM]) (oral) ongoing from 17-May-2021 for Palpitations and Atrial flutter with rapid ventricular response, at a dose of UNK dosage form and DILTIAZEM (CARDIZEM [DILTIAZEM]) (intravenous) ongoing from 17-May-2021 for Palpitations and Atrial flutter with rapid ventricular response, at a dose of UNK dosage form. On 18-May-2021, PALPITATIONS (palpitations) and ATRIAL FLUTTER (atrial flutter with RVR) had resolved.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): In May 2021, Heart rate: high heart beats per minute (High) > 120 bpm and high heart beats per minute (High) > 100 bpm. On 17-May-2021, Computerised tomogram thorax: negative (Negative) Negative for a pulmonary embolism (PE). On 17-May-2021, Electrocardiogram: inconclusive (Inconclusive) new onset atrial flutter with rapid ventricular response (RVR), no ST changes. On 17-May-2021, Full blood count: normal (normal) normal. On 17-May-2021, Heart rate: high heart beats per minute (High) 115-150 bpm. On 17-May-2021, Mean arterial pressure: inconclusive (Inconclusive) > 75.     For mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular), the reporter did not provide any causality assessments.   The patient denied any history of sleep apnea. He denied fevers, night sweats, shaking, chills recently and denied any other complaints of chest pain, dyspnea or lightheadedness. He was not an alcohol user and had no new prescriptions. He had no history of thyroid disease and had not been diagnosed with chronic obstructive pulmonary disease.  No concomitant medications were reported. Patient was hemostable throughout his emergency department stay. The patient got converted to sinus rhythm overnight and got discharged to home on 18-May-2021.  Based on the current available information and temporal association between the use of the product and the start date of the event, a causal relationship cannot be excluded.   Patient´s cardiovascular risk factors are an important confounder for the event atrial flutter.; Sender's Comments: Based on the current available information and temporal association between the use of the product and the start date of the event, a causal relationship cannot be excluded.   Patient´s cardiovascular risk factors are an important confounder for the event atrial flutter.