A-fib (atrial fibrillation); Goosebumps; Feeling dizzy; Shortness of breath; Heart palpitations; arm and muscle twitching; Chest is twitching; Weakness; Anxiety; High blood pressure 177/111; Nausea; Chills; Fever; Fatigue; This spontaneous case was reported by a non-health professional and describes the occurrence of ATRIAL FIBRILLATION (A-fib (atrial fibrillation)) in a 63-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 042B21A and 046A21A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     No Medical History information was reported.   On 24-Mar-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) 1 dosage form. On 21-Apr-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) dosage was changed to 1 dosage form. In May 2021, the patient experienced ATRIAL FIBRILLATION (A-fib (atrial fibrillation)) (seriousness criterion medically significant), PILOERECTION (Goosebumps), DIZZINESS (Feeling dizzy), DYSPNOEA (Shortness of breath), PALPITATIONS (Heart palpitations), MUSCLE TWITCHING (arm and muscle twitching), CHEST DISCOMFORT (Chest is twitching), ASTHENIA (Weakness), ANXIETY (Anxiety), HYPERTENSION (High blood pressure 177/111), NAUSEA (Nausea), CHILLS (Chills), PYREXIA (Fever) and FATIGUE (Fatigue). The patient was treated with METOPROLOL at an unspecified dose and frequency; BISOPROLOL at an unspecified dose and frequency and LOSARTAN at an unspecified dose and frequency. At the time of the report, ATRIAL FIBRILLATION (A-fib (atrial fibrillation)), PILOERECTION (Goosebumps), DIZZINESS (Feeling dizzy), DYSPNOEA (Shortness of breath), PALPITATIONS (Heart palpitations), MUSCLE TWITCHING (arm and muscle twitching), CHEST DISCOMFORT (Chest is twitching), ASTHENIA (Weakness), ANXIETY (Anxiety), HYPERTENSION (High blood pressure 177/111), NAUSEA (Nausea), CHILLS (Chills), PYREXIA (Fever) and FATIGUE (Fatigue) had not resolved.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Blood pressure measurement: 177/111 (High) 177/111. On an unknown date, Electrocardiogram: atrial fibrillation (abnormal) Atrial Fibrillation and atrial fibrillation (abnormal) Atrial Fibrillation.         Concomitant medications were reported unknown.  Patient reported developing the reactions after second dose. The symptoms started 4 months ago and are ongoing and getting worse. It was reported that patient never had A-fib (atrial fibrillation) before. The patient stated that the symptoms are getting worse and had to go to ER 3 times but was not hospitalized. An EKG showed that the patient had atrial fibrillation. EKG done both times he went to ER and showed Atrial fibrillation. Patient is wearing heart monitor right now. The Patient stated that he was initially put on metoprolol (beta blocker) then switched to bisoprolol that caused nightmares and it was also stopped and was advised to start losartan today.  Company comment This case concerns a 63 year old male with no relevant medical history who experienced serious unexpected event of atrial fibrillation after the second dose of Modern Covid 19 vaccine. Rechallenge was unknown. The benefit-risk relationship is not affected by this report.; Sender's Comments: This case concerns a 63 year old male with no relevant medical history who experienced serious unexpected event of atrial fibrillation after the second dose of Modern Covid 19 vaccine. Rechallenge was unknown. The benefit-risk relationship is not affected by this report.