warm feeling; breaking out in cold sweat/cold sweat having from Saturday and today; had atrial fibrillation; This spontaneous case was reported by a consumer and describes the occurrence of ATRIAL FIBRILLATION (had atrial fibrillation) in an 87-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 004FZ1A, 031M20A and 038k20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     Concurrent medical conditions included Penicillin allergy, Lung disease (taking prednisone for lung disease), Sore throat and Headache. Concomitant products included PREDNISONE for Lung disease.   On 30-Jan-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 27-Feb-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 13-Nov-2021, received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 17-Nov-2021, the patient experienced ATRIAL FIBRILLATION (had atrial fibrillation) (seriousness criterion medically significant). On 20-Nov-2021, the patient experienced FEELING HOT (warm feeling) and COLD SWEAT (breaking out in cold sweat/cold sweat having from Saturday and today). The patient was treated with DIGOXIN ongoing from 17-Nov-2021 for Atrial fibrillation and Pulse rate increased, at a dose of 0.025 milligram. On 17-Nov-2021, ATRIAL FIBRILLATION (had atrial fibrillation) had resolved. At the time of the report, FEELING HOT (warm feeling) and COLD SWEAT (breaking out in cold sweat/cold sweat having from Saturday and today) had not resolved.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On 18-Nov-2021, Heart rate increased: increased (High) Increased.     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   Patient was concerned about the cold sweat she was having from Saturday and today. Stated she always have constant sore throat and headache prior the vaccines. Haven't contacted her doctor yet, would talk to him on Monday. On Thursday, her pulse rate went up, nurse advised her to get hydrated, rest and also took Digoxin to lower pulse rate. On Friday she was fine. This case concerns an 87-year-old, female patient with a history of Lung disease, who experienced the unexpected serious AESI event of Atrial fibrillation. The event occurred approximately 4 days after booster dose of mRNA-1273 vaccine. Patient was treated with digoxin. The rechallenge was not applicable since the events happened after the booster dose and no information on additional dosing is expected. The medical history Lung disease and patient´s age remain as confounders. The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report. Case was assessed as serious due to important medical event and medical judgement.; Sender's Comments: This case concerns an 87-year-old, female patient with a history of Lung disease, who experienced the unexpected serious AESI event of Atrial fibrillation. The event occurred approximately 4 days after booster dose of mRNA-1273 vaccine. Patient was treated with digoxin. The rechallenge was not applicable since the events happened after the booster dose and no information on additional dosing is expected. The medical history Lung disease and patient´s age remain as confounders. The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report. Case was assessed as serious due to important medical event and medical judgement.