covid arm from shoulder to elbow; they said it was an allergic reaction to something; A fib; This spontaneous case was reported by a consumer and describes the occurrence of ATRIAL FIBRILLATION (A fib) in a 68-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. 023M20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     The patient's past medical history included Cardioversion (Patient reports a month after the vaccine she got A fib for 7 days and this was the first occurrence since 2019 when  she had cardioversion) and Atrial fibrillation ((this was the first occurrence since 2019 when she had cardioversion)) in 2019.    On 17-Feb-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On an unknown date, the patient experienced ATRIAL FIBRILLATION (A fib) (seriousness criterion medically significant), VACCINATION COMPLICATION (covid arm from shoulder to elbow) and HYPERSENSITIVITY (they said it was an allergic reaction to something). At the time of the report, ATRIAL FIBRILLATION (A fib) had resolved and VACCINATION COMPLICATION (covid arm from shoulder to elbow) and HYPERSENSITIVITY (they said it was an allergic reaction to something) outcome was unknown.        The action taken with mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown) was unknown.       No concomitant medication reported. No treatment information was provided.  patient received the first dose of Moderna vaccine on 17Feburary2021 Lot#023M20A . Patient reports a week after having a bad case of Covid arm from her shoulder to elbow. Patient reports she went to see general practioner and they said it was an allergic reaction to something. Patient reports a month after the vaccine she got A fib for 7 days and this was the first occurrence since 2019 when  she had cardioversion. Patient reports no Covid diagnosis or history of pericarditis or myocarditis. Patient reports being concomitant medications but did not wish to provide.  Company comment- This case concerns a 68-year-old, female patient with a history of Atrial fibrillation with Cardioversion (in 2019), who experienced the unexpected serious AESI event of Atrial fibrillation. The event occurred approximately 1 month after the first dose of mRNA-1273 vaccine. The rechallenge was unknown since the event happened after the first dose and no information on additional dosing is available. The medical history of Atrial fibrillation with Cardioversion remains as a confounder. 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 a 68-year-old, female patient with a history of Atrial fibrillation with Cardioversion (in 2019), who experienced the unexpected serious AESI event of Atrial fibrillation. The event occurred approximately 1 month after the first dose of mRNA-1273 vaccine. The rechallenge was unknown since the event happened after the first dose and no information on additional dosing is available. The medical history of Atrial fibrillation with Cardioversion remains as a confounder. 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.