Delay 2nd dose 2 weeks, after the 42 days post 1st shot; Had an episode of Afib; Elevated Heart rate 4 hours after first dose; Chest pain; Chest pressure; Palpitation; This spontaneous case was reported by a consumer (subsequently medically confirmed) and describes the occurrence of ATRIAL FIBRILLATION (Had an episode of Afib) in a 50-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. 036A21A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     The patient's past medical history included Atrial fibrillation and Costochondritis on 09-Mar-2021. Concomitant products included METOPROLOL, LOSARTAN, SOTALOL, LEVOTHYROXINE, LIOTHYRONINE and METFORMIN for an unknown indication, DABIGATRAN ETEXILATE MESILATE (PRADAXA) and EMPAGLIFLOZIN (JARDIANCE).    On 20-Mar-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) 1 dosage form. On 20-Mar-2021, the patient experienced HEART RATE INCREASED (Elevated Heart rate 4 hours after first dose), CHEST PAIN (Chest pain), CHEST DISCOMFORT (Chest pressure) and PALPITATIONS (Palpitation). On 07-Apr-2021, the patient experienced ATRIAL FIBRILLATION (Had an episode of Afib) (seriousness criterion medically significant). On an unknown date, the patient experienced PRODUCT DOSE OMISSION IN ERROR (Delay 2nd dose 2 weeks, after the 42 days post 1st shot). At the time of the report, ATRIAL FIBRILLATION (Had an episode of Afib), HEART RATE INCREASED (Elevated Heart rate 4 hours after first dose), CHEST PAIN (Chest pain), CHEST DISCOMFORT (Chest pressure) and PALPITATIONS (Palpitation) outcome was unknown and PRODUCT DOSE OMISSION IN ERROR (Delay 2nd dose 2 weeks, after the 42 days post 1st shot) had resolved.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On 09-Mar-2021, Chest X-ray: normal (normal) normal. On 09-Mar-2021, Electrocardiogram: normal (normal) normal.     The action taken with mRNA-1273 (Moderna COVID-19 Vaccine) (Intramuscular) was unknown.       Her cardiologist changed her medications from: metoprolol Tartrate 50mg BID, Levothyroxine 200MCG QD, Liothyronine 25mcg 1/2 T Bid, Losartan 100mg QD, Metformin 1000mg BID To:  DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On 09-Mar-2021, Chest X-ray: normal normal. On 09-Mar-2021, Electrocardiogram: normal normal.  Losartan 25mg QD, Jardiance 25mg 1/2 T QD, Metoprolol t 50 mg TID, Pradaxa 150mg BID  Today(26-Apr-2021) evening she is supposed to stop her Metoprolol Tartrate 50mg TID and start Sotalol 80mg 1/2T BID.  Company comment:This case concerns a 50-year-old female with a serious unexpected event of atrial fibrillation, and non-serious unexpected heart rate increased, chest pain, chest discomfort, palpitations, and product dose omission in error. Event onset 19 days after first dose mRNA-1273. Events resolved. Based on 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.; Sender's Comments: This case concerns a 50-year-old female with a serious unexpected event of atrial fibrillation, and nonserious unexpected heart rate increased, chest pain, chest discomfort, palpitations, and product dose omission in error. Event onset 19 days after first dose mRNA-1273. Events resolved. Based on 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.