Atrial fibrillation/irregular heart beat; pulse rate increased; blood pressure top number dropped; weakness; dizzy; a little bit nauseated; felt tired; started feeling bad; This spontaneous case was reported by a consumer and describes the occurrence of ATRIAL FIBRILLATION (Atrial fibrillation/irregular heart beat) in an 81-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 091D21A, 031M20A and 013L20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     The patient's past medical history included Hypertension. Concomitant products included ACETYLSALICYLIC ACID (BABY ASPIRIN) for an unknown indication.   On 20-Jan-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 17-Feb-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 28-Oct-2021, received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 30-Oct-2021, the patient experienced VACCINATION COMPLICATION (started feeling bad), ASTHENIA (weakness), DIZZINESS (dizzy), NAUSEA (a little bit nauseated) and FATIGUE (felt tired). In November 2021, the patient experienced ATRIAL FIBRILLATION (Atrial fibrillation/irregular heart beat) (seriousness criteria hospitalization and medically significant). On an unknown date, the patient experienced HEART RATE INCREASED (pulse rate increased) and BLOOD PRESSURE DECREASED (blood pressure top number dropped). The patient was hospitalized for 2 days due to ATRIAL FIBRILLATION. The patient was treated with PARACETAMOL (TYLENOL [PARACETAMOL]) for Adverse event, at a dose of 1 dosage form and APIXABAN (ELIQUIS) for Adverse event, at a dose of 1 dosage form. At the time of the report, ATRIAL FIBRILLATION (Atrial fibrillation/irregular heart beat), HEART RATE INCREASED (pulse rate increased) and BLOOD PRESSURE DECREASED (blood pressure top number dropped) had resolved and VACCINATION COMPLICATION (started feeling bad), ASTHENIA (weakness), DIZZINESS (dizzy), NAUSEA (a little bit nauseated) and FATIGUE (felt tired) outcome was unknown.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Blood pressure measurement: low (Low) blood pressure top number dropped. On an unknown date, Electrocardiogram: abnormal (abnormal) the patient had an irregular heart beat. On an unknown date, Heart rate: high (High) increased..     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   The patient felt like she had a side effect for the Moderna booster. On 28 Oct 2021 she had the Moderna booster, it was a Thursday, and she was fine till Saturday night when she started feeling bad, they had weakness, were a little bit nauseated and a little bit dizzy, felt tired, and her blood pressure top number dropped and their pulse rate increased. Next week they went to see their PCP, who performed and EKG and found the patient had an irregular heart beat. The patient was hospitalized for 2 days and HCPs were evaluating performing a procedure to correct her hearts rhythm but then realized the heart had corrected itself. HCP indicated the patient to wear a heart monitor for 30 days but still this has not occurred. The patient state it had Atrial fibrillation.  Company Comment: This is a spontaneous case concerning a 81-year-old female patient with hypertension, who experienced the serious  unexpected event of special interest Atrial fibrillation. The event occurred approximately between 7 and 14 days after the third dose  of mRNA-1273.  The benefit-risk relationship of mRNA-1273 is not affected by this report.   This case was linked to MOD-2021-396222 (Patient Link).; Sender's Comments: This is a spontaneous case concerning a 81-year-old female patient with hypertension, who experienced the serious  unexpected event of special interest Atrial fibrillation. The event occurred approximately between 7 and 14 days after the third dose  of mRNA-1273.  The benefit-risk relationship of mRNA-1273 is not affected by this report.