arrhythmia/already had them but it was deepened/ heart arrhythmia; was feeling really bad; pain in the legs; had to sit because it's balance is not very good; great tiredness/was really really really tired; This spontaneous case was reported by a consumer and describes the occurrence of ARRHYTHMIA (arrhythmia/already had them but it was deepened/ heart arrhythmia) in a 79-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     The patient's past medical history included Breast cancer and Cancer surgery. Concurrent medical conditions included Arrhythmia. Concomitant products included METOPROLOL for an unknown indication.   On 26-Feb-2021, the patient received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 28-Sep-2021, received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On an unknown date, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 30-Sep-2021, the patient experienced FATIGUE (great tiredness/was really really really tired). On an unknown date, the patient experienced ARRHYTHMIA (arrhythmia/already had them but it was deepened/ heart arrhythmia) (seriousness criterion medically significant), FEELING ABNORMAL (was feeling really bad), PAIN IN EXTREMITY (pain in the legs) and BALANCE DISORDER (had to sit because it's balance is not very good). At the time of the report, ARRHYTHMIA (arrhythmia/already had them but it was deepened/ heart arrhythmia), FEELING ABNORMAL (was feeling really bad), PAIN IN EXTREMITY (pain in the legs), BALANCE DISORDER (had to sit because it's balance is not very good) and FATIGUE (great tiredness/was really really really tired) outcome was unknown.            Metoprolol was also used treatment medication.  Company Comment : This case concerns a 79-year-old, female patient with previous relevant medical history of breast cancer. arrhythmia and use of concomitant medication metoprolol, who experienced the unexpected event of arrhythmia. The event occurred 2 days after the third dose of mRNA-1273. The rechallenge was not applicable since the event happened after the third dose. The medical history of breast cancer. arrhythmia and use of concomitant medication metoprolol remain a confounder. The benefit-risk relationship of mRNA-1273 is not affected by this report. Further information was request.   This case was linked to MOD-2021-354379, MOD-2021-354326 (Patient Link).; Sender's Comments: This case concerns a 79-year-old, female patient with previous relevant medical history of breast cancer. arrhythmia and use of concomitant medication metoprolol, who experienced the unexpected event of arrhythmia. The event occurred 2 days after the third dose of mRNA-1273. The rechallenge was not applicable since the event happened after the third dose. The medical history of breast cancer. arrhythmia and use of concomitant medication metoprolol remain a confounder. The benefit-risk relationship of mRNA-1273 is not affected by this report. Further information was request.