This spontaneous case was reported by a consumer and describes the occurrence of ARRHYTHMIA (during the night, approximately 1 and a half hour after fever started, patient started to experience a rapid irregular heart beat) in a 55-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. 046L21A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below. Concurrent medical conditions included Penicillin allergy, Latex allergy, Multiple sclerosis (The patient did not have any change in status such as improvement or worsening.) since 1995, Heart disorder, Stomach function disorder and Arrhythmia (it was a problem since a few years ago, even before the vaccine was given). Concomitant products included PANTOPRAZOLE for Stomach function disorder, OCRELIZUMAB (OCREVUS), GABAPENTIN, ROPINIROLE, BACLOFEN, BUPROPION HYDROCHLORIDE (WELLBUTRIN), DIAZEPAM, HYDROCHLOROTHIAZIDE, TRIAMTERENE (TRIAMTERENE & HCTZ), DICYCLOMINE [DICYCLOVERINE], PROMETHAZINE and MELOXICAM (MOBIC) for an unknown indication. On 17-Mar-2022, the patient received dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 17-Mar-2022, the patient experienced ARRHYTHMIA (during the night, approximately 1 and a half hour after fever started, patient started to experience a rapid irregular heart beat) (seriousness criterion medically significant), HEADACHE (experienced again headache) and PYREXIA (very high fever). The patient was treated with ACEBUTOLOL ongoing since an unknown date at an unspecified dose and frequency; PARACETAMOL (TYLENOL [PARACETAMOL]) at an unspecified dose and frequency and IBUPROFEN at an unspecified dose and frequency. On 17-Mar-2022, ARRHYTHMIA (during the night, approximately 1 and a half hour after fever started, patient started to experience a rapid irregular heart beat) and PYREXIA (very high fever) had resolved. At the time of the report, HEADACHE (experienced again headache) was resolving. DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On 17-Mar-2022, Heart rate: rapid irregular heart beats (abnormal) rapid irregular heart beats. The patient did not have a history of Myocarditis/pericarditis. The patient never had COVID positive test or diagnosis. The patient did not receive any other vaccines within 1 month prior to Moderna COVID-19 vaccine. The patient mentioned she experienced rapid irregular heart beats even before getting any of the vaccines, because she had heart problems; but this time, when she took her medication (Acebutolol) to control it, it got worse, for approximately 3 hours. The patient still had a bit of headache. As per the patient the rapid irregular heart beats was related to booster dose. There were no cardiac lab values (ECG, cardiac test, cardiac enzymes) and the cause was not determined. The adverse events did not cause the patient to seek medical care (office visit, urgent care, ER, hospitalized). The patient had rapid irregular heart beats even after 3rd dose also. Company Comment: This spontaneous case concerns a 55 year old female with relevant medical history of  multiple sclerosis (stable condition), heart disorder and cardiac arrhythmia, who experienced serious (medically significant), unexpected, AESI event of arrhythmia  which occurred on the same post vaccination with the booster dose (4th dose) of mRNA-1273 vaccine. On the same day post vaccination with the booster dose of mRNA-1273 vaccine (4th dose) this patient experienced non-serious, expected events of headache and pyrexia. This patient was treated with Acebutolol and  paracetamol. The patient did not seek any medical care (office visit, urgent care, ER, hospitalized) and also no laboratories /diagnostic procedures were reported. This patient had low grade fever after the first two doses of mRNA-1273 vaccine which resolved spontaneously as well as for the third dose however for the third dose it was continuous and needed medication and resolved after 3 days. So for the event of pyrexia it is positive re-challenge. Event of arrhythmia, headache and fever was also reported for the 3rd dose of the mRNA-1273 vaccine hence possible positive re-challenge for the event headache, fever and arrhythmia. The outcome of the event arrhythmia and fever is resolved. And for the event headache it is resolving. The above medical history of Cardiac arrhythmia, cardiac disorder and multiple sclerosis are considered as confounders for the event arrhythmia while for the event of fever and headache her medical condition of multiple sclerosis is a confounder. The benefit -risk relationship of mRNA -1273 (Moderna Covid-19 Vaccine) is not affected by this report. This case was linked to MOD-2022-517635, MOD-2022-517697, MOD-2022-517699 (Patient Link). Sender's Comments: This spontaneous case concerns a 55 year old female with relevant medical history of  multiple sclerosis (stable condition), heart disorder and cardiac arrhythmia, who experienced serious (medically significant), unexpected, AESI event of arrhythmia which occurred on the same post vaccination with the booster dose (4th dose) of mRNA-1273 vaccine. On the same day post vaccination with the booster dose of mRNA-1273 vaccine (4th dose) this patient experienced non-serious, expected events of headache and pyrexia. This patient was treated with Acebutolol and  paracetamol. The patient did not  seek any medical care (office visit, urgent care, ER, hospitalized) and also no laboratories /diagnostic procedures were reported. This patient had low grade fever after the first two doses of mRNA-1273 vaccine which resolved spontaneously as well as for the third dose however for the third dose it was continuous and needed medication and resolved after 3 days. So for the event of pyrexia it is positive re-challenge. Event of arrhythmia, headache and fever was also reported for the 3rd dose of the mRNA-1273 vaccine hence possible positive re-challenge for the event headache, fever and arrhythmia. The outcome of the event arrhythmia and fever is resolved. And for the event headache it is resolving. The above medical history of Cardiac arrhythmia, cardiac disorder and multiple sclerosis are considered as confounders for the event arrhythmia while for the event of fever and headache her medical condition of multiple sclerosis is a confounder. The benefit -risk relationship of mRNA -1273 (Moderna Covid 19 Vaccine) is not affected by this report.