Palpitations; Decrease exercise tolerance; fluttering heart; leg started to swelled; shortness of breath/ Difficulty breathing / Dyspnoea; This spontaneous case was reported by a consumer and describes the occurrence of CARDIAC FLUTTER (fluttering heart) in a 65-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 939904, 031L20A. and 011J20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     The patient's past medical history included Pulmonary embolism. Concurrent medical conditions included DVT. Concomitant products included APIXABAN (ELIQUIS) for DVT.   On 12-Jan-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 09-Feb-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 26-Oct-2021, received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 29-Oct-2021, the patient experienced CARDIAC FLUTTER (fluttering heart) (seriousness criterion medically significant), PERIPHERAL SWELLING (leg started to swelled) and DYSPNOEA (shortness of breath/ Difficulty breathing / Dyspnoea). On an unknown date, the patient experienced PALPITATIONS (Palpitations) and EXERCISE TOLERANCE DECREASED (Decrease exercise tolerance). The patient was treated with DIPHENHYDRAMINE HYDROCHLORIDE (BENADRYL [DIPHENHYDRAMINE HYDROCHLORIDE]) ongoing since an unknown date at an unspecified dose and frequency. At the time of the report, CARDIAC FLUTTER (fluttering heart) and PERIPHERAL SWELLING (leg started to swelled) had not resolved, DYSPNOEA (shortness of breath/ Difficulty breathing / Dyspnoea) and PALPITATIONS (Palpitations) had resolved and EXERCISE TOLERANCE DECREASED (Decrease exercise tolerance) outcome was unknown.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Blood pressure measurement: 138/75 138/75 mmHg. On an unknown date, Body temperature: 97 97 degree F. On an unknown date, Heart rate: 94 94 beats per minute. On an unknown date, Respiratory rate: 70 70 breaths per minute.     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter considered DYSPNOEA (shortness of breath/ Difficulty breathing / Dyspnoea), PALPITATIONS (Palpitations) and EXERCISE TOLERANCE DECREASED (Decrease exercise tolerance) to be not related. No further causality assessments were provided for CARDIAC FLUTTER (fluttering heart) and PERIPHERAL SWELLING (leg started to swelled).    Patient was taking Eliquis from years.  Patient went to the ER were they checked for blood clots and did not find blood clot, also  performed an ultrasound and since the patient is allergic to the contrast chemical was indicated  Benadryl and steroids.  Patient has no previous heart conditions nor has had covid-19.  Patient had inquired if symptoms are common and whether it would prevent from getting. another Moderna vaccine in the future.  Patient had difficulty in breathing and palpitations. Patient went to emergency room for Palpitations and Dyspnea on 18 Nov 2021 and had a doctor office visit on 23 Nov 2021.  Causality for dyspnea, palpitations and decreased exercise tolerance was likely not related to the vaccine and anxiety was other possible cause of the events.  Company comment: This case concerns a 65-year-old, female patient with medical history of pulmonary embolism and deep vein thrombosis, who experienced the unexpected, serious (Medically significant) and AESI of cardiac flutter, among other non-serious events. The event of cardiac flutter (described as fluttering heart) occurred approximately 3 days after receiving a dose, considered as the third dose of mRNA-1273. Follow up information by patient's physician stated that the patient suffered palpitations along with dyspnoea, that could be explained by patient's anxiety and decreased tolerance to exercise. The patient was concerned for a blot clot, however this was ruled out on images. Patient's physician does not considered that the reaction was not related to the vaccine. The rechallenge was not applicable as events occurred after third dose and no further dosing was reported. The medical history of pulmonary embolism and deep vein thrombosis remain as confounders. The benefit-risk relationship of mRNA-1273 is not affected by this report.   This case was linked to MOD-2021-400069 (Patient Link).   Most recent FOLLOW-UP information incorporated above includes: On 04-Jan-2022: Follow up contains new events, secondary reporter, patient details, laboratory data, concomitant medication and date of vaccination was added and outcome of the event dyspnea was updated.; Sender's Comments: This case concerns a 65-year-old, female patient with medical history of pulmonary embolism and deep vein thrombosis, who experienced the unexpected, serious (Medically significant) and AESI of cardiac flutter, among other non-serious events. The event of cardiac flutter (described as fluttering heart) occurred approximately 3 days after receiving a dose, considered as the third dose of mRNA-1273. Follow up information by patient's physician stated that the patient suffered palpitations along with dyspnoea, that could be explained by patient's anxiety and decreased tolerance to exercise. The patient was concerned for a blot clot, however this was ruled out on images. Patient's physician does not considered that the reaction was not related to the vaccine. The rechallenge was not applicable as events occurred after third dose and no further dosing was reported. The medical history of pulmonary embolism and deep vein thrombosis remain as confounders. The benefit-risk relationship of mRNA-1273 is not affected by this report.