BLOOD CLOT IN LEFT THIGH; SWELLING; FATIGUE; This spontaneous report received from a consumer concerned a 55 year old female. Initial information received on 13-APR-2021, processed along with information received via telephone communication on 15-APR-2021 The patient's weight was 100 pounds, and height was 152.4 centimeters. The patient's concurrent conditions included progressive multiple sclerosis, swelling of the left leg and foot, and non smoker, and other pre-existing medical conditions included the patient had no known drug allergies. the patient was not a pregnant at the time of reporting. The patient had no family history of thrombosis, no prior deep vein thrombosis (DVT). The patient received covid-19 vaccine ad26.cov2.s (suspension for injection, route of admin not reported, batch number: 1808609, expiry: UNKNOWN) dose was not reported, 1 total, administered on 22-MAR-2021 for prophylactic vaccination. Concomitant medications included baclofen, escitalopram oxalate, and risedronate sodium (received 2 doses and stopped after the event).  The patient's husband reported that in MAR-2021, the patient had symptoms of fatigue.  On 23-MAR-2021, the patient had swelling and fatigue.  Her fatigue was resolved on 24-MAR-2021 and swelling was resolving.  On 05-APR-2021, the patient developed left thigh swelling. On 06-APR-2021, the patient's swelling was not going down and she visited her primary care physician. The physician could not determine the cause of swelling (left thigh very noticeable clot swollen around where clot was located) and the physician suspected clot and performed an AD dimer (fibrin degradation) test to look for potential blood clotting. The patient also had a venous duplex ultrasound that showed left leg clot.  At 19:00 on the same day, the patient's physician advised to visit emergency room and determined there was a clot on left thigh and she was prescribed Eliquis Starter Pack (apixaban) for 3 months treatment and recommended another scan in several months. On 06-APR-2021, the patient's vitals were reported as blood pressure 124/70 mmHg, heart rate 88 bpm and oxygen saturation of 99%. Laboratory data included prothrombin time (PT) 13.4 sec , International normalized ratio (INR) 1.2 and  activated partial thromboplastin time  (APTT) 34.8 sec. (Normal range not reported for all) d-dimer and platelet count results were not available and Covid-19 test was not performed. Body max index was 19.5 kg/m2. On an unspecified date the patient was feeling better but had not recovered from the event. The action taken with covid-19 vaccine ad26.cov2.s was not applicable. The patient recovered from fatigue on 24-MAR-2021, had not recovered from blood clot in left thigh, and recovering from swelling. This report was serious (Other Medically Important Condition).; Sender's Comments: V0: This case concerns a 55-year- old female subject, immobile due to progressive multiple sclerosis who developed a left thigh swelling 12 days after Janssen COVID-19 vaccine was administered intramuscularly for the prevention of symptomatic SARS-CoV-2 virus infection.  The patient's concurrent conditions included body max index of 19.5 kg/m2, progressive multiple sclerosis and swelling of the left leg and foot. She is a non-smoker, no known drug allergies, no family history of thrombosis, or prior deep vein thrombosis (DVT). Symptoms started with swelling of the left thigh and fatigue. The fatigue resolved the next day. The patient's swelling in the left thigh was not going down and she visited her primary care physician who checked her D-dimer dimer (level not known to patient) and scheduled a venous duplex ultrasound that showed a clot in the left thigh.  Her vital signs were blood pressure of  124/70 mmHg, heart rate 88 bpm and oxygen saturation of 99%. and she was prescribed Eliquis Starter Pack (apixaban) for 3 months treatment and recommended another scan in several months. Laboratory data included PT 13.4 sec, INR 1.2 and APTT 34.8 sec. D-dimer and platelet count results were not available and Covid-19 test was not performed. The patient is feeling better but had not recovered from blood clot in the left thigh, and is recovering from the swelling. Given underlying progressive multiple sclerosis with immobility and preexisting swelling of the left leg and foot on the same lower extremity, the clotting of the left thigh is considered not related to Janssen COVID-19 vaccine. Additional information has been requested.