BLOOD CLOT ATTACHED FROM THE LEFT KNEE UP TO STOMACH; This spontaneous report received from a patient concerned an 83 year old female. The patient's weight was 243 pounds, and height was 66 inches.  The patient's concurrent conditions included: severe arthritis, and sleep disorder. The patient had no known allergies. The patient did not have a family history of blood clot, nor did the patient travel recently and was not sitting around. The patient received covid-19 vaccine ad26.cov2.s (suspension for injection, route of admin, and batch number were not reported, expiry: UNKNOWN) dose was not reported, 1 total administered to left arm on 03-MAY-2021 for prophylactic vaccination. The batch number was not reported and has been requested. Concomitant medications included trazodone for sleep, acetylsalicylic acid (aspirin), allopurinol, ascorbic acid (Vitamin C), colecalciferol (Vitamin D3), lisinopril, lovastatin, oxybutynin, ropinirole, verapamil, glucamide MSM and triamterene.  On an unspecified date several months ago, the patient experienced swollen left foot (toes to ankle) and she mentioned it to her physician who didn't say or do anything about the foot. The swelling remained persistent and she mentioned it to her primary care doctor (PCP) several times, yet testing was not done at the time. She stated that the foot was painful and a little bit red. She noted that the pain in her foot and ankle was dull and present even when she was sitting and it became very painful when she was standing or walking. The patient stated that she had to use a cane due to the pain. On AUG-2021, the patient had her whole left leg, from thigh down to toes got swollen and when the swelling worsened, she went back to the physician. On an unspecified date in SEP-2021, the patient underwent an Ultrasound and reported that it showed no blood clot. The redness in the foot had also spread to her entire left leg. The patient performed therapy and exercises at her home, she was icing, elevating the leg and wearing compression stockings to help relieve the swelling. None of these interventions helped so the patient went back to her physician who referred her to a Spine Specialist. The patient had a spinal x-ray and pelvic/abdominal CT scan done, both test results were normal except severe arthritis was shown in X-ray, although this was previously diagnosed.  On 12-OCT-2021, the patient consulted a vascular doctor who performed an in-depth ultrasound of both legs and stomach. She was told about the results, that a blood clot was attached and has been there a while, from the left knee up to her stomach. The patient was started on Xarelto 15 mg twice a day. It was reported that on the 22nd day of treatment, the dose of Xarelto will be reduced to 20 mg once per day. The patient reported that she started experiencing a sharp pain in her knee which comes and goes especially when she is walking or standing. The patient reported that she has an ultrasound scheduled for next Tuesday and would have weekly ultrasounds to monitor the progress.  The action taken with covid-19 vaccine ad26.cov2.s was not applicable. The patient had not recovered from blood clot attached from the left knee up to stomach. This report was serious (Other Medically Important Condition).; Sender's Comments: V0:20211028558-COVID-19 VACCINE AD26.COV2.S- Blood clot attached from the left knee up to stomach. This event(s) is considered unassessable. The event(s) has a compatible/suggestive temporal relationship, is unlabeled, and has unknown scientific plausibility. There is no information on any other factors potentially associated with the event(s).