Got worse and the leg swelled and was also having foot swelling because blood flow wasn't there; sore arm; blood clot; pain behind left knee; This is a spontaneous report from a contactable consumer (patient's wife). An 81-year-old male patient received the first dose of BNT162B2 (PFIZER-BIONTECH COVID-19 VACCINE, solution for injection), via an unspecified route of administration, administered in right shoulder on 23Feb2021 (Batch/Lot Number: EN6202) (at the age of 81-years-old) as single dose for covid-19 immunisation. Medical history included diabetes mellitus, stage 3 renal failure (was not bad, was in stage 5 couple of years ago, was a month or so away from full failure, was in the hospital and had dialysis, was able to get reversed with medication and diet), high blood pressure, and blood cholesterol increased/high cholesterol. The reporter stated that with regards to patient's height, he shrunk a little. Patient had no prior vaccinations within 4 weeks, didn't get flu shots. Concomitant medications include about 12 other unspecified drugs, takes statins, has high blood pressure and high cholesterol. Patient started experiencing pain behind his left knee about 6 or 7 days later in Mar2021 (also reported as 04 or 05Apr2021, pending clarification). By that Sunday, it had gotten worse. The leg swelled and was also having foot swelling because blood flow wasn't there. There was no TIA's no stroke, no heart racing, no sweating, was all pain. Got leg up and kept elevated until they could get to the doctor and get imaging. Patient went Monday 08Mar2021 to see the doctor. Patient was brought to the doctor and was then sent to the imaging place. Radiology read the imaging around 5:15pm on the Monday, 08Mar2021. The radiologist said there was 2.5" blood clot behind the left knee. He had never had a blood clot before. Patient was then sent to the Emergency Room at (Name) Hospital, in (Name) where they live. Was kept there several hours. Did a lot more blood tests. He was immediately put on 10mg ELIQUIS 10mg, once in the morning, and once at night. He is now taking 5mg Eliquis in the morning, and 5mg at night. He has not had any of these problems before the Pfizer shot. The reporter knew that not a lot had been reported. It seemed it should be safe. The correlation and timing having the shot on 23Feb2021 and all of a sudden a week and a half later this happened. It was 10 days later. There was nothing they can do about it. Just continuing with the ELIQUIS, trying to dissolve the blood clot, will be staying on it for 4 to 6 months. It was a pretty big blood clot and were worried about it breaking up and traveling. The patient had no other symptoms, had a sore arm, that was it. Patient going back to (Name) 01Jun2021 and he will have another CT scan or ultra sound when they get back. The reporter did not want to need to get the leg amputated. Patient was only in the ER, was not admitted to the hospital. Patient was concerned because of his age. If it doesn't dissolve, they know there will be a lot more trouble. The event of pain left knee, blood clot and sore arm required a visit to the emergency room and physician's office. The patient received the second dose of BNT162B2 on 01Apr2021 (lot number: ER8730) in the right arm. The outcome of events was unknown.