DVT in right leg; Swelling in right leg; Discomfort in right leg while walking; This is a spontaneous report from a contactable consumer (patient). A 58-year-old male patient received BNT162B2 (PFIZER-BIONTECH COVID-19 VACCINE), first dose via an unspecified route of administration, administered in the left arm on 10Mar2021 at 16:30 (batch/lot number: EN6208) as a single dose for COVID-19 immunisation (wanted the protection). Medical history included deep vein thrombosis (DVT) from Jul2018 and ongoing. No family medical history relevant to the adverse events. Concomitant medication included warfarin taken for maintenance level for history of DVT from Aug2020 and ongoing. The patient experienced DVT in right leg on 02Apr2021, swelling in right leg on 15Mar2021 and discomfort in right leg while walking on 15Mar2021. The patient reported that he had a reaction to the COVID-19 vaccine and he believes that it was directly related to the COVID-19 vaccine. He reported that he has a history of blood clots in his right leg going back to 2018. He reported that his most recent DVT was in Aug2020 and he was cleared in Nov2020. He reported that he received the COVID-19 vaccine on 10Mar2021. He reported that on 15Mar2021, he started to notice his right leg swelling. He reported that he called his physician on 29Mar2021 and that his physician ordered an ultrasound. He reported that he had the ultrasound on 02Apr2021 and a deep vein thrombosis was found again. Swelling in right leg: he reported that the swelling has gone down. He reported that the reason that he mentioned that he has a history of blood clots was because of his warfarin dosage. He reported that he was first diagnosed with a deep vein thrombosis in Jul2018. He reported that he went through that cycle and that his maintenance level of warfarin was maybe 10 or 12 mg of warfarin per day. He reported that he had another deep vein thrombosis in Aug2020, and since Aug2020 he has been on 17 mg of warfarin daily. He reported that the blood level on warfarin maintenance was supposed to be between 2 and 2.5. He reported that now because of the deep vein thrombosis he experienced in Aug2020, his doctor wanted his level to be at 2.5 to 3.5. He stated that he doesn't think that at that blood level for his warfarin that he should have gotten a blood clot. He reported that something compromised or interfered with the warfarin, resulting in a deep vein thrombosis. Discomfort when walking: he reported that it feels like his leg was compressing. He reported that the discomfort seems to have gotten better. He was queried for details of warfarin prescription. He reported that the 10 or 12 mg warfarin tablets that he had been taking when the blood clot was diagnosed in Aug2020 have been consumed and he no longer has the bottle. He reported that he did not have the bottle with him for his current warfarin prescription. He reported that the warfarin was not auto-refilled by the pharmacy. He reported that the warfarin was only prescribed a week or two at a time. Investigations: he reported that since Aug2020 when the deep vein thrombosis returned, he started doing self INR tests at home. He reported that he constantly self-monitors his INR at home and his levels were consistently at 3.0 to 3.1. He reported that he monitors his INR at least once a week, sometimes twice a week depending on how he feels. He reported that sometimes his INR goes over 4 and he can feel it. He reported that his system was a little off when it feels too high so he knows how to adjust. He reported that he last checked his INR today, 15Apr2021, and that it was 4.0. He reported that he typically checks his INR on Wednesday. He reported that he keeps a log of his INR levels. He reported that he did not see his physician on 29Mar2021, but that he called his physician and his physician wrote a prescription for the caller to get an ultrasound. Vaccination facility type was pharmacy. Vaccine was not administered at military facility. The adverse events did not require a visit to the emergency room and physician office. No prior vaccinations (within 4 weeks). He reported that he doesn't usually get vaccines. BNT162B2 second dose (lot number: ER5729) was administered on an unspecified date; anatomical location: left arm. The outcome of the events was recovering.