INR dropped drastically; This is a spontaneous report from a contactable consumer (patient). A 63-year-old female patient received the first dose of BNT162B2 (PFIZER-BIONTECH COVID-19 VACCINE, lot number: ER8730 and expiration date not provided) via an unspecified route of administration, administered in the left arm on 24Mar2021 10:00 as a single dose for COVID-19 immunisation. Medical history included Left ventricular assist device insertion from 2016 and ongoing, she had a pacemaker with defibrillator placed in 2005. There were no concomitant medications. The patient did not receive any other vaccine within 4 weeks prior to the COVID vaccine. The patient reported that she was calling about the Pfizer BioNTech COVID-19 Vaccine. She said she was a LVAD (Left Ventricular Assist Device) patient. She said the LVAD helped to pump blood in her heart, and the thickness of her blood was important for the LVAD pump to work.  She said after she got her first Pfizer BioNTech COVID-19 Vaccine on 24Mar2021 and her INR dropped drastically on 25Mar2021. She said she spent a week in the hospital to build her INR level back up until she could go home at an INR number that was safe. She said she talked with the doctor, who was the head of the heart failure unit at the hospital. She said both the doctor and her felt the Pfizer BioNTech COVID-19 Vaccine shot caused her INR problem. She said she now knows that she needs to bump up her warfarin dose if her INR were to drop after she gets the second Pfizer BioNTech COVID-19 Vaccine. She said she thought it was important that Pfizer know what happened to her. She reported that her pacemaker with defibrillator was triggered, and she was shocked 3 times, so she called the hospital and was told to go to the hospital's Emergency Room. She clarified she had her first COVID-19 Vaccine on 24Mar2021, and the next day (25Mar2021) her INR dropped to 1.4. She said her normal INR range was between 2-3. She said she went to the Emergency Room on 28Mar2021, and was admitted into the hospital. She said her INR was checked while she was in the Emergency Room, and her INR had dropped even further to .07 on 28Mar2021. She said when she was admitted to the hospital on 28Mar2021 and she was put on a slow IV drip of heparin as a bridge to warfarin. She said she eventually restarted warfarin on Wednesday, 31Mar2021, along with the IV Heparin drip. She said between the IV Heparin drip and the Warfarin, her INR reached 1.76 on 02Apr2021 and she was discharged from the hospital on 02Apr2021. She reported when she got home, she took a larger Warfarin dose for the first 3 days. She clarified she increased her Warfarin to 7.5 mg a day for 3 days, and used her current Warfarin prescription, adjusting the amount of Warfarin tablets she took. She said she then went back to her regular Warfarin dosing: 3.5 mg daily on Monday, Wednesday, and Friday, and 4 mg daily on Tuesday, Thursday, Saturday and Sunday. She stated her Warfarin was dispensed in pharmacy bottles. She said she had 2 separate Warfarin doses she used at home, so she could adjust her Warfarin dosing, as needed, based on her INR levels. She reported she had her INR checked on Monday, 05Apr2021, and her INR had increased to 2.4. She said she was going later today (08Apr2021) to have her INR rechecked at the hospital's lab. She stated since she and her doctors were now aware of her INR situation after the first Pfizer BioNTech COVID-19 Vaccine, her doctors will monitor her the day before she gets her second Pfizer BioNTech COVID-19 Vaccine, and for several days afterwards. The event required a visit to emergency room. The outcome of the event was recovered on 01Apr2021.