This is a spontaneous report from a Pfizer sponsored program IBCC (Inbound Call Center for HCPs) via a contactable nurse (patient, self-reported). A 67-year-old female patient (age at vaccination: 67 year) received second dose of BNT162B2 (PFIZER-BIOTECH COVID-19 mRNA VACCINE, Solution for injection, lot number: ER8730 and expiry date was not reported), via an unspecified route of administration, in Arm Left, on 30Mar2021 at 13:00, as a single dose for COVID-19 immunization. The patient's medical history included ongoing diabetes (under control now with medication, not out of control. Onset probably 2010) and ongoing overweight (Onset after she had her 3 kids, she started putting it on, at one point she was up to 349 lbs but has been dropping weight so now she is down to 235lbs). Historical vaccine included first dose of BNT162B2 (PFIZER-BIOTECH COVID-19 mRNA VACCINE, Solution for injection, lot number: EN6203 and expiry date was not reported), via an unspecified route of administration, in Arm Left, on 09Mar2021 (like in the morning), as a single dose for COVID-19 immunization. The patient took Pfizer-BioNTech Covid-19 Vaccine because mostly because of her age and she lived with grandchildren both son daughter-in-law in community "didn". Concomitant medications were not reported. The patient reported that she got the second dose of the Pfizer-BioNTech COVID-19 Vaccine on 30Mar2021 and have been having blisters in the mouth since beginning of April and have been getting bigger and has been spreading to her left cheeks. She said the blisters were fingerprint size. She was not sure if this was from the vaccine, but said she never had this before, only after getting the vaccine. She was asking for additional information about this, and if similar cases have been reported. She was more interested to know how to treat the blisters. She really didn't feel any side effects for a few days and then all of a sudden, she started getting fingerprint-sized blisters inside her left cheek. They were like clear blisters, they blisters would break and now they were tannish-yellow pus like looking color, become bigger and then more blisters would come. Now the blisters were pretty much taking over the left side of the inside of her cheek and down by the gums on an unknown date in Jun 2021. She had a biopsy was done on 03Jun2021 and blood work was done on 04Jun2021 because they thought maybe she had an autoimmune syndrome; but the biopsy and bloodwork came back negative and in normal range. Outcome was worsened as the blisters were getting bigger, getting worse; was now red on the edges and sort of feels like a pinching, stinging-type feeling. They were thinking maybe the Pfizer-BioNTech Covid-19 Vaccine triggered something in her, and her immune system was not up to fighting it. The doctor asked her questions like if she was sexually active which she was not, had not been in over 30 years; things like that, so that removed a lot of possibilities; she had an appointment, but she wanted to rule out stuff. Outcome of the events was not recovered. Causality was reported as yes, because from when she first started getting the blisters, then they were like clear blisters, they would break and now (at the time of reporting) they are tannish-yellow pus like looking colour, and she believes they are from the Pfizer-BioNTech COVID-19 vaccine. The relatedness of the Pfizer-BioNTech COVID-19 vaccine and adverse reaction was reported as related.