Caller provides this rash was from head to toe; Dose 1 and dose 2 is of moderna and dose 3 of pfizer; This is a spontaneous report received from contactable reporter(s) (Nurse) from medical information team. The reporter is the patient.  A 71-year-old female patient received BNT162b2 (BNT162B2), on 29Oct2021 as dose 3 (booster), single (Lot number: 320308D, Expiration Date: Feb2022) at the age of 71 years for covid-19 immunisation. The patient's relevant medical history included: "Hyperparathyroidism", start date: 2021 (unspecified if ongoing); "Allergies: chocolate" (unspecified if ongoing), notes: it gives her palpitations  specifically she experiences SVT's; "Allergies: caffeine" (unspecified if ongoing), notes: it gives her palpitations  specifically she experiences SVT's; "Palpitations" (unspecified if ongoing), notes: it gives her palpitations  specifically she experiences SVT's; "SVT's." (unspecified if ongoing), notes: it gives her palpitations  specifically she experiences SVT's; "MSG" (unspecified if ongoing), notes: it gives her palpitations  specifically she experiences SVT's. There were no concomitant medications. Past drug history included: Avelox antibiotic, reaction(s): "Allergies: She has no allergies but the only medication she has something with was Avelox antibiotic". Vaccination history included: moderna covid vaccine (Moderna First Dose: Date 02Feb2021, Caller is unsure if the Lot number is 004M20A or 009M20A), administration date: 02Feb2021, when the patient was 70-year-old, for COVID-19 Immunization; moderna covid vaccine (feeling very sick, Moderna Second Dose: Date 02Mar2021 Lot 026L20A.), administration date: 02Mar2021, when the patient was 70-year-old, for COVID-19 Immunization, reaction(s): "feeling very sick", "102°F temperature".  The following information was reported: INTERCHANGE OF VACCINE PRODUCTS (non-serious) with onset 29Oct2021, outcome "unknown", described as "Dose 1 and dose 2 is of moderna and dose 3 of pfizer"; RASH (disability, medically significant) with onset 07Nov2021, outcome "recovered" (16Nov2021), described as "Caller provides this rash was from head to toe". The event "caller provides this rash was from head to toe" required emergency room visit. The patient underwent the following laboratory tests and procedures: Body temperature: 102 Fahrenheit; Growth retardation: 5 feet 4 inches, notes: about 5 feet 4 inches, she has shrunk, she was 5 feet 6 inches; 5 feet 6 inches; they did all this testing: everything came back negative; Weight: 162lbs or 163lbs at that time.. Therapeutic measures were taken as a result of rash. Clinical course: The patient stated that she was calling about the Pfizer covid Booster, not the new one with the omicron. She was in the process of signing up to have booster, she was hoping to do the Pfizer Covid Bivalent vaccine. Her first 2 COVID vaccines were Moderna, but her last covid booster was by Pfizer on 29Oct2021. The reason she was  calling because she tried to sign up, and she did have a reaction but she never reported it, it was 10 days after the Pfizer COVID booster vaccine from head to toe she had a rash she went to urgent care, they prescribed her Benadryl and a steroid dose pack. She had the steroid for a week, and it took like a week for the rash to go away. The doctor said it had nothing to do with the vaccine, but then another said it had to do with the booster. She never reported it. The pharmacist said she needed to check with the doctor as it could have been a severe reaction. She then read on the CDC information that it was a side effect; it did say severe reaction side effect and that she should have reported it. The patient wanted to know it was considered a severe reaction. she wanted information; she has already called the doctor, but she doubted they would come back. The patient reported about rash: The rash stopped about a week after around 16Nov2021, once the itch was gone, she felt like she recovered. Seriousness Criteria: It was disabling also because she was on Benadryl every 4-6 hours and when she is on that she could not function, it made her sleepy, so if she had to drive, she would have not been able to. The patient got a Tetanus shot because she stepped on a nail, this was well after the rash, she got the tetanus shot in either Apr2022 or May2022. Medical Conditions: last year she got diagnosed with Hyperparathyroidism, she was diagnosed in an arthritis clinic in either Oct2021 or Sep2021. She also would like to add that she went to the clinic in Nov2021 after the rash, it was an arthritis clinic, they were looking for autoimmune diseases, they did all this testing and everything came back negative, however she is a little over average for scleroderma, she has not been diagnosed with scleroderma but they have been watching her for 2 more symptoms, if she experienced that then she needs to call, right now it was unknown if she has an autoimmune disease. The patient was probed for the Benadryl Lot number, she responded saying that with the Benadryl she had no problems, she was told to take it every 4 - 6 hours, when she was itching, she also put on Caladryl. Benadryl Lot is unknown, she another that is unused, but she does not have the one she consumed.  Allergies: She had no allergies but the only medication she has something with was Avelox antibiotic, she also cannot eat chocolate, caffeine, or MSG as it gives her palpitations specifically, she experienced SVTs.; Sender's Comments: Based on possible temporal association the causal association of suspect  drug BNT162b2 and for the reported events -Rash and interchange of vaccine products- cannot be excluded. The impact of this report on the benefit/risk profile of the Pfizer product is evaluated as part of Pfizer procedures for safety evaluation, including the review and analysis of aggregate data for adverse events. Any safety concern identified as part of this review, as well as any appropriate action in response, will be promptly notified to RAs, Ethics Committees, and Investigators, as appropriate