Non-immunoglobulin E-mediated allergy associated with Pfizer-BioNTech coronavirus disease 2019 vaccine excipient polyethylene glycol; Anaphylactic reaction; This is a literature report. A 56-year-old woman received her first dose of the Pfizer-BioNTech messenger RNA (mRNA) vaccine (BNT162b2). Approximately 5 minutes after administration, she felt dizzy, lightheaded, dyspneic, throat tightening, and abdominal pain. Initial blood pressure was 145/94 mm Hg; a repeat reading minutes was later done with a blood pressure of 70/42 mm Hg and a pulse rate of 150 beats/minute. Her physical examination was notable for faint end-expiratory wheezes. Her systolic blood pressure further fell to the 50s and a code blue was called for additional resources. Intramuscular epinephrine was administered immediately after code blue team arrival and after 2 minutes, her blood pressure recovered to 176/77 mm Hg. Although her symptoms transiently improved, she continued to have waves of chest tightness and dyspnea requiring 2 subsequent doses of 0.3 mg of intramuscular epinephrine followed by a 20-mg bolus of epinephrine intravenously, and initiation of an epinephrine infusion at 0.1 mg/kg/min. In addition, she received lactated ringer's solution, racemic epinephrine and albuterol nebulizer, famotidine, diphenhydramine, and methylprednisolone (Methylprednisolone), and was admitted to the intensive care unit. In the intensive care unit, her vital signs improved, and she was weaned off of the epinephrine infusion 3 hours later. She did not require supplemental oxygen and her wheezing resolved. Tryptase level was collected approximately 90 minutes after the index event, which was 6 ng/mL (reference range less than 11.5 ng/mL). There were no further objective signs of a biphasic or protracted anaphylactic reaction, and she was ultimately discharged from the hospital after 5 days with epinephrine injector pens. The patient was instructed not to receive the second dose of the Pfizer-BioNTech vaccine and was enrolled in the vaccine adverse event reporting system. Allergy testing was pursued on a follow-up clinic visit after 21 days. The patient received skin prick testing (SPT) to undiluted BNT162b2 vaccine, polyethylene glycol (PEG) (a small lipophilic excipient in both Pfizer-BioNTech and Moderna vaccines), and polysorbate 80 (a known cross-reactant to PEG). Histamine and normal saline were used as positive and negative controls, respectively. Whole blood was obtained from the patient, and activation markers, which are up-regulated on basophils during a hypersensitivity reaction, were measured in vitro using flow cytometry. Blood was heparinized, stored in a 4 C cold room on a rocker, and aliquoted and analyzed within the same day. Dilutions of dimyristoyl glycerol-polyethylene glycol (DMG-PEG) 2000 (Avanti Polar Lipids) were prepared and then stored at 4 C. Using sterile tubes, 100 mL of heparinized blood was stimulated with 100 ml of saline, a DMG-PEG dilutant (0.002 mg/ml), or with 1 mL of BNT162b2. Cells were stained with a viability dye (Zombie NIR Fixable Viability Kit) and an antibody panel consisting of anti-CD63-FITC, anti-HLA-DR-PR, and anti-CD123-PerCP/Cy5.5 (Becton Dickinson) using standardized published procedures. Cells were counted by means of flow cytometry with the BD FACSCanto II Cell Analyzer (Becton Dickinson Immunocytometry Systems) and analyzed with FlowJo Software (FlowJo LLC). Using the patient's sera, enzyme-linked immunosorbent assay was performed to detect anti-PEG antibodies. Enzyme-linked immunosorbent assays were developed with anti-PEG human-6.3-Immunoglobulin G (IgG) with a cutoff optical density (OD405) of 0.4 and anti-PEG human-6.3-IgE with a cutoff OD405 of 0.2. Anti-PEG IgE and IgG standards were used (Academia Sinica). The SPT result was interpreted as negative to BNT162b2, DMGPEG, and polysorbate 80. Basophils were detectable in the patient's samples and activated basophils were gated as CD63-positive (CD63+), CD123-positive, and HLA-DR-negative. For blood stimulated with DMG-PEG 2000 or BNT162b2, 29.1% and 23.3% of basophils, respectively, were CD63+, compared with 3.129% in the saline control group. Samples of the patient's sera tested with anti-PEG human-6.3-IgG and anti-PEG human-6.3-IgE resulted in titers below cutoff values. Polyethylene glycol is a vaccine stabilizer used in the Pfizer-BioNTech and Moderna mRNA COVID-19 vaccines and is thought to be a major contributing allergen. It has never previously been included in approved vaccines, but it serves as an excipient in a number of medicines, foods, and cosmetics. Activation of the patient's basophils on exposure to the vaccine excipient PEG implicates PEG as a potential allergen. However, given the low anti-PEG titers, the reaction seems to be non-IgE- and non-IgG-mediated anaphylaxis. The tryptase from the index event was negative; however, negative tryptase after a case of anaphylaxis to the Pfizer vaccine has been described. Skin prick testing performed on the patient to the vaccine, DMG-PEG 2000, and polysorbate 80 all had negative results. Although we did not test multiple molecular weights of PEG, there is evidence that PEG2000 (molecular weight 2000 Da) found in the vaccine can result in negative SPT despite clinical anaphylaxis to PEG. Furthermore, higher molecular weight PEGs may have a lower reactivity threshold. Anaphylaxis to PEGylated compounds is unusual but has been reported in the literature, including 53 unique cases identified by the Food and Drug Administration Adverse Event Reporting System between 1989 and 2017, and similarly, 37 cases identified between 1977 and 2016. Case reports of suggested IgE-mediated hypersensitivity to PEG provide some insights; however, PEG-specific IgE is often not detectable, which may, in some circumstances, result from a lack of assay sensitivity. In other cases, PEGylated lipid nanoparticles may activate host immune defense through non-IgE pathways such as Mas-related G protein-coupled receptor X2-mediated direct mast cell and basophil degranulation and complement activation-related pseudoallergy to PEG. Recent data reported the tolerability of a second dose COVID-19 mRNA vaccine with an antihistamine or steroid premedication in those with convincing immediate hypersensitivity reactions to the first dose. This suggests a role for possible second or booster dose of mRNA vaccine or alternative Janssen COVID-19 vaccine under an allergist's supervision for those with vaccine-associated PEG hypersensitivity.  Follow-up attempts are completed. No further information is expected.; Sender's Comments: Based on the information given in narrative, the causal relationship between the events Reaction to excipient and Anaphylactic reaction with the suspect drug BNT162B2 cannot be excluded. The impact of this report on the benefit/risk profile of the Pfizer drug 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 Regulatory Authorities, Ethics Committees and Investigators, as appropriate.