How to perform allergy assessment with mRNA COVID vaccine?To the Editor:Vaccines against coronavirus-2019 (SARS-CoV-2) are the solution to the current pandemic, However anaphylactic reaction occurs following administration of Pfizer-BioNTech COVID-19 Vaccine (1). Previous sensitization to Polyethylene glycol (PEG) may be a cause of anaphylaxis to the first dose of vaccine (2). However, the molecular weight (MW) of PEG seems play a part both in the process of sensitization and in HSR severity. Low MWs penetrate skin and mucosa to a greater degree, increasing the risk of sensitization (3).We report the case of a 53-year-old female who had a history of urticaria 30 minutes after administration of oral PEG 3350 for Bowel preparation for colonoscopy. Her usual treatment is perindopril and leflunimide, which contain PEG 8000, and is well-tolerated.We performed intradermal injection (IDR) with 0.02 ml of COVID-19 vaccine, diluted 10-1. After 20 min, the diameter of the wheal has increased by 2 folds and was accompanied by an erythema (picture 1).We also perfumed a skin prick test with PEG 3350 which was positive (wheal 6mm, erythema 8mm).Positive (Histamine 10 mg/ml) and negative tests were used as recommended.IDR testing was performed in two healthy volunteers to confirm the non-irritation of the used dose (or concentration). One the two volunteers has received the first dose of COVID-19 vaccine 9 days before testing, but not the other. Both of IDR were negative.We also performed a skin prick test with PEG 3350 which was positive (wheal 6mm, erythema 8mm).We speculated that this concentration for intradermal test is not irritated and can be used to performed allergy assessment for BNT162b2 Pfizer- BioNTech mRNA. In a case of positive reaction with skin testing we do not recommend vaccination and advise to wait for another formulation of vaccine.1. Castells MC, Phillips EJ. Maintaining Safety with SARS-CoV-2 Vaccines. N Engl J Med [Internet]. 20202. Garvey LH, Nasser S. Anaphylaxis to the first COVID-19 vaccine: is polyethylene glycol (PEG) the culprit? Br J Anaesth. 2020;3. Wenande E, Garvey LH. Immediate-type hypersensitivity to polyethylene glycols: a review. Clin Exp Allergy J Br Soc Allergy Clin Immunol. 2016;46(7):907‑22.Sébastien Lefèvre, MD, Regional Institute for allergic disease, Metz Regional Hospital, Metz, FranceLaurianne Moumane, MD, Regional Institute for allergic disease, Metz Regional Hospital, Metz, FrancePascale Monfort, MD, Pharmacy Department, Metz Regional Hospital, Metz, FranceValentin Philippe, MD, Pharmacy Department, Metz Regional Hospital, Metz, FranceNadia Ouamara, PhD, Clinical Research Unit, Metz Regional Hospital, Metz, FranceCamille Cano, MD, Regional Institute for allergic disease, Metz Regional Hospital, Metz, FranceEtienne Beaudouin, MD, Regional Institute for allergic disease, Metz Regional Hospital, Metz, France