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ABSTRACTBasophil activation test (BAT) with COVID-19 mRNA vaccine seems particularly suitable for detecting sensitization to polyethylene glycol (PEG) in patients with PEG allergy. It was the aim of this study to determine the cutoffs for BAT using BNT162B2 (Comirnaty®) in a larger group of PEG allergic patients and controls. 10 PEG allergic patients and 10 controls were studied. BAT was performed using anti-CCR3 for basophil identification and anti-CD63 to assess basophil activation. Incubations with BNT162B2 at four different concentrations were performed. Basophil activation was significantly higher in PEG allergic patients compared to controls at the higher concentrations used. ROC curves showed best results with a sensitivity of 60% and specificity of 100% with a cutoff of 5% CD63+ basophils at a concentration of 4.5 µg/ml. Controls showed no positive results. In our group of PEG allergic patients, a concentration of 4.5 µg/ml BNT162B2 with a cutoff of 5% CD63+ basophils was the most suitable condition for identifying patients with a sensitization to PEG. Allergological work-up of PEG allergic patients including BAT with PEGylated lipid nanoparticles might play a role in the future when these substances will be used for other vaccines and cancer immunotherapies.