Priya Sellaturay et al.

Allergy to PEG is rare,5,6 but reactions can be severe or even fatal.7 PEG is an excipient of many drugs, but hypersensitivity de- pends on its molecular weight (MW).

… Although the Pfizer/BioNTech vaccine contains a number of excipients, PEG 2000 is the only one reported to cause anaphylaxis. All mRNA vaccines are likely to contain PEG, which is used to stabilize the lipid nanoparticles, and the Moderna COVID-19 vaccine also contains PEG 2000.

… Undiagnosed PEG-allergic patients are at risk of anaphylaxis to mRNA vaccines containing PEG, so should be identified before vaccination.