Inactive ingredient triggered a severe allergic reaction to Pfizer COVID-19 vaccine

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Some severe allergic reactions to the Pfizer COVID vaccine may actually be caused by one particular inactive ingredient, polyethylene glycol (PEG), according to a UK case report. PEG is an inactive substance that is used in the vaccine to protect the active ingredient (mRNA) and help it penetrate cells. On the first day of the UK campaign for COVID-19 vaccination, there were reports of two cases of anaphylaxis - a severe, potentially life-threatening allergic reaction - within minutes of receiving the Pfizer/BioNTech mRNA vaccine. Subsequently, further cases of suspected anaphylaxis to the Pfizer vaccine were reported. This research reveals that an allergy to PEG caused one of the cases. The authors say it is unclear if all cases of anaphylaxis after the Pfizer vaccine are due to PEG allergy, but they stress that PEG allergy is rare and that COVID-19 vaccines remain safe.

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New Insights on Cause of Anaphylaxis Following COVID-19 Vaccination

On the first day of the UK campaign for COVID-19 vaccination, there were reports of two cases of anaphylaxis—a severe, potentially life-threatening allergic reaction—within minutes of administration of the Pfizer/BioNTech mRNA vaccine. Subsequently, further cases of suspected anaphylaxis to the Pfizer vaccine were reported. A new report published in Clinical & Experimental Allergy reveals that an allergy to the ingredient polyethylene glycol (PEG) is a cause of anaphylaxis to the vaccine. However, this is in the context of millions of doses safely administered.

The authors note that very few people are allergic to PEG, and they provide a guide to identifying those who are at risk. “This preliminary report confirms PEG as a cause of anaphylaxis to the Pfizer/BioNTech vaccine, for the first time,” they wrote. “COVID-19 vaccine anaphylaxis and PEG allergy are both rare, so proof of PEG as the cause in one case of vaccine anaphylaxis is important. However, it is important to emphasise that PEG allergy is rare and that COVID-19 vaccines remain safe.”

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Clinical & Experimental Allergy
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Organisation/s: Cambridge University Hospitals NHS Foundation Trust, UK
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