What Causes Allergic Reaction In Pfizer Coronavirus Vaccine? KEY POINTS Researchers believe it could be caused by polyethylene glycol PEG has been described as a high-risk hidden allergen An Alaskan health care worker suffered severe allergic reactions after receiving a dose of Pfizer and BioNTech’s coronavirus vaccine, prompting health care experts to consider whether tighter guidelines are needed. The middle-aged woman had an anaphylactic reaction within minutes of receiving the vaccine at Barlett Regional Hospital in Juneau, Alaska. She reportedly developed a rash over her face and torso as well as an elevated heart rate. The woman had no history of allergies.