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A COVID-19 Vaccine Ingredient Can Trigger Allergic Reactions—But It's Rare

A stylistic still life shot of a vial labeled "COVID-19 vaccine" on a pink stand and blue background.

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Key Takeaways

  • Polyethylene glycol (PEG) and polysorbate are additives in the COVID-19 vaccines. They protect the mRNA as it enters the human cell. 
  • PEG and polysorbate are found in many products including skincare, cosmetics, medications, and laxatives.
  • Hypersensitivity to PEG leading to a severe allergic reaction has been reported, but it is not common.

Allergic reactions to vaccines are not common, but they are possible. To be on the safe side, when you get in line to receive your COVID-19 vaccine, you will be asked if you are allergic to any ingredients in the shot. While the ingredients in vaccines are safe, it is possible that someone might have sensitivities that could make them more likely to have an allergic reaction.

With the COVID-19 vaccines, experts point to a specific ingredient that, although rare, can trigger an allergic reaction in people who are sensitive to it: polyethylene glycol (PEG).

What Is PEG?

PEG, also called macrogol, is a lipid compound that acts as a solvent, thickener, softener, and moisture carrier in many common products including medications, toothpaste, shampoo, skincare, and laxatives.

PEG is an additive in both the Moderna and Pfizer-BioNTech COVID-19 vaccines. It protects the mRNA as it is transferred into human cells.

PEG in Vaccines

PEG, as well as another compound additive called polysorbate (which is used in the Johnson & Johnson vaccine), can trigger an immediate allergic reaction, though it is rare. The reaction may include anaphylactic symptoms including rashes, shortness of breath, rapid heartbeat, and a decrease in blood pressure.

The creation of the COVID-19 vaccines is the first time PEG has been used as an additive in a widely used immunization platform. That means that we do not have any data confirming that vaccines containing PEG have caused any severe allergic reactions. However, PEG and polysorbate have been noted as suspected causes of anaphylactic reactions to the Moderna and Pfizer-BioNTech vaccines.

How Common is PEG Sensitivity?

A 2016 study published in Analytical Chemistry found that about 7% of people who have been exposed to PEG from various products have a level of antibodies to the compound that is high enough to cause an anaphylactic reaction.

In a presentation at the American Academy of Allergy, Asthma, and Immunology Annual Meeting, John Kelso, MD, a practicing allergist in the Division of Allergy, Asthma, and Immunology at Scripps Clinic and voluntary health sciences clinical professor at the University of California San Diego School of Medicine, said that “Even in the rare patient who is PEG allergic, we still don’t know if there is enough PEG in the vaccine to actually cause a reaction. It is certainly possible, but remains a speculation to date.”

Severe allergic reactions to the COVID-19 vaccines do not happen often. In his presentation, Kelso reported that since COVID vaccine distribution began, only about 5 in 1 million people have reported an anaphylactic reaction to the vaccine, which he says is “really quite rare.”

How to Find Out If You're Allergic to PEG

According to allergists, there are a few ways to find out if you are sensitive to PEG. One of the easiest is to take a small amount of the laxative Miralax, which is mostly comprised of PEG, to see if you develop any symptoms of an allergy.

If you have experienced an allergic reaction to any vaccine you've had in the past, make sure you talk to your healthcare provider before you get a COVID shot.

Getting a COVID Vaccine If You Have Sensitivities

The Centers for Disease Control and Prevention (CDC) recommends that people who have a known allergy to PEG do not get a COVID-19 vaccine that contains the compound, which rules out the Moderna and Pfizer-BioNTech vaccines. For people who are sensitive to PEG, the Johnson & Johnson vaccine could be an option.  

For people who are sensitive to polysorbate, the CDC recommends not getting a COVID-19 shot that contains the ingredient—in this case, the Johnson & Johnson vaccine. If you are not sure which vaccine is safe for you, talk with your doctor.

You should also know that the CDC requires all vaccination sites have at least three doses of epinephrine, such as an EpiPen, on hand at all times. All sites also need to have staff who are trained to recognize the signs and symptoms of a severe allergic reaction and are prepared to administer treatment if necessary.

What This Means For You

Allergic reactions to vaccines, including the COVID-19 vaccines, are rare. However, they are possible—especially if you have known allergies to certain ingredients or have had a reaction to a vaccine before.

If you have a hypersensitivity to either PEG or polysorbate, talk to your doctor or allergist before receiving the COVID-19 vaccine. 

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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