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Allergies to mRNA-based vaccines are rare and mild, study finds

Experts hope these results help consumers better understand the safety of the COVID-19 vaccines

Photo (c) Violeta Stoimenova - Getty Images
A new study conducted by researchers from the Stanford University School of Medicine explored the likelihood of consumers developing an allergic reaction to the mRNA-based COVID-19 vaccines

According to the findings, reactions to these types of vaccines are rare. When consumers do experience reactions, they’re usually mild. 

“It’s nice to know these reactions are manageable,” said Dr. Kari Nadeau, an author of the study, published in JAMA Network Open. “Having an allergic reaction to these new vaccines is uncommon, and if it does happen, there’s a way to manage it.” 

Understanding allergic reactions

For the study, the researchers analyzed medical records of 22 individuals (20 women) who experienced possible allergic reactions and were among recipients of the first 39,000 doses of the Pfizer and Moderna vaccines given to health care providers at Stanford. The 22 recipients had symptoms within three hours of receiving the shot, but only 17 met diagnostic criteria for an allergic reaction. Three received epinephrine, and all 22 fully recovered. Fifteen of the 22 had documented histories of previous allergic reactions.

In follow-up testing on 11 of the individuals, the researchers concluded they were likely reacting negatively to polyethylene glycol (PEG), one of the inert ingredients in both the Pfizer and Moderna vaccines. PEG is used as a stabilizer in household products, medications and cosmetics; the authors suggest that because women use these products more often, they possibly have more sensitivity to PEG.

The allergic reactions experienced by the vaccine recipients weren't severe, suggesting that recipients who do get symptoms such as hives, swelling or shortness of breath can still get the second dose, as long as it's under medical supervision, according to the researchers.

The researchers hope that consumers’ biggest takeaway is that the active ingredients in the mRNA-based COVID-19 vaccines aren’t likely to be linked with allergic reactions. 

“What’s important is what we didn’t find, as much as what we did find,” Dr. Nadeau said. “It does not seem that mRNA itself causes the allergic reactions.” 

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