Polyethylene Glycol Linked to Rare Allergic Reactions Seen With mRNA COVID-19 Vaccines

Andrew D. Bowser

September 27, 2021

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A common inert ingredient may be the culprit behind the rare allergic reactions reported among individuals who have received mRNA COVID-19 vaccines, according to investigators at a large regional health center that was among the first to administer the shots.

Blood samples from 10 of 11 individuals with suspected allergic reactions reacted to polyethylene glycol (PEG), a component of both the Pfizer and Moderna mRNA vaccines, according to a report in JAMA Network Open.

In total, only 22 individuals had suspected allergic reactions out of nearly 39,000 mRNA COVID-19 vaccine doses administered, the investigators reported, noting that the reactions were generally mild and all fully resolved.

Those findings should be reassuring to individuals who are reticent to sign up for a COVID-19 vaccine because of fear of an allergic reaction, said study senior author Kari Nadeau, MD, PhD, director of the Parker Center for Allergy and Asthma Research at Stanford University, Stanford, California.

"We're hoping that this word will get out, and then that the companies could also think about making vaccines that have other products in them that don't include polyethylene glycol," Nadeau said in an interview with Medscape Medical News.

PEG is a compound used in many products, including pharmaceuticals, cosmetics, and food. In the mRNA COVID-19 vaccines, PEG serves to stabilize the lipid nanoparticles that help protect and transport mRNA. However, its use in this setting has been linked to allergic reactions in this and previous studies.

No immunoglobulin E (IgE) antibodies to PEG were detected among the 22 individuals with suspected allergic reactions to mRNA COVID-19 vaccine, but PEG immunoglobulin G (IgG) was present. That suggests non-IgE mediated allergic reactions to PEG may be implicated for the majority of cases, Nadeau said.

This case series provides interesting new evidence to confirm previous reports that a mechanism other than the classic IgE-mediated allergic response is behind the suspected allergic reactions that are occurring after mRNA COVID-19 vaccine, said Aleena Banerji, MD, associate professor at Harvard Medical School and clinical director of the Drug Allergy Program at Massachusetts General Hospital in Boston.

"We need to further understand the mechanism of these reactions, but what we know is that IGE mediated allergy to excipients like PEG is probably not the main cause," Banerji, who was not involved in the study, said in an interview with Medscape.

In a recent research letter published in JAMA Internal Medicine , Banerji and co-authors reported that all individuals with immediate suspected allergic reactions to mRNA COVID-19 vaccine went on to tolerate the second dose, with mild symptoms reported in the minority of patients (32 out of 159, or about 20%).

"Again, that is very consistent with not having an IgE-mediated allergy, so it seems to all be fitting with that picture," Banerji said.

The case series by Nadeau and co-authors was based on review of nearly 39,000 mRNA COVID-19 vaccine doses administered between December 18, 2020 and January 26, 2021. Most mRNA vaccine recipients were Stanford-affiliated healthcare workers, according to the report.

Among recipients of those doses, they identified 148 individuals who had anaphylaxis-related ICD-10 codes recorded over the same time period. In a review of medical records, investigators pinpointed 22 individuals as having suspected allergy and invited them to participate in follow-up allergy testing.

A total of 11 individuals underwent skin prick testing, but none of them tested positive to PEG or to or polysorbate 80, another excipient that has been linked to vaccine-related allergic reactions. One of the patients tested positive to the same mRNA vaccine they had previously received, according to the report.

Those same 11 individuals also underwent basophil activation testing (BAT). In contrast to the skin testing results, BAT results were positive for PEG in 10 of 11 cases (or 91%) and positive for their administered vaccine in all 11 cases, the report shows.

High levels of IgG to PEG were identified in blood samples of individuals with an allergy to the vaccine. Investigators said it's possible that the BAT results were activated due to IgG via complement activation-related pseudoallergy, or CARPA, as has been hypothesized by some other investigators.

The negative skin prick testing results for PEG, which contrast with the positive BAT results to PEG, suggest that the former may not be appropriate for use as a predictive marker of potential vaccine allergy, according to Nadeau.

"The take-home message for doctors is to be careful," she said. "Don't assume that just because the person skin-tests negative to PEG or to the vaccine itself that you're out of the woods, because the skin test would be often negative in those scenarios."

The study was supported by a grants from the Asthma and Allergic Diseases Cooperative Research Centers, a grant from the National Institutes of Health, the National Institute of Allergy and Infectious Disease SARS Vaccine study, the Parker Foundation, the Crown Foundation, and the Sunshine Foundation.

Nadeau reports receiving grants from the National Institute of Allergy and Infectious Diseases; the National Heart, Lung, and Blood Institute; the National Institute of Environmental Health Sciences; and Food Allergy Research & Education; serving as director of the World Allergy Organization; serving as advisor for Cour Pharma; serving on the national scientific committee of Immune Tolerance Network and the National Institutes of Health clinical research centers; and being cofounder of Before Brands, Latitude, Alladapt, and IgGenix outside the submitted work.

Nadeau reports having patents pending for an oral formula for decreasing food allergy risk and treatment for food allergy, for granulocyte-based methods for detecting and monitoring immune system disorders issued, for methods and assays for detecting and quantifying pure subpopulations of white blood cells in immune system disorders, and for microfluidic device and diagnostic methods for allergy testing based on detection of basophil activation.

Banerji has disclosed no relevant financial relationships.

JAMA Netw Open. Published online  September 17, 2021. Full text

Andrew D. Bowser is a freelance medical writer in Philadelphia.

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