Covid vaccine: Study looks at safety of second dose after reports of anaphylaxis

For those who are managing allergies, it can be a little worrisome to hear that some people have experienced allergic reactions after receiving the vaccine, particularly if they have a history of severe reactions themselves. Earlier this year, the Centers for Disease Control and Prevention announced that people who had an allergic reaction to a first dose of an mRNA should not get a second. However, study authors said their results suggest this “largely unstudied” strategy might not be needed.

In the study, examination on whether it was safe to proceed with a second mRNA vaccine after a dose one reaction.

Investigators from MGH, Vanderbilt University Medical Center, University of Texas Southwestern Medical Center and Yale School of Medicine combined data from patients who sought allergy specialist care at their hospitals after a reaction to their first mRNA COVID-19 vaccine dose.

The study involved 189 patients, with 32 patients (17 percent) who experienced anaphylaxis after their first dose of the vaccine.


A total of 159 patients (84 percent) went on to receive a second dose.

All 159 patients, including 19 individuals who had experienced anaphylaxis following the first dose, tolerated the second dose.

Thirty-two patients (20 percent) reported immediate and potentially allergic symptoms associated with the second dose.

Thirty-two patients (17 percent) experienced anaphylaxis.

Of the patients with first-dose reactions, 130 (69 percent) were to the Moderna vaccine, and 59 (31 percent) were to Pfizer.

The Pfizer and Moderna vaccines both contain mRNA wrapped in lipid nanoparticles (LNPs) that help carry it to human cells but also act as an adjuvant, a vaccine ingredient that bolsters the immune response, said a report published in Science. 

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It continued: “The LNPs are ‘PEGylated’—chemically attached to PEG molecules that cover the outside of the particles and increase their stability and life span.

“PEGs are also used in everyday products such as toothpaste and shampoo as thickeners, solvents, softeners, and moisture carriers, and they’ve been used as a laxative for decades.

“PEGs were long thought to be biologically inert, but a growing body of evidence suggests they are not.”

As much as 72 percent of people have at least some antibodies against PEGs, according to a 2016 study led by Samuel Lai, a pharmaco-engineer at the University of North Carolina, Chapel Hill.

In the study about seven percent have a level that may be high enough to predispose them to anaphylactic reactions, he found.

Other studies have also found antibodies against PEG, but at lower levels.


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