Why do people have COVID-19 mRNA vaccination-associated acute reactions?

Published: December 28, 2021

There has been significant interest in acute-onset hypersensitivity reactions associated with the widespread use of COVID-19 mRNA vaccines, particularly since this was the first time this technology has been used in millions of people.  
Macy and co-workers from Kaiser Permanente Southern California recently published an article in The Journal of Allergy and Clinical Immunology: In Practice on the population-based incidence, severity, and risk factors associated with treated acute-onset COVID-19 mRNA vaccination-associated hypersensitivity reactions. They were able to identify all 391,123 unique Kaiser Permanente Southern California (KPSC) members who received COVID-19 mRNA vaccinations, first and/or second doses, between December 15, 2020, and March 11, 2021, at a KPSC facility. The comprehensive electronic healthcare record system KPSC uses enabled the researchers to identify all treated acute-onset hypersensitivity reactions. Most previous publications had relied on patient reporting, possibly missing some reactions.

There were only 104 reactions after first doses and 32 after second doses. After accounting for the number of individuals who had first and second doses, reactions were noted to be only half as common with second doses. Most reactions were mild, associated with transient itchy rashes, and resolved quickly with antihistamine therapy. Only two reactions were determined to be anaphylaxis, and both of these were mild anaphylaxis and not considered to be life-threatening.

Treated acute-onset COVID-19 mRNA vaccine-associated hypersensitivity events were relatively rare and mostly mild. There were no deaths. Reactions were more common with first COVID-19 mRNA vaccine doses than second doses. Reactions were more likely to occur in younger females. Reactions were associated with the typical risk factors seen in a condition called multiple drug intolerance syndrome. Reactions were determined to be very unlikely to be primarily immunologically- or allergically-mediated. This report, like others, supports our growing confidence in the overall safety of COVID-19 mRNA vaccines.

The Journal of Allergy and Clinical Immunology: In Practice is an official journal of the AAAAI, focusing on practical information for the practicing clinician.

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