Moderna’s vaccine, like Pfizer’s, is designed around a molecule called messenger RNA, or mRNA, that is injected into the upper arm. Once inside human cells, mRNA instructs the manufacture of a protein called spike, which then teaches the immune system to recognize and prevent the coronavirus if it invades the body. Each vaccine contains a handful of other ingredients that wrap the fragile mRNA in a protective blister of fat and help keep the recipe stable in transit.
None of the ingredients in any of the vaccines have been identified as a common allergen. But several experts cautiously pointed to polyethylene glycol, or PEG, which appears in both recipes, although in slightly different formulations, as a possible culprit. PEG is found in a variety of pharmaceutical products, including ultrasound gel, laxatives and injectable steroids, and allergies to it are extremely rare.
Dr. Kuruvilla said that it is still possible that something else is responsible, and further investigations are needed to discover the cause of this small number of events.
Dr. Kimberly Blumenthal, an allergist and immunologist at Massachusetts General Hospital, noted that anaphylaxis can sometimes be difficult to confirm without blood tests looking for an enzyme called tryptase, which is released during allergic reactions. It is essential, she added, that protocols are in place so that similar cases can be investigated in more detail.
According to data records from its final-stage clinical trials, Moderna has not reported any link between its vaccine and anaphylaxis. But when products emerge from closely monitored studies for wide distribution, rare side effects can occur.
Recent allergic reactions linked to the very similar Pfizer vaccine have sparked heated discussions during consultative panel discussions this month by the FDA and CDC, with experts noting that anaphylaxis appeared to be occurring with an unusual frequency soon in the distribution. (Under normal circumstances, allergic reactions to vaccines are believed to occur at a rate of about one in a million.)
Denise Grady and Noah Weiland contributed reporting.