Moderna’s vaccine, like Pfizer’s, is designed around a molecule called messenger RNA, or mRNA, which is injected into the upper arm. Once inside human cells, mRNA directs the production of a protein called a peak, which then teaches the immune system to recognize and counteract the coronavirus, if it invades the body. Each vaccine contains a handful of other ingredients that envelop the fragile mRNA in a protective fatty bubble and help keep the recipe stable during transport.
None of the ingredients in the two vaccines have been identified as a common allergen. But several experts have cautiously pointed to polyethylene glycol, or PEG, which appears in both recipes, albeit in slightly different formulations, as a possible culprit. PEG is found in a multitude of pharmaceuticals, including ultrasound gel, laxatives, and injectable steroids, and allergies to it are extremely rare.
Dr Kuruvilla said there was still the possibility that something else was responsible and that further investigation was needed to determine the cause of this handful of events.
Dr Kimberly Blumenthal, allergist and immunologist at Massachusetts General Hospital, noted that anaphylaxis can sometimes be difficult to confirm without a blood test that looks for an enzyme called tryptase, which is released during allergic reactions. It is essential, she added, that there are protocols in place so that similar cases can be investigated further.
According to data repositories from its advanced clinical trials, Moderna has not reported any link between its vaccine and anaphylaxis. But when the products emerge from closely watched studies into wide distribution, rare side effects can occur.
Recent allergic reactions to Pfizer’s very similar vaccine have sparked heated discussion during advisory group discussions this month by the FDA and CDC, with experts noting anaphylaxis appeared to be occurring at an unusual frequency so early. in distribution. (Under normal circumstances, allergic reactions to vaccines are thought to occur at a rate of about one in a million.)
Denise Grady and Noah Weiland contributed reporting.