Moderna’s vaccine, like Pfizer’s, is designed around a molecule called messenger RNA, or mRNA, that’s injected into the upper arm. Once inside human cells, the mRNA instructs the manufacture of a protein called spike, which then teaches the immune system to recognize and thwart the coronavirus, should it ever invade the body. Each vaccine contains a handful of other ingredients that sheath the fragile mRNA in a protective greasy bubble and help keep the recipe stable in transit.

None of the ingredients in either vaccine have been identified as common allergens. 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 bevy of pharmaceutical products, including ultrasound gel, laxatives and injectable steroids, and allergies to it are extremely rare.

Dr. Kuruvilla said it remained possible that something else was responsible, and more investigation was needed to nail down the cause of this smattering of events.

Dr. Kimberly Blumenthal, an allergist and immunologist at Massachusetts General Hospital, noted that anaphylaxis can sometimes be difficult to confirm without blood work that hunts for an enzyme called tryptase, which is released during allergic reactions. It’s essential, she added, for there to be protocols in place so similar cases can be investigated further.

According to data filings from its late-stage clinical trials, Moderna did not report any links between its vaccine and anaphylaxis. But when products emerge from closely monitored studies into broad distribution, rare side effects can occur.

The recent allergic reactions linked to Pfizer’s very similar vaccine prompted heated discussions during advisory panel discussions held this month by the F.D.A. and the C.D.C., with experts noting that anaphylaxis seemed to be occurring at an unusual frequency so soon into 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.