Allergic-like reactions to mRNA vaccines occurred only in high-risk group
A multi-center, randomized, placebo-controlled trial compared systemic allergic reaction rates to Pfizer-BioNTech and Moderna COVID-19 vaccines between people with prior severe allergic reactions or mast cell disorders and a comparison group without such history. The trial reported that clinically-determined systemic allergic reactions within 90 minutes or 48 hours occurred only in the allergic/mast cell disorder group, but at rates similar to placebo, and did not reach planned enrollment, limiting statistical precision. The authors state that reactions to placebo and the absence of an identified immunologic mechanism suggest non-allergic or non-immunologic causes.