On Apr. 7, 2021, the NIH announced that a clinical trial was underway to determine whether people who are highly allergic or have a mast cell disorder are at increased risk for an immediate, systemic allergic reaction to the Moderna or Pfizer-BioNTech COVID-19 vaccines.

A systemic allergic reaction to a vaccine occurs in one or more parts of the body beyond the injection site. If such an allergic reaction occured in study participants, investigators assessed whether the reactions were more frequent in participants who were highly allergic or had a mast cell disorder than in participants with no allergic history.