A clinical trial is 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 occurs in study participants, investigators will assess whether the reactions are more frequent in participants who are highly allergic or have a mast cell disorder than in participants with no allergic history. In addition, investigators will examine the biological mechanism behind the reactions and whether a genetic pattern or other factors can predict who is at most risk. Volunteers will be considered highly allergic if they previously had an allergic reaction related to food, insect stings or allergen immunotherapy that required treatment with epinephrine; or previously had immediate allergic reactions to a vaccine or to one or more drugs.