The National Institutes of Health (NIH) has launched a clinical trial to better understand allergic reactions related to the Moderna and Pfizer- BioNTech COVID-19 vaccines. The trial, which will involve 3,400 adults, 60% of whom have a history of severe allergic reactions or a diagnosis of mast cell disorder, will also consider the biological mechanism behind the reactions to see if a genetic pattern or other factors can better predict who is at risk of a reaction.