The National Institutes of Health (NIH) is searching for participants in a study of rare but severe allergic reactions to Pfizer's COVID-19 vaccine.

Daniel Rotrosen, director of the Division of Allergy, Immunology and Transplantation at the National Institute of Allergy and Infectious Diseases, told The Washington Post that researchers need people who have a history of severe anaphylaxis.

“This is not a simple study design,” Rotrosen said. “We expect to be looking at highly allergic individuals. They will be not necessarily so easy to recruit, either. A lot still needs to be done to be sure we have the optimal study design. That said, we’re trying to move as quickly as we can, for obvious reasons.”