The U.S. National Institutes of Health said on Wednesday it had begun a mid-stage study to determine the risk of allergic reactions to COVID-19 vaccines made by Moderna Inc and Pfizer Inc.

Several allergic reaction incidents, including serious episodes, known as anaphylaxis, have been reported in the U.S. after vaccinations of Pfizer/BioNTech and Moderna shots.

In January, the U.S. Centers for Disease Control and Prevention said allergic reactions are occurring at a rate of 11.1 per 1 million vaccinations.

The study, funded by the National Institute of Allergy and Infectious Diseases, will enroll 3,400 adults between the ages of 18 to 69, with about 60% participants having a history of severe allergic reactions to food, insect stings or immunotherapy.

The goal of the trial is to access the proportion of participants who have a systemic allergic reaction within 90 minutes after injection.

The agency expects to report data later this summer.