(Bethesda, MD) -- U.S. health officials are researching why some people have severe allergic reactions to getting a dose of a COVID-19 vaccine. The National Institutes of Health is starting a trial looking at the vaccines made by Pfizer and Moderna. Some people have reported symptoms that include hives, swelling and wheezing within four hours after getting the shot. Those impacted are usually people who have a history of allergies. The N-I-H study will include 34-hundred adults from 18 to 69.