Dr. Fauci warned that vaccinated individuals could experience worse outcomes

"Safety study, Mark, is to see if I inject it in the arm, does it have some sort of idiosyncratic or bad reaction? There's another element to safety, and that is if you vaccinate someone and they make an antibody response and then they get exposed and infected, does the response that you induce actually enhance the infection and make it worse?
And the only way you'll know that is if you do an extended study, not in a normal volunteer who has no risk of infection, but in people who are out there in a risk situation. This would not be the first time if it happened that a vaccine that looked good in initial safety actually made people worse. There was the history of the respiratory syncytial virus vaccine in children which paradoxically made the children worse. One of the HIV vaccines that we tested several years ago actually made individuals more likely to get infected. So you can't just go out there and give it unless you feel that in the field, when someone is getting infected and exposed, being vaccinated doesn't make them worse. That's why you gotta do a trial."