Fauci explains why vaccines could make disease worse.

"[...] it's gonna take a year to year and a half to know if we have a vaccine that we can use."

"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."