Vaccine challenge trials, in which healthy volunteers are infected with a pathogen to determine whether a vaccine works, can be done faster and with fewer participants than traditional efficacy studies. But there are serious ethical considerations. Johns Hopkins vaccine researcher Dr. Anna Durbin talks with Stephanie Desmon about this method of getting to a COVID-19 vaccine, her experience with a dengue challenge trial, and what we know so far about whether COVID-19 antibodies confer immunity.