Partnership for Research on Ebola Vaccination (PREVAC) is a randomized, double-blind, two-stage trial comparing three experimental Ebola vaccine strategies with identical placebo regimens. The first stage of the study was initiated with two arms: the Ad26.ZEBOV experimental vaccine given as a first dose followed eight weeks later by MVA-BN-Filo as a booster dose, which is being compared with an identical placebo regimen. The second stage added two other experimental vaccine strategies — rVSVΔG-ZEBOV-GP prime followed eight weeks later by either the same vaccine or a placebo — compared with an identical placebo regimen.