PREVAIL I: Partnership for Research on EBOLA Vaccines in Liberia is a randomized, placebo-controlled phase 3 trial of the chimpanzee adenovirus 3 (ChAD3-EBO-Z) and the recombinant vesicular stomatitis virus (rVSV∆G-ZEBOV-GP) vaccines initiated in Liberia. A phase 2 substudy was embedded to evaluate safety and immunogenicity. As Ebola declined in Liberia, the phase 3 component was eliminated. The phase 2 substudy enrolled 1,500 participants in Liberia after one-year safety and immunogenicity were reported. In 2015, a new outbreak of Ebola cases in Liberia led to an amendment to PREVAIL I to permit vaccination of contacts of cases using a "ring vaccination" strategy. Long-term follow-up continues.