Background: During the Ebola epidemic of 2014-5, there was no proven therapy or prevention available beyond horizontal infection control measures. After the Canadian government donated 800 vials of the vaccine candidate VSV-EBOV to World Health Organization (WHO), the University Hospitals of Geneva agreed to perform a WHO-coordinated phase I/II double-blind, placebo-controlled randomized trial to assess the candidate's safety and immunogenicity healthy adult volunteers. The doses of 10 E7 pfu and 5 x 10 E7 pfu were tested in 35 and 16 volunteers, respectively, before a safety-driven study hold in late 2014. At trial resumption in early 2015, the remaining volunteers (n=51) received 3 x 10 E5 pfu. Thirteen volunteers received placebo. Among vaccinees, early immune responses were impressive, even at the lowest dose of 3 x 10 E5 pfu, and persisted well up to 6 months (last time point assessed). The trial successfully ended 12 months after injection. This prospective observational study will assess immune durability in this group of vaccinees at yearly time points up to five years post-vaccination.