A Multicenter Study of the Immunogenicity of Recombinant Vesicular Stomatitis Vaccine for Ebola-Zaire (rVSV∆G-ZEBOV-GP) for Pre-Exposure Prophylaxis in Individuals at Potential Occupational Risk for Ebola Virus Exposure (PREPARE) compares the durability of the immune response three years after primary vaccination with rVSV∆G-ZEBOV-GP vaccine (V920) with and without a homologous booster immunization at 18 months.