The inclusion criteria for the efficacy trial are:  Have had, within the previous 72 hours, a high-risk contact with an Ebola patient confirmed by RT-PCR; Be 18 years of age or older at the time of inclusion; Have no symptoms of EVD; Give consent to participate in the efficacy trial; Agree not to participate in any other therapeutic or vaccine study until the end of the trial follow-up.  The criteria for non-inclusion in the efficacy trial are:  Have a history of EVD (self-report); Have been vaccinated with ERVEBO prior to the start of the study; Have participated in another therapeutic or vaccine study within 15 days prior to inclusion.  Inclusion criteria for the immunology ancillary study are:  High Risk Arm:  Be included in the efficacy trial; Be available for extended follow-up as specified in the protocol; Specifically consent to the immunology ancillary study.  Control arm:  Be 18 years of age or older at the time of inclusion; Have no symptoms of EVD; Eligible for ERVEBO vaccination according to national program criteria; Be available for extended follow-up as specified in the protocol; Consent specifically for the ancillary immunology study.  The criteria for non-inclusion in the immunologic ancillary study are:  All efficacy trial non-inclusion criteria; HIV positive; Pregnant women.