Inclusion Criteria:  Must have previously received the 2-dose Ebola vaccine regimen in Kenya or Uganda in Cohort 2a of the VAC52150EBL2002 study. Must be aged 18 - 50 years at time of randomisation in the VAC52150EBL2002 study. Must consent to participate in the study by signing or thumbprinting an informed consent form (ICF), indicating that the participant understands the purpose and procedures of the study, as well as the potential risks and benefits of participation. Must be willing/able to ensure that participants adhere to the prohibitions and restrictions specified in this protocol Must be available and willing to participate for the duration of the study visits. Must be in reasonably good medical condition (absence of acquired immunodeficiency syndrome [AIDS]-defining illnesses or clinically significant disease) Must be on a stable regimen of HAART, with a HIV viral load of <50 copies/mL and a CD4+ T-cell count of >350 cells/µL at screening. Must be willing to continue HAART throughout the study. Potential participants must be healthy on the basis of clinical laboratory tests performed at screening. Female subjects of childbearing potential must use adequate birth control measures consistent with local regulations, from at least 14 days before vaccination until the end of the study. Must have a negative pregnancy test at screening and immediately prior to vaccination. Must have a means to be contacted  Exclusion Criteria:  Participants in the VAC52150EBL2002 trial who were not in the Cohort 2a, or were allocated to the placebo arm. Known allergy or history of anaphylaxis or other serious adverse reactions to vaccines or vaccine products, chicken or egg proteins and aminoglycosides (e.g. gentamicin). Presence of acute illness or axillary temperature ≥38ºC on the day of vaccination. Participants with such symptoms will be deferred. Women who are breast-feeding or known to be pregnant or planning to become pregnant during the study. Clinically significant history of skin disorder, allergy, symptomatic immunodeficiency, cardiovascular disease, respiratory disease, endocrine disorder, liver disease, renal disease, gastrointestinal disease, neurological illness. Received a blood transfusion or other blood products within 8 weeks of enrolment. Potential participants who have been vaccinated with live-attenuated vaccines within 30 days before and after the study vaccination, and with inactive vaccine within 15 days before and after the study vaccination. Receipt of any disallowed therapies before the planned administration of the study vaccine. Subjects who, in the opinion of the investigator, are unlikely to adhere to the requirements of the study or are unlikely to complete the study. Any other finding which, in the opinion of the investigator, would increase the risk of an adverse outcome.