Safety and Immunogenicity of Ervebo® and Zabdeno® Booster Vaccines Against Ebola Virus Following Previous Vaccination with the Zabdeno/Mvabea® or Ervebo® Vaccine Schedules in DRC

Inclusion Criteria: * Subjects who received either the Ervebo® vaccine (MSD), or the full Zabdeno, Mvabea® vaccine regimen (J\&J) more than 4 months prior to recruitment * Subjects between 18 and 50 years of age at time of randomization * Subject must be willing and able to provide informed consent * The subject must be in possession of an identification card (or other identification document) * Agreement to refrain from blood donation and other vaccinations 30 days after booster vaccination * Agreement to share and discuss participant's medical history, medical records and concomitant medications when relevant Exclusion Criteria: * Participants who previously experienced active Ebola Virus Disease (EVD) * Receipt of any vaccine (licensed or experimental) within 30 days prior to recruitment * Receipt of an additional booster dose of either Ervebo®, Zabdeno®, or any experimental Ebola vaccine * Incorrect or incomplete primary vaccination scheme with the Zabdeno, Mvabea® (J\&J) vaccine * Administration of immunoglobulins and/or any blood products within three months prior to recruitment. * Fever (\>38°C) within last 24 hours prior to recruitment. * Any confirmed or suspected immunosuppressive or immunodeficient state (incl. cancer and HIV); asplenia; recurrent severe infections and use of immunosuppressant medication within the last 6 months, except topical or short-term oral steroids. * Severe and/or uncontrolled cardiovascular disease, respiratory disease, gastrointestinal disease, liver disease, renal disease, endocrine disorder and neurological illness (mild/moderate well controlled comorbidities are allowed) * History of anaphylaxis, allergic disease or reactions to any component of the study vaccines * History of bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venipuncture * History of any thrombotic disorder, thrombocytopenia, thrombotic thrombocytopenia syndrome (TTP), or heparin-induced thrombocytopenia and thrombosis (HITT) * Any other significant disease, disorder, planned surgery, or finding which may significantly affect the ability of the volunteer to participate in the study or impair interpretation of the study data * Suspected or known alcohol or drug dependency * Subject is not readily available by telephone, email or physical address The non-vaccinated control group will also adhere to all the above in- and exclusion criteria, with exemption of: * Agreement to refrain from blood donation and other vaccinations 30 days after study vaccination * Subjects who received either the Ervebo® vaccine, or the full Zabdeno, Mvabea® vaccine regimen more than 4 months prior to recruitment The latter is rather introduced as an additional exclusion criteria: * Subjects who received either the Ervebo® vaccine or the full Zabdeno, Mvabea® vaccine regimen