Evaluation of the Immunogenicity and Safety of Ad26.COV2.S's COVID-19 Vaccine

Inclusion Criteria: * Age between 18 and 45 years old or 55 years and older * Be eligible to receive one of the study vaccines as part of the trial * Understand and agree to comply with study procedures (visits, telephone calls) * Agree not to participate in any other vaccine study during the time of the study * Give written informed consent prior to any examination performed as part of the trial Exclusion Criteria: * Positive SARS-CoV-2 antigenic test * Positive SARS-CoV-2 polymerase chain reaction (PCR) results less than 48 hours old * History of infection by SARS-CoV-2 confirmed by antigenic test or PCR within 3 months prior to inclusion * Symptoms compatible with infection to SARS-CoV-2: sick or febrile participants (body temperature ≥ 38.0°C) * Pregnant or breastfeeding woman * Known chronic disease impacting the participant's immune response (uncured cancer, human immunodeficiency virus (HIV), hepatitis B virus (HBV) or hepatitis C virus (HCV) infection) * Anti-coagulant treatment * Immunosuppressive treatment * Contraindication to the proposed vaccine (according to RCP) * Previously received at least one injection of a SARS-CoV-2 vaccine * Patient having received immunoglobulin or another blood product within 3 months prior to inclusion * A history of serious adverse vaccine reactions (anaphylaxis and associated symptoms such as rash, breathing difficulties, laryngeal edema, or a history of allergic reaction that may be exacerbated by a component of the SARS-CoV-2 vaccine) * Any condition that, in the opinion of the investigator, may adversely affect the well-being of the participant and interfere with the purpose of the study