Phase II, non-randomized, open-label, comparative, national, multicenter trial in Mali, aimed to assess the humoral vaccine immune response induced by Ad26.COV2.S vaccine in 200 adults one month after receiving the complete vaccination schedule of SARS-CoV-2 vaccine.

The main objective of this phase II trial is to evaluate the humoral immune response induced by the Ad26.COV2.S vaccine in adults one month after complete vaccination regimen against SARS-CoV-2, compared younger (up to 45 years old) and elderly (55+ years old) populations.

200 participants will be included, 200 participants for each vacine. The age categories are 18 - 45 years, 55 - 64 years and 65+ years. The number of participants per age group will be distributed as follows 1:1:0,5. There will be no comparison between population on different vaccines.

Ad26.COV2 vaccine (200 participants) 18-45 years old, 80 participants 55-64 years old, 80 participants 65 years old or older, 40 participants

Participants in Ad26.COV2 arm receive intramuscularly as a single dose of 0.5mL.

Humoral vaccine immune responses, induced by Ad26.COV2.S vaccine, will be measured by ELISA at D0, M1, M2, M6, M12 and M24.

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