Cohort Assessing the Immunogenicity and the Safety of the COVID-19 Vaccine Janssen in Healthy Volunteers Based on 2 Age Groups: 65 Years or Older - 55 to 65 Years. Covicompare-Janssen

Inclusion Criteria: * Healthy adults, or stable medical condition for adults with pre-existing medical conditions. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during 3 months before enrolment, nor expected to require any significant change in therapy or hospitalization for worsening disease in foreseeable future. * Understands and agrees to comply with the study procedures (visits, phone calls) and provides written free informed consent. * Able to comply with study procedures based on Investigator judgement. * Affiliated to a social security system, (except state medical aid) Exclusion Criteria: * Subject is ill or febrile (body temperature ≥ 38.0°C) within 72 prior hours or and/or symptoms suggestive of COVID-19 within the past 14 days at enrolment visit (ill or febrile participants may be re-scheduled within the inclusion period when no longer presenting symptoms) * History of documented COVID-19 (PCR+, antigenic test+ or chest CT Scan + or serology SARS-CoV- 2+) prior to the vaccine administration * Subjects with positive serology to SARS-CoV-2 at the enrolment visit. * Subjects who already received another anti-SARS-CoV-2-vaccine * Subjects who received BCG (Bacille Calmette and Guérin vaccine) given within the last year. * An immediate family member or household member of study staff. * Use of immunosuppressive drugs like e.g. corticosteroids at a dosage \> 10mg/day (excluding topical preparations and inhalers) within 3 months prior to enrolment or 6 months for chemotherapies * Received immunoglobulin or other blood product within 3 months prior to enrolment or planned receipt of immunoglobulin or a blood product through study completion. * Received any vaccination within 4 weeks prior to first injection or plan to receive a licensed vaccine 4 weeks after the last injection (exception for flu vaccine within 2 weeks). * History of severe adverse reactions to vaccine administration, including anaphylaxis and related symptoms, such as rash, respiratory difficulty, angioedema and abdominal pain to vaccines, or history of allergic reaction likely to be exacerbated by any component of the anti-SARS-CoV-2- vaccine. * History of severe allergic event * Participation in another investigational clinical study within 4 weeks before the enrolment visits or planned before the study completion. * Known HIV (human immunodeficiency virus), active HCV (Hepatitis C virus) or HBV (Hepatitis B virus) infection * Any pathological condition, such as cancer, which may be susceptible of reducing immunity response * History of heparin-induced thrombocytopenia (HIT) * Any bleeding disorder considered as contraindication to intramuscular injection or phlebotomy * The use of investigational Ig, investigational monoclonal antibodies or convalescent serum are not allowed during the study * Any condition which in the opinion of the investigator may interfere with the aim of the study * Pregnant or breastfeeding or positive pregnancy urine test at enrolment visit. * Woman in childbearing without efficacious contraception (in the opinion of the investigator) for 31 days after treatment * People under legal protection measure (tutorship, curatorship or safeguard measures)