An Experimental Medicine Multicenter Trial to Evaluate the Safety and Immunogenicity of Experimental Versus Authorized SARS-CoV-2 Vaccine Candidates as a Booster Dose in Healthy Participants Previously Vaccinated With Authorized mRNA SARS-CoV-2 Vaccines.

Inclusion Criteria: 1. Male and female subjects aged 18 to 65 years inclusive at the time of the screening visit 2. Having received at least two doses of COVID-19 mRNA vaccines in the past with the last dose received more than 6 months prior to IMP administration in the trial 3. In healthy condition or with stable health status which is defined as an existing disease that has not required a significant change in treatment or hospitalization for worsening before enrolment, and for which neither a significant change in treatment or hospitalization for worsening is expected in the near future. 4. For woman of childbearing potential: a negative Beta-HCG blood test measure during the screening visit, and a negative highly sensitive pregnancy urinary test the day of the vaccination visit * if heterosexually active female, consistently using a highly effective method of contraception with partner from at least 21 days prior to enrolment through 4 months after the IMP administration. Highly effective contraception is defined as using any of the following methods: * Combined hormonal contraception with inhibition of ovulation (estrogen and progesterone containing); * Intrauterine device (IUD); * Intrauterine hormone releasing system (IUS); * Hormonal contraception (progesterone only); * Successful vasectomy in the male partner (considered successful if a participant reports that a male partner has (i) documentation of azoospermia by microscopy, or (ii) a vasectomy more than 2 years ago with no resultant pregnancy despite unprotected sexual activity post vasectomy); * Or not be of reproductive potential, such as having reached menopause (no menses for 1 year without an alternative medical cause) or having undergone hysterectomy, bilateral oophorectomy, or tubal ligation. * Agree not to seek pregnancy including through alternative methods, such as artificial insemination or in vitro fertilization until 4 months after the IMP administration. 5. Participant who was born male, if heterosexually active male, using an effective method of contraception with their partner from the IMP administration until 4 months thereafter. All male participants also agree not to donate sperm during this period. 6. Negative nasopharyngeal antigenic test for SARS-CoV-2 on the day of screening and before randomisation 7. Participant who has normal biological values: * Hemoglobin ≥ 11.0 g/dL for participants who were born female, ≥ 13.0 g/dL for participants who were born male; * White blood cell count = 3,300 to 12,000 cells/mm3; * Total lymphocyte count ≥ 800 cells/mm3; * Platelets = 125,000 to 550,000/mm3; * ALT, AST, and alkaline phosphatase \< 1.25 times the institutional upper limit of normal; * Creatinine \<1.1x institutional upper limit of normal of the laboratory; * Normal urine test: absence of glucose, protein and haemoglobin * Negative HIV antigen/antibody test; * Negative Hepatitis B surface antigen (HBsAg); * Negative anti-Hepatitis C virus antibodies (anti-HCV), or negative HCV polymerase chain reaction (PCR) if the anti-HCV is positive; 8. Willingness to undertake SARS-CoV-2 testing according to study protocol, and receive SARS-CoV-2 test results 9. Willingness and availability to be followed for the planned duration of the study in one of the dedicated trial centres 10. Informed and signed written consent form before performance of any trial-related screening procedures 11. Agree to be registered in the French Health Ministry computerised file (for France only) 12. Being covered by Health Insurance (for France only) Exclusion Criteria: 1. Acute febrile infection (body temperature ≥ 38.0°C) within the previous 72 hours and/or presenting symptoms suggestive of COVID-19 or SARS-CoV-2 infection within the previous 28 days or having been in contact with an infected individual for the last 14 days before the inclusion visit 2. Any medical condition that could impair the immune response: clinically significant medical condition, physical examination findings, clinically significant abnormal laboratory results, or past medical history with clinically significant implications for current health. A clinically significant condition or process includes but is not limited to: * A process that would affect the immune response; * A process that would require medication that affects the immune response; * Any contraindication to repeated injections or blood draws; * A condition that requires active medical intervention or monitoring to avert grave danger to the participant's health or well-being during the study period; * A condition or process for which signs or symptoms could be confused with reactions to vaccine, or * Any condition specifically listed among the exclusion criteria below. 3. Pregnancy or breastfeeding that is currently ongoing, or positive pregnancy test at screening visit and the day of the vaccination 4. Immunodeficiency 5. Asthma: a condition that requires active medical intervention or monitoring to avert grave danger to asthma other than mild, well-controlled asthma. (Symptoms of asthma severity as defined in the most recent National Asthma Education and Prevention Program (NAEPP) Expert Panel report). Exclude a participant who: * Uses a short-acting rescue inhaler (typically a beta 2 agonist) daily, or * Uses moderate/high dose inhaled corticosteroids, or * In the past year has either of the following: (i) Greater than 1 exacerbation of symptoms treated with oral/parenteral corticosteroids or (ii) Needed emergency care, urgent care, hospitalization, or intubation for asthma. 6. Diabetes type 1 or type 2, including cases controlled with diet alone. (Not excluded: history of isolated gestational diabetes). 7. Thyroidectomy, or thyroid disease requiring medication during the last 12 months 8. Hypertension: * If a person has been diagnosed with hypertension, exclude for blood pressure that is not we