Immunogenicity and Safety of a Booster Vaccination With a Recombinant Protein RBD Candidate Vaccine Against SARS-CoV-2 in Healthy Adults Volunteers Fully Vaccinated Followed by an Extension Period to Study a Fourth Dose Administration.

Part A: Inclusion Criteria: * Male or female, by birth, ≥ 18 years old at Screening. * Willing and able to comply with scheduled visits, laboratory tests, complete diaries, and other study procedures. * Body Mass Index (BMI) between 18 to 40 kg/m2. * Has received a complete COVID-19 vaccination programme (two administrations, prime and boosting) at least 182 days and with a maximum of 365 days before Screening with Comirnaty vaccine. * Has a negative COVID-19 polymerase chain reaction (PCR) test at Screening. * Willing to avoid all other vaccines within 4 weeks before and after vaccination in this study (Day 0). Seasonal influenza vaccination is allowed if it is received at least 14 days before or after vaccination. * Willing to refrain from blood donation during the study. * Women of childbearing potential must have a negative urine pregnancy test at Screening and prior to vaccination. * Women of childbearing potential must be willing to use highly effective contraceptive methods or have practiced sexual abstinence from the screening visit until 8 weeks after the vaccination. * Males who are not sterilised, must be willing to avoid impregnating female partners from Screening until 8 weeks after vaccination. * Willing and able to provide written informed consent prior the initiation of any study procedures. Part A: Exclusion criteria: * Pregnant or lactating or intending to become pregnant or plans to breastfeed during the study. * Positive pregnancy test at Screening or v…