Immunogenicity After COVID-19 Vaccines in Adapted Schedules

Inclusion Criteria: 1. Male, female, or X (non-binary gender) subjects, 18-55y inclusive on the day of signing of the ICF 2. Provision of signed and dated informed consent form 3. Available at all provided timepoints of the study and is not planning to move abroad for the whole duration of the study 4. In good general health as evidenced by medical history and/or physical examination or adults with pre-existing medical conditions who are in stable condition. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 3 months before enrollment. 5. Willing and able to comply with all study procedures 6. Participants born female must be either: * of childbearing potential and using effective contraception for at least 1 month prior to screening and agree to use such a method during study participation until 1 months following the last study dose administration. * of non-childbearing potential. Exclusion Criteria: 1. Previous clinical or microbiological confirmed diagnosis of COVID-19. 2. Febrile illness within 72hours before first vaccination (this is a temporary exclusion criterion). 3. Unstable, severe, progressive disease in the past 3 months. 4. History of malignancy during the past 5 years. 5. History of severe adverse reaction associated and/or anaphylaxis with a vaccine. 6. Known allergic reactions of any severity to polyethylene glycol \[PEG\] or to polysorbate (due to potential cross-reactive hypersensitivity with the vaccine ingredient PEG). 7. Primary or secondary immunodeficiency disorders (e.g. immunosuppressive disease or therapy, human immunodeficiency virus (HIV) infection…). 8. Chronic administration (defined as more than 14 days in total) of immunosuppressant or other immune-modifying drugs during the period starting 6 months prior to the first vaccine dose. For corticosteroids, this will mean prednisone 20 mg/day, or equivalent. Inhaled, nasal, opthalmic and topical steroids are allowed. 9. Pregnancy or lactation. 10. History of drug or alcohol abuse. 11. Anything in the opinion of the investigator that would prevent volunteers from completing the study or put the volunteer at risk, including relevant psychiatric diagnosis. 12. Previous vaccination or planned to accept other vaccination during this study with any coronavirus vaccine outside this study. 13. Previous vaccination or planned to accept other vaccination during this study with any coronavirus vaccine outside this study, with the exception of a third COVID-19 vaccine during fall/winter '21-'22. 14. Receipt of blood/plasma products or immunoglobulin, from 60 days before study intervention administration or planned receipt throughout the study. 15. Participation in another clinical trial with an IMP or a new medical device within 28 days prior to study entry and/or during study participation. 16. Participant is an employee of the investigator or study site, with direct involvement in the proposed study or other studies under the direction of that investigator or study site, as well as first degree family members and household members of the employees or the investigator, or an employee of the sponsor.