Immunogenicity After COVID-19 Vaccines in Adapted Schedules

The goal of this clinical trial is to compare different Coronavirus Disease 2019 (COVID-19) vaccination schedules in healthy adults that have not yet been exposed to SARS-CoV-2, the virus causing COVID-19. The main questions it aims to answer are: 1. Is it possible to adapt COVID-19 vaccination schedules while maintaining an adequate humoral immune response? 2. Is it possible to adapt COVID-19 vaccination schedules while maintaining an acceptable safety profile? Participants will be vaccinated twice with a COVID-19 vaccine (on day 0, and on day 28 or 84). After each vaccination, they will collect information about adverse events in a diary for 14 days. Information about the occurrence of events such as hospitalizations and infections with SARS-CoV-2 will be collected by the investigator for up to 364 days after the first vaccination. Blood samples will be taken on different timepoints and used to assess immunity against SARS-CoV-2. Researchers will compare 8 vaccination schedules to see if the immune response and safety profile is similar. Each participant will receive 1 of the following 8 vaccine schedules: * BNT162b2 (30µg) on day 0, followed by BNT162b2 (30µg) on day 28 * BNT162b2 (20µg) on day 0, followed by BNT162b2 (20µg) on day 28 * BNT162b2 (30µg) on day 0, followed by BNT162b2 (30µg) on day 84 * BNT162b2 (30µg) on day 0, followed by mRNA-1273 (100µg) on day 28 * BNT162b2 (30µg) on day 0, followed by ChAdOx1-S \[recombinant\] on day 28 * BNT162b2 (6µg, intradermal administration) on day 0, followed by BNT162b2 (6µg, intradermal administration) on day 28 * mRNA-1273 (100µg) on day 0, followed by mRNA-1273 (100µg) on day 28 * mRNA-1273 (50µg) on day 0, followed by mRNA-1273 (50µg) on day 28