Immunogenicity and Safety Study of SCB-2019 Vaccine as a Booster to COVID-19 Vaccine in Adults

Inclusion Criteria: * Male and female participants at least 18 years of age; * Individuals willing and able to comply with study requirements, including all scheduled visits, vaccinations, laboratory tests, and other study procedures; * Individuals willing and able to give an informed consent, prior to screening; * Healthy participants or participants with pre-existing medical conditions who are in a stable medical condition; * Individuals who received primary vaccination series with one of the selected COVID-19 vaccines (Comirnaty, Vaxzevria, or CoronaVac) ≥3 months prior to enrollment; or received primary series and a booster dose (≥3 months after primary series) of CoronaVac, ≥3 months prior to enrollment. Exclusion Criteria: * Individuals with fever \>37.5°C (axillary), or any acute illness at baseline (Day 1) or within 3 days prior to randomization; * Individuals with laboratory-confirmed SARS-CoV-2 infection at Visit 1 (determined by positive Rapid Antigen Test or RT-PCR); * Individuals who have received an investigational or licensed COVID-19 vaccine prior to Day 1 (except for primary series with Comirnaty, Vaxzevria, CoronaVac vaccines, or primary and booster dose of CoronaVac), or plan to receive COVID-19 vaccine during the study period; * Individuals who have a history of severe adverse reaction associated with a vaccine or severe allergic reaction (e.g., anaphylaxis) to any component of the study vaccines; * Individuals who have received any other investigational product within 30 days prior to Day 1 or intend to participate in another clinical study at any time during the conduct of this study.