Inclusion Criteria:  Male and female participants ≥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 series with SCB-2019 vaccine or one of the selected authorized or investigational COVID-19 vaccines (Comirnaty, COVID-19 Vaccine Janssen, or CoronaVac) ≥6 months prior to enrollment. Female participants of childbearing potential may be enrolled in the study, if the participant has practiced highly effective contraception for 30 days prior to vaccination, and has a negative pregnancy test on the day of vaccination and has agreed to continue highly effective contraception for 3 months after the last vaccination.  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, COVID-19 Vaccine Janssen, or CoronaVac vaccines or with SCB-2019 vaccine as part of CLO-SCB-2019-003 study), 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.