The purpose of this study is to assess the immunogenicity and safety of the investigational SCB-2019 vaccine, administered as a booster dose, to adults who:

• Received primary series with one of the selected authorized or investigational COVID-19 vaccines at least 3 months prior to enrollment.
• Received primary series and a booster dose of CoronaVac at least 3 months prior to enrollment.

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.