we has selected a new candidate vaccine which is monovalent.

The purpose of this study is to assess the immunogenicity and safety of SCB-2023 trivalent vaccine compared to the prototype SCB-2019 monovalent vaccine in participants previously vaccinated with 3 doses of inactivated COVID-19 vaccine ≥6 months prior to enrollment.

Inclusion Criteria:

• Male or female ≥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 three doses of inactivated COVID-19 vaccine.

Exclusion Criteria:

• Body temperature >37.8°C (axillary), or any acute illness at baseline.
• Confirmed SARS-CoV-2 infectionor with known history of COVID-19.
• Individuals who have received an investigational or licensed COVID-19 vaccine prior to Day 1 (except for inactivated COVID-19 vaccine).
• Any confirmed or suspected immunosuppressive or immunodeficient condition resulting from disease or immunosuppressive/cytotoxic therapy.
• Any progressive unstable or uncontrolled clinical conditions.
• Individuals who are pregnant, or breastfeeding, or planning to become pregnant while enrolled in this study or during the study period.
• History of severe adverse reaction associated with a vaccine or severe allergic reaction.
• History of malignancy within 1 year before screening.
• Individuals who have received any other investigational product.
• Individuals who have received any other licensed vaccines within 14 days prior to enrollment.
• Treatment with Rituximab or any other anti-CD20 monoclonal antibodies.
• Known bleeding disorder that would, in the opinion of the investigator, contraindicate i.m. injection.
• Administration of intravenous immunoglobulins and/or any blood products.
• Any condition that, in the opinion of the investigator, would interfere with the primary study objectives or pose additional risk to the participant.