A phase Ⅱ, observer-blind, randomized, controlled, investigator-initiated clinical trial to evaluate the safety and immunogenicity of a booster vaccination with Recombinant COVID-19 vaccine (Sf9 Cell) in a population aged 18 years and above who have completed homologous primary vaccination with either Inactivated or mRNA or Viral Vector COVID-19 vaccines at least 6 months prior to enrolment.

A phase Ⅱ, observer-blind, randomized, controlled, investigator-initiated clinical trial to evaluate the safety and immunogenicity of a booster vaccination with Recombinant COVID-19 vaccine (Sf9 Cell) in a population aged 18 years and above who have completed homologous primary vaccination with either Inactivated or mRNA or Viral Vector COVID-19 vaccines at least 6 months prior to enrolment. The study uses a non-inferiority design to compare between schedules with Recombinant COVID-19 Vaccine (Sf9 Cell) versus COVID-19 Vaccine (Vero Cell), Inactivated as the booster dose. Participants, laboratory and analysing statisticians will remain blind to treatment allocation.

A total of 450 participants will be enrolled (participants aged ≥ 60 years account for approximately 10%), consisting of 3 cohorts:

150 participants who received 2 doses of Inactivated COVID-19 vaccines with the second dose at least 6 months (≥180 days) prior to enrolment.

150 participants who received 2 doses of mRNA COVID-19 vaccines with the second dose at least 6 months (≥180 days) prior to enrolment.

150 participants who received 2 doses of Viral Vector COVID-19 vaccines with the second dose at least 6 months (≥180 days) prior to enrolment.

Each of 3 cohorts of 150 participants will be randomized 1:1 to receive a single dose of Recombinant COVID-19 vaccine (Sf9 Cell) (test group) or the COVID-19 Vaccine (Vero Cell), Inactivated (control group).

Inclusion Criteria:

• 1. Participant is willing and able to give written informed consent for participation in the trial.
• 2. Male or Female, aged 18 years or above and in good health as determined by a trial clinician.
• 3. Female participants of childbearing potential must be willing to ensure that they or their partner use effective contraception from 1 month prior to first immunisation continuously until 3 months after boost immunisation. See Section "Contraception and Pregnancy" for definition of child-bearing potential and definition of effective contraception.
• 4. In the Investigator's opinion, is able and willing to comply with all trial requirements.
• 5. Willing to allow investigators to discuss the volunteer's medical history with their General Practitioner and access all medical records when relevant to study procedures.
• 6. Agreement to refrain from blood donation during the study.
• 7. Participants who have completed homologous primary vaccination with either Inactivated or mRNA or Viral Vector COVID-19 vaccines (full approval, CMA or EUA) and is at least 6 months post second vaccination.

Exclusion Criteria:

• 1. Receipt of any vaccine (licensed or investigational) other than the study intervention within 30 days before and after each study vaccination (one week for licensed seasonal influenza vaccine or pneumococcal vaccine)
• 2. Prior or planned receipt of any other investigational or licensed vaccine or product likely to impact on interpretation of the trial data (e.g. adenovirus vectored vaccines, any coronavirus vaccines)
• 3. Positive SARS-CoV-2 RT-PCR at screening.
• 4. Participants who are pregnant at enrolment or planning to become pregnant during the first 3 months following vaccination.
• 5. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines.
• 6. Any confirmed or suspected immunosuppressive or immunodeficient state; asplenia; recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting ≤14 days).
• 7. History of allergic disease or reactions likely to be exacerbated by any component of study vaccines.
• 8. Any history of anaphylaxis.
• 9. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the skin and cervical carcinoma in situ).