This clinical trial is designed to assess the safety, tolerability and immunogenicity of a single dose of ChulaCov19 BNA159 and ChulaCov19 BNA159.2 vaccines as a booster dose, given at least 3 months after receipt of a previous booster dose of any authorized/approved COVID-19 vaccine.

This is a phase II, randomised open-label trial in which 150 healthy males and non-pregnant females aged 18-64 years, will be recruited from multi-sites in Australia. This is a 2-part study (Part A and Part B). In Part A, the randomisation will be a 2:1 design to receive either ChulaCov19 BNA159 vaccine or Comirnaty Pfizer/BNT vaccine. In Part B, participants will receive only ChulaCov19 BNA159.2 (Bivalent, COMVIGEN) vaccine. Participants in part A and B will be followed up using a combination of an-site and telephone visits for assessment of safety and immunogenicity for 6 months post-vaccination.

Inclusion Criteria:

1. Must be a male or female aged 18 - 64 (inclusive) at the time of enrolment

2. Must have completed a primary course of 2 doses of any approved COVID-19 vaccine and 3 months or more have passed since receipt of last booster dose (1 or 2 prior booster doses for a total of 3 or 4 doses) as described in Table 1

3. Must be able to communicate effectively with study personnel and considered reliable, willing, and cooperative in terms of compliance with the protocol requirements

4. Participants must sign the written informed consent form prior to undertaking any protocol-related procedures

5. SARS-CoV-2 rapid antigen test is negative at Day 1 (the day of receiving the study booster dose)

6. Does not intend to receive any other authorized/approved COVID-19 vaccine at the time of enrolment and up to 3 months of the study

7. Males must be surgically sterile (>30 days since vasectomy with no viable sperm), practice true abstinence or, if engaged in sexual relations with a female of childbearing potential, the participants and their partner must use an acceptable, highly effective, dual contraceptive method* from Screening and for a period of at least 90 days after vaccination

8. A female participant is eligible if she is not pregnant, or breastfeeding indicated by one of the following conditions:

   1. With childbearing potential: she agrees to use an effective contraceptive method or abstinence from at least 4 weeks prior to the study intervention administration until at least 90 days after the study intervention administration, or
   2. With non-childbearing potential. To be considered of non-childbearing potential, a female must be post-menopausal for at least 1 year or surgically sterile. If the participant is < 1 year post-menopausal, an FSH test may be conducted to establish childbearing potential.

9. Participants must be in general good health* based on medical history and physical examination, as determined by the PI at Screening.

10. Participants must agree to refrain from donating blood, plasma, ova, sperm, or organs during the whole study.

Exclusion Criteria:

1. History of a systemic hypersensitivity or life-threatening reaction to a vaccine containing any of the same or similar substances.
2. History of test-confirmed by PCR or rapid antigen test to SARS-CoV-2 COVID-19 infection within 3 months prior to randomisation.
3. Presence of clinically significant medical history*, unstable chronic or acute disease that, in the opinion of the PI, may increase the risk of exposure to the investigational vaccine
4. History of having any significant side effects after receipt of any other COVID-19 vaccine eg. endocarditis, pericarditis or myocarditis. History of any severe reactogenic side effects or other medical illness that were thought to be associated with vaccine.
5. Presence of an acute illness* or with fever at 38.00 C or more within 72 hours prior to vaccination.
6. Bleeding disorders or taking an anticoagulant or anti-platelet agent that may contraindicate for intramuscular injection based on Investigator's judgment
7. Inadequate venous access to allow the collection of blood samples.
8. Received any prophylactic or therapeutic vaccine, biologic product, device or blood product, within 4 weeks of vaccination or 5 half-lives (whichever is longer) or anticipate doing so in the follow-up period defined for this study. For influenza vaccine, however, can be administered up to 14 days prior to randomization and following Visit 3 (Day 29+3) blood sample collection.
9. History of ever had an anaphylaxis reaction to food, medication, or vaccination.
10. Participant is immunosuppressed as caused by disease or immunosuppressive therapy or anticipated need to use of any chemotherapy or immunosuppressive agents* within the next 6 months.
11. Participation in any of the other investigational trials of vaccines, therapeutic, or medical devices 12 weeks before or during the 6 months of this study.
12. Received immunoglobulins and/or any blood or blood products within 3 months before vaccination day or plans to receive any blood or blood products at any time during the study.