The purpose of this research study is to evaluate the safety of and the body's immune response to single and multiple vaccinations in healthy men and women. We want to find out what effects GBP511 has on you and your health. We are doing this study to find out:

If the study drug has any side effects when given as single and multiple vaccinations.

If reactogenicity, an indication that the immune systems is working and is preparing to protect the body against future infection, occurs. This is a normal physical inflammatory response that occurs after vaccination, manifesting as localised reactions that may include pain or redness at the injection site, or systemic symptoms such as headache or fever.

If immunogenicity which is when GBP511 causes your body to make antibodies against the non-harmful viral proteins contained within GBP511, called 'neutralising antibodies (Nab),' occurs. And if GBP511 will cause the specialised cells of the body's immune system to mount a defence against the non-harmful viral proteins contained within GBP511, called 'cell-mediated immunity (CMI)'.

In this study, GBP511 will be administered with and without adjuvant, CAS-1. An adjuvant is an ingredient used in some vaccines that help produce a stronger immune response in the people receiving the vaccine and thus helps the vaccine work better.

This study will recruit approximately 368 participants in total and will be conducted in two stages:

Stage 1 will look at two vaccinations with GBP511 when given with and without the CAS-1 adjuvant; and Stage 2 will look at either one or two vaccinations with two candidate GBP511 vaccines chosen from Stage 1 of the study.

Participants will be required to attend Linear's clinical unit on two separate occasions to receive their two vaccinations. On the first day of dosing (Visit 2/Day 1) and second day of dosing (Visit 5/Day 29), we will be testing a single dose of the study drug in up to 168 healthy volunteers who will be divided into 3 groups. Group 1, Groups 2 and 3 will have 56 people each. These 3 groups will be divided into treatment subgroups, with a total of 6 test subgroups and 3 control subgroups. In all groups, neither you or the study doctor will know what subgroup you are part of, and therefore what treatment (GBP511 or Comirnaty+ placebo) you will receive.

Inclusion Criteria:

Age

1. For Stage 1, participant must be 18 to 55 years of age inclusive, at the time of signing the informed consent.

   For Stage 2, participant must be 18 years of age and older, at the time of signing the informed consent.

   Type of Participant and Disease Characteristics

2. Participants who are healthy as determined by medical evaluation including medical history, vital signs, physical examination, clinical laboratory tests, and medical judgement of the investigator.

3. Participants who are willing and able to attend all scheduled visits and comply with all study procedures.

4. Body mass index (BMI) within the range of 18.5-32.0 kg/m2 (revised) at screening

5. Participants who received a primary series and at least one booster dose of an authorized COVID-19 vaccine at least 24 weeks prior to the 1st study vaccination.

   Sex and Contraceptive/Barrier Requirements

6. All participants must agree to be abstinent from heterosexual intercourse or agree to consistently use at least one acceptable method of contraception from at least 4 weeks prior to the 1st study vaccination to 12 weeks after the last study vaccination (See Appendix 10.4 for detailed contraceptive methods).

7. Women of childbearing potential (WOCBP) with a negative urine or serum pregnancy test at screening.

   * Female participants who are surgically sterile (e.g., having undergone a full hysterectomy, bilateral salpingectomy, or bilateral oophorectomy) or postmenopausal with at least 12 months amenorrhoea not considered to be caused by any other medical condition, are not subject to a pregnancy test.

   Informed Consent

8. Participants who are capable of giving signed informed consent as described in Appendix 10.1.3 which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol before initiation of any trial-specific procedures.

Exclusion Criteria:

Medical Conditions

1. Any clinically significant respiratory symptoms (e.g., cough, sore throat), febrile illness (tympanic temperature >38°C), or acute illness within 24 hours prior to the 1st study vaccination. Prospective participants with these conditions cannot be included until 24 hours after resolution.

2. Concurrent or a history of virologically or serologically confirmed SARS-CoV-2 infection, or suspected SARS-CoV-2 infection as determined by the investigator, within 24 weeks prior to the 1st study vaccination.

3. History of virologically or serologically confirmed SARS-CoV-1, or MERS disease.

4. History of congenital, hereditary, acquired immunodeficiency, or autoimmune disease.

5. Any positive test results for hepatitis B surface antigen, hepatitis C antibody, or human immunodeficiency virus (HIV) at screening.

6. History of bleeding disorder or thrombocytopenia which is contraindicating intramuscular vaccination in the investigator's opinion.

7. History of hypersensitivity and severe allergic reaction (e.g., anaphylaxis) to any vaccines or components of the study intervention.

8. History of neurologic diseases (e.g., Guillain-Barre syndrome, myelitis, or encephalomyelitis).

9. History of myocarditis, pericarditis or myopericarditis, as assessed by the investigator, indicating probable or possible myocarditis, pericarditis, or myopericarditis, or demonstrating clinically significant abnormalities that could affect participant safety or the interpretation of study findings.

10. History of malignancy within 1 year prior to first study vaccination, except for cutaneous non-melanoma malignancy, melanoma-in-situ, or cervical carcinoma in situ that have been fully treated with completed follow-up prior to screening. Malignancy considered to carry minimal risk of recurrence may also be permitted at the discretion of the investigator.

11. Significant unstable chronic or acute illness that, in the opinion of the investigator, might pose a health risk to the participant if enrolled, or could interfere with the protocol-specified activities, or interpretation of study results.

12. Any other conditions which, in the opinion of the investigator, might interfere with the evaluation of the study objectives (e.g., alcohol or drug abuse, neurologic or psychiatric conditions).

13. Female participants who are pregnant or breastfeeding.

    Prior/Concomitant therapy

14. Receipt of any COVID-19 vaccination including investigational vaccines within 24 weeks prior to the 1st study vaccination or planned receipt at any time during the study period.

15. Receipt of any vaccinations intended to prevent SARS-CoV-1 or MERS, including investigational vaccines.

16. Receipt of any vaccine within 4 weeks prior to the 1st study vaccination or planned receipt of any vaccine through 4 weeks after the last study vaccination, except for influenza vaccination, which may be received at least 2 weeks prior to the 1st study vaccination.

17. Receipt of any immunoglobulins and/or blood-derived products within 12 weeks prior to the 1st study vaccination or planned receipt at any time during the study period.

18. Receipt of immunosuppressive therapy, such as any use of anti-cancer chemotherapy or radiation therapy; or chronic use of systemic corticosteroid (≥10 mg prednisone/day or equivalent more than 14 consecutive days) within 12 weeks prior to the 1st study vaccination and throughout the entire study period. The use of intranasal, inhaled, topical, or intra-articular corticosteroid is permitted.

    Prior/Concurrent Clinical Study Experience

19. Participation in another clinical study and receipt of study intervention within 4 weeks prior to the 1st study vaccination, or concurrent, planned participation in another clinical study with study intervention during this study period.

    Other Exclusions

20. Investigators, or study staff who are directly involved in the conduct of this study or supervised by the investigator, and their respective family members.

21. Donation of ≥450mL of blood product within 4 weeks prior to screening, or planned donation of blood product from enrollment through 12 weeks after the last study vaccination.