Inclusion Criteria:  Participant is able and willing to provide written informed consent prior to any study procedure. Participant is 18 to 59 years of age. Participant is male or non-pregnant female. Participant has had no known exposure to SARS-CoV-2 virus in the last 14 days and has a negative RT-PCR SARS-CoV-2 laboratory test 24-48 hours prior to initial vaccination.  SARS-CoV-2 antibodies:  Negative for SARS-CoV-2 antibodies, or Positive for SARS-CoV-2 antibodies with no known clinical history of COVID-19 infection (asymptomatic) (Note: up to a maximum of 5 such participants per treatment arm). Participant is determined by the Investigator to be healthy on the basis of medical history, physical examination, vital signs, and routine laboratory tests. Participant agrees to comply with study procedures, including the collection of venous blood, and to be available for all study visits. Women of child-bearing potential must have a negative urine pregnancy test during screening and a negative serum pregnancy test on Day -1 (prior to the first dose) and be neither breastfeeding nor intending to become pregnant during study participation. Women of child-bearing potential must agree to use highly effective contraceptive methods at least 28 days prior to study entry, for the duration of study participation, and for 120 days after the last dose of study vaccine. Men who are potentially fertile must agree to use barrier protection for the duration of their participation in the study and until 120 days after administration of the last dose of study vaccine when they engage in sexual relations with women who are of child-bearing potential, pregnant, or lactating; they also agree to request their female partners to use an effective method of contraception if they are of child-bearing potential. Participant has an oral temperature of less than 37.5°C at screening and prior to dosing.  Participant has a screening ECG with none of the following clinically significant findings:  PR-interval >210 msec QRS-duration >120 msec QT-interval >500 msec QTcF-interval >450 msec (males), >470 msec (females) Pathologic Q wave Significant ST-T wave changes Second or third-degree atrioventricular heart block. Participant agrees to refrain from donating blood or plasma, outside of the study, for the duration of study participation, and for 28 days after the last dose of study vaccine. Participant agrees not to consume any alcohol within 48 hours prior to each study vaccine administration.  Exclusion Criteria:  Participant has a history of chronic respiratory disease, hypertension, significant cardiovascular disease, autoimmune disease (including hypothyroidism without defined non-autoimmune cause), immunodeficiency, clotting disorder, history of thrombosis, or malignancy (except for adequately treated malignancies with an expected 5-year survival rate of >90%, e.g., carcinoma in-situ of the breast or cervix, squamous or basal cell carcinoma of the skin). Participant has any medical disease or condition, or psychiatric condition, which in the opinion of the Investigator would preclude study participation (would place the participant at an unacceptable risk of injury, render the participant unable to meet the requirements of the protocol, or may interfere with the evaluation of responses). Participant has a positive test result for hepatitis B surface antigen, hepatitis C virus antibody, or human immunodeficiency virus types 1 or 2 antibodies at screening. Alcohol consumption of >21 units per week (males) or >14 units per week (females) (1 unit of alcohol equals 1/2 pint [285 mL] of beer or lager, 1 glass [125 mL] of wine, or 1/6 gill [25 mL] of spirits). Strenuous exercise (e.g., heavy lifting, weight, or fitness training) within 96 hours (4 days) of screening and the first dosing visit. Participant has participated in another investigational study involving an investigational product within 30 days, or 5 half-lives, whichever is longer, before the first study vaccine administration in the current study. Participant is currently enrolled in, or plans to participate in, another clinical trial with an investigational product that will be received during the study-reporting period. Participant has a history of any vaccine or drug hypersensitivity reactions (including skin reactions or anaphylaxis), or other known clinically significant allergies. Participant has a history of chronic use (>14 continuous days in the 6 months preceding screening) of any medications that may be associated with impaired immune responsiveness including, but not limited to: systemic corticosteroids exceeding 10 mg/day of prednisone equivalent, allergy injections, immunoglobulin, interferon, immunomodulators, cytotoxic drugs, or other immuno-suppressive drugs. The use of low dose topical, ophthalmic, inhaled, and intranasal steroid preparations is permitted (not more than the equivalent of 10 mg prednisone a day). Use of any prescription medications within 14 days or 5 half-lives (whichever is longer) of first study vaccine administration (Day 1), use of over-the-counter medications, or herbal supplements within 7 days. The use of occasional paracetamol (up to 4 g per day) and hormone replacement therapy, oral, implantable, transdermal injectable, or intrauterine contraceptives is permitted. Nutritional supplements may be permitted but must be discussed with the Sponsor's medical monitor prior to participant enrolment. Participant has received immunoglobulins and/or any blood or blood products within 90 days before the first study vaccine administration (Day 1) or at any time during the study. Participant has received a vaccine within 28 days of the first dose of study vaccine administration. Participant has a history of alcohol abuse or other recreational drug (excluding cannabis) use within 6 months before the first study vaccine administration. Participant has a positive result for the urine drugs of abuse test at screening or prior to the first study vaccine administration (Day 1). Participant has received any other SARS-CoV-2 vaccine or experimental coronavirus vaccine at any time prior to the study. Participant is pregnant, lactating, or is expecting to conceive/father children during the study. Participant has any clinically significant abnormal findings on screening biochemistry, haematology blood tests, or urinalysis; participants with Gilbert's syndrome will be permitted to enter the study.