ChulaCov19 mRNA Vaccine in Healthy Adults

Inclusion Criteria:

1. Participants must be able to communicate effectively with study personnel and considered reliable, willing, and cooperative in terms of compliance with the protocol requirements.
2. Participants must sign the written informed consent form prior to undertaking any protocol related procedures.
3. Participants must have a body mass index (BMI) at screening, calculated as the body mass divided (in kilograms [kg]) by the square of the body height (in meters [m]) of 18.0-30.0 kg/m2, inclusive, at screening.
4. Participants must have haematology, clinical chemistry, coagulation (for all participants in Phase 1, and, only if applicable, for participants in Phase 2), and urinalysis test results that are not deviating from the normal reference range by age and gender to a clinically relevant extent at screening.
5. 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 child-bearing potential, the participants and their partner must use an acceptable, highly effective, double-barrier contraceptive method* from screening and for a period of at least 60 days after the last dose of investigational vaccine.

6. Women of child-bearing potential must practice true abstinence or, if engaged in sexual relations with a male, they must agree to use highly effective (failure rate of < 1% per year when used consistently and correctly), double-barrier contraceptive measures* throughout the study and intend to continue use of contraception for at least 60 days following the last vaccination.

   * The PI is to assess the adequacy of methods of contraception on a case-by-case basis. These criteria do not apply if the participants are in a same-sex relationship.

7. Women of child-bearing potential must have a negative serum pregnancy test (beta human chorionic gonadotropin [β-HCG]) at screening and a negative urine-based test within 24 hours prior to each investigational vaccine administration.

8. Women of non-childbearing potential must:

   • be classified as being postmenopausal (defined as having a history of amenorrhea of at least one year), or
   • if having a history of amenorrhea of less than one year they must have an FSH level > 40 milli-international units per milliliter (mIU/mL), or
   • have a documented status of being surgically sterile (hysterectomy, bilateral oophorectomy, or tubal ligation/salpingectomy).
9. Participants must be in general good health based on medical history and physical examination, as determined by the PI.
10. Body temperature measured orally must be less than 37.8ºC.
11. Pulse must be no greater than 100 beats per minute.
12. Systolic blood pressure (SBP) must be between 85 to 150 millimetres of mercury (mm Hg), inclusive.
13. Participants must agree to refrain from donating blood, plasma, ovules, sperm, or organs during the whole study.

Adult Participants (Group 1 of Phase 1 and Phase 2) only 14. Must be a male or female aged 18-55 years (inclusive) at the time of enrolment.

Elderly Participants (Group 2 of Phase 1 and Phase 2) only 15. Must be a male or female aged 65-75 years (inclusive) at the time of enrolment.

Exclusion Criteria:

1. Presence of clinically significant medical history, unstable chronic or acute disease, or physical, or laboratory findings that, in the opinion of the PI may potentially increase the expected risk of exposure to the investigational vaccine, compromise the safety of the participant, or interfere with any aspect of study conduct or interpretation of results. This will include any thrombocytopenia or bleeding disorder contraindicating IM vaccination
2. Presence of self-reported or medically documented significant medical or psychiatric condition(s).

3. Presence of an acute illness, as determined by the participating site PI or appropriate sub-PI, with or without fever (oral temperature ≥ 38.0 ºC) within 72 hours prior to each vaccination.

   Presence of birthmarks, tattoos, wound, or other skin conditions over the deltoid region of both arms that, in the PI's opinion, could reasonably obscure and interfere with evaluation of local ISRs.

4. Inadequate venous access to allow collection of blood samples.
5. Breastfeeding or planning to breastfeed from the time of the first vaccination through 60 days after the last vaccination, or pregnant as confirmed by a positive serum β-HCG pregnancy test at screening or positive urine pregnancy test at subsequent clinic visits at timepoints as delineated in the study schedule.
6. Received any prophylactic or therapeutic vaccine, or licensed or unlicensed vaccine, drug, biologic, device, blood product, or medication, within 4 weeks of first vaccination or 5 half-lives (whichever is longer), or anticipate to do so in the follow-up period defined for this study.
7. Participant has previously participated in an investigational study involving LNPs (a component of the investigational vaccine assessed in this trial).
8. History of severe allergy (requiring hospital care), severe reaction to any drug or prior vaccination, or any known or suspected allergies or sensitivities to any component of the investigational vaccine or placebo.
9. Participant is immunosuppressed as caused by disease (such as HIV).

10. Chronic use (more than 14 continuous days) of or anticipated need to use, within the next 6 months, of any medications that may be associated with impaired immune responsiveness or with immunosuppression*.

    * Including, but not limited to, systemic corticosteroids exceeding 10 mg/day of prednisone equivalent, allergy injections, immunoglobulin, interferon, immunomodulators, cytotoxic drugs, or other similar or toxic drugs during the preceding 6-month period prior to vaccine administration (Day 1). The use of low dose topical, ophthalmic, inhaled and intranasal steroid preparations will be permitted.

11. History of hepatitis B or hepatitis C infection.
12. Receipt of immunoglobulins or blood products within 3 months of first vaccination.
13. Requirement for antipyretic or analgesic medication on a daily or every other day basis from enrolment through 72 hours after vaccination.
14. Current use of any prescription or over-the-counter medications within 7 days prior to vaccination, unless approved by the PI.
15. History of alcohol or drug abuse or psychiatric disorder that in the opinion of the PI could affect the participant's safety or compliance with study.
16. Participant unwilling to abstain from blood donation during the course of the study, and/or participation in any research study involving blood sampling (more than 450 mL /unit of blood), or blood donation to any blood bank during the 2 months prior to the screening visit.
17. Participant unwilling to abstain from donating plasma, ovules, sperm, or organs during the course of the study.
18. Close contact with anyone known to have SARS-CoV-2 infection within 30 days prior to vaccine administration.
19. History of COVID-19 diagnosis.
20. On current treatment with investigational agents for prophylaxis of COVID-19.
21. Planning to travel outside Thailand from enrolment through 28 days after the second vaccination.
22. Residing in a nursing home or other skilled nursing facility or having a requirement for skilled nursing care.

23. Is a participant at high risk of SARS-CoV2 exposure in the opinion of the PI (e.g., healthcare workers, active health care workers with direct patient contact, emergency response personnel).

    Elderly Participants (Group 2 of Phase 1 and Phase 2) only

24. Chronically smoking (defined as ≥10 Pack years [packs/day × years smoked]) within the 12 months prior to enrolment.
25. Presence of comorbidities that can be associated with an increased risk of severe COVID-19.