The primary objective of the study is to measure the anti-SARS-CoV-2 neutralizing antibody titres in adult participants, in particular elderly, so as to demonstrate immunogenic superiority of MVC-COV1901 to the active control, AZD1222 vaccine, in terms of the GMT of neutralizing antibodies at 14 days after the second dose of the study intervention. This study also assesses the safety and tolerability of the study intervention and explores the immunogenicity in terms of antigen-specific immunoglobulin as well as the potential efficacy of MVC-COV1901 in preventing COVID-19.

This is a Phase III, parallel-group, prospective, randomized, double-blind, active-controlled, two- arm study to be conducted in approximately 250 participants aged 18 years and above who are generally healthy or with stable pre-existing medical conditions. The participants, investigators, the site personnel and the Sponsor staff who are involved in the blinded conduct of the study will be blinded to the study intervention assignment. Preparation and administration of the study intervention will be performed by authorized unblinded site personnel who do not participate in the evaluation of the participants.

Eligible participants will be randomized to receive either MVC-COV1901 or AZD1222 vaccine in a 1:1 ratio. Randomization of participants will be stratified by study site and age (≥ 18 to < 65 years and ≥ 65 years), at least 40% of the participants shall be ≥ 65 years.

Approximately 30 participants in the age 18 to <65 group and 10 participants in the age ≥ 65 years will be included in the CMI Subset for CMI assessments.

Inclusion Criteria:

1. Male or female participant aged 18 years and above at randomization.

2. Healthy adult or adult with pre-existing medical conditions who is in a stable condition. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease 3 months before enrollment and expected to remain stable for the duration of the study.

3. Female participant:

   1. A female participant is eligible if the participant is a woman of non-childbearing potential, i.e., surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal.
   2. If the participant is a woman of childbearing potential, she must agree to practice sexual abstinence or agree to use medically effective contraception from 14 days before screening to 30 days following the last administration of study intervention. Highly effective methods of contraception include:

   i. Implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine hormonal-releasing system ii. Established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository iii. Azoospermic partner (vasectomized or due to medical cause), provided the partner is the sole sexual partner of the female participant and the absence of sperm has been confirmed (from medical records/examination/history).

   c. Have a negative pregnancy test

4. Participant is willing and able to comply with all required study visits and follow-up required by this protocol.

5. Participant or the participant's legal representative must understand the procedures of the study and provide written informed consent.

Exclusion Criteria:

1. Pregnant or breastfeeding or have plan to become pregnant within 30 days after the last administration of the study intervention.
2. Employees at the investigator's site, of the Sponsor or delegate (e.g., contract research organization) who are directly involved in the conduct of the study
3. Currently receiving or received any investigational intervention within 30 days prior to the first dose of the study intervention.
4. Administered any licensed live-attenuated vaccines within 28 days or other licensed non- live-attenuated vaccines within 7 days prior to the first dose of the study intervention.
5. Administered any blood product or intravenous immunoglobulin administration within 12 weeks prior to the first dose of the study intervention.
6. Currently receiving or anticipate receiving concomitant immunosuppressive or immune- modifying therapy (excluding inhaled, topical skin and/or eye drop-containing corticosteroids, low-dose methotrexate, or < 2 weeks of daily receipt of prednisone less than 20 mg or equivalent) within 12 weeks prior to the first dose of the study intervention.
7. Currently receiving or anticipate receiving treatment with tumor necrosis factor (TNF)-α inhibitors, e.g., infliximab, adalimumab, etanercept within 12 weeks prior to the first dose of the study intervention.
8. Major surgery or any radiation therapy within 12 weeks prior to the first dose of the study intervention.
9. Has received any other investigational or approved COVID-19 vaccine.
10. Immunosuppressive illness or immunodeficient state, including hematologic malignancy, history of solid organ, bone marrow transplantation, or asplenia.
11. A history of malignancy with potential risk for recurrence after curative treatment, or current diagnosis of or treatment for cancer (exceptions are squamous and basal cell carcinomas of the skin and treated uterine cervical carcinoma in situ, at the discretion of the investigator).
12. Bleeding disorder considered a contraindication to intramuscular (IM) injection or phlebotomy.
13. Documented SARS-CoV-2 infection prior to the first dose of study intervention, or an individual with positive anti-SARS-CoV-2 antibody test at screening.
14. A history of cerebral venous sinus thrombosis, heparin-induced thrombocytopenia, thrombosis with thrombocytopenia syndrome (TTS), antiphospholipid syndrome, or capillary leak syndrome.
15. Participant who, in the investigator's judgement, is not in a stable condition and by participating in the study could adversely affect the safety of the participant, interfere with adherence to study requirements or evaluation of any study endpoint. This may include a participant with ongoing acute diseases, severe infections, autoimmune disease, laboratory abnormality or serious medical conditions in the following systems: cardiovascular, pulmonary, hepatic, neurologic, metabolic, renal, or psychiatric.
16. A history of hypersensitivity to any vaccine or a history of allergic disease or reactions likely to be exacerbated by any component of the MVC-COV1901 or AZD1222.
17. Body (oral, rectal, or ear) temperature ≥ 38.0°C or acute illness (not including minor illnesses such as diarrhea or mild upper respiratory tract infection at the discretion of the investigator) within 2 days before the first dose of study intervention.