A Study o Evaluate the Safety, Tolerability, and Immunogenicity of MVC-COV1901, CT-COV-21 Sub-study

In this sub-study, approximately 400 adults participants who are generally healthy or with stable pre-existing health conditions aged ≥ 20 to < 65 years will be enrolled. The first 250 participants enrolled in this sub-study will receive MVC-COV1901(Scale A) and the remains will receive MVC-COV1901(Scale B). Each participant will receive 2 doses of study intervention, administered 28 days apart via IM injection in the deltoid region, preferably of the nondominant arm, at Day 1 and Day 29. The study procedure and visit schedule refer to the main study protocol and will be the same as the study procedure for the Immunogenicity Subset.

Inclusion criteria

• Male or female participant ≥ 20 to < 65 years, or ≥ 65 years of age at randomization.
• Healthy adults or adults with pre-existing medical conditions who are in 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.
• Female participant must:

• Be either 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;
• Or, if of childbearing potential, be abstinent or agree to use medically effective contraception from 14 days before screening to 30 days following the last injection of study intervention. Acceptable forms include:

i. Implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine 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 c. Have a negative pregnancy test

• Participant is willing and able to comply with all required study visits and follow-up required by this protocol.
• Participant has not travelled overseas within 14 days of screening and will not have any oversea travelling throughout the study period.
• Participant or the participant's legal representative must understand the procedures of the study and provide written informed consent.

Exclusion criteria

• Pregnant or breast feeding or have plan to become pregnant in 30 days after last administration of study intervention.
• Employees at the investigator's site, of the Sponsor or the contract research organization (CRO) directly involved in the conduct of the study.
• Currently receiving or received any investigational intervention within 30 days prior to the first dose of study intervention.
• 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 study intervention.
• Administered any blood product or intravenous immunoglobulin administration within 12 weeks prior to the first dose of study intervention.
• Currently receiving or anticipate to receive 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 study intervention.
• Currently receiving or anticipate to receive treatment with tumor necrosis factor (TNF)-α inhibitors, e.g. infliximab, adalimumab, etanercept within 12 weeks prior to the first dose of study intervention.
• Major surgery or any radiation therapy within 12 weeks prior to the first dose of study intervention Medical Conditions
• Immunosuppressive illness or immunodeficient state, including hematologic malignancy, history of solid organ, bone marrow transplantation, or asplenia.
• A history of autoimmune disease (systemic lupus, rheumatoid arthritis, scleroderma, polyarthritis, thyroiditis, Guillain-Barré syndrome, etc.).
• 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).
• Bleeding disorder considered a contraindication to intramuscular injection or phlebotomy.
• Human immunodeficiency virus (HIV) antibody positive participants with CD4 count < 350 cells/mm3 or a detectable HIV viral load within the past year (low level variations from 50-500 viral copies/mL or equivalent which do not lead to changes in antiretroviral therapy [ART] are permitted).
• Hepatitis B surface antigen (HBsAg) positive participant with positive hepatitis B e antigen (HBeAg) or abnormal liver function.
• Hepatitis C virus (HCV) antibody positive participants with detectable HCV ribonucleic acid (RNA) viremia in recent 12 weeks.
• Participant with ongoing acute diseases or serious medical conditions which will interfere with adherence to study requirements, or the evaluation of any study endpoint. Acute diseases or serious medical conditions include cardiovascular (e.g. New York Heart Association Grade III or IV), pulmonary (e.g. chronic obstructive pulmonary disease stage III or IV), hepatic (e.g. Child-Pugh Class C), neurologic (e.g. dementia), metabolic (e.g. diabetes mellitus with hemoglobin A1c [HbA1c] > 8%), renal (Stage 3 or worse chronic kidney disease), psychiatric condition (e.g. alcoholism, drug abuse), current severe infections, medical history, physical findings, or laboratory abnormality that in the investigators' opinion are not in stable condition and participating in the study could adversely affect the safety of the participant.

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• Participant with previous known or potential exposure to SARS-CoV-1 or 2 viruses (EXCEPT for those who have been tested negative and completed the 14-day self-managements/ home quarantines/ home isolations) or received any other COVID-19 vaccine.
• Participant with 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.
• 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.