A Study to Evaluate the Safety, Tolerability, and Immunogenicity of COVID-19 Vaccine, CT-COV-21 Extension Study

Inclusion Criteria 1. Participants who are in the placebo arm of the main study (CT-COV-21), and are unblinded due to urgent condition other than safety events (i.e. on request from participants with high risk of acquiring and transmitting infection) after Day 119, are eligible. 2. Female participant must: 1. 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; 2. 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 3. Participant is willing and able to comply with all required study visits and follow-up required by this protocol. 4. Participant or the participant's legal representative must understand the procedures of the study and provide written informed consent Exclusion Criteria 1. Pregnant or breast feeding or have plan to become pregnant in 30 days after last administration of study intervention. 2. Employees at the investigator's site, of the Sponsor or the contract research organization (CRO) directly involved in the conduct of the study. Prior/Concomitant Therapy 3. Currently receiving or received any investigational intervention within 30 days prior to the first dose of 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 study intervention. 5. Administered any blood product or intravenous immunoglobulin administration within 12 weeks prior to the first dose of study intervention. 6. 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. 7. 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. 8. Major surgery or any radiation therapy within 12 weeks prior to the first dose of study intervention 9. Participant with previous known or potential exposure to SARS-CoV-1 or 2 viruses or received any other COVID-19 vaccine. 10. 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. 11. 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.