Under the limited resources, to re-plan the ongoing clinical trials of this product.

This is a phase II, observer-blind, multiple-centre, randomized, placebo-controlled study to evaluate the immunogenicity, safety, tolerability and lot consistency of 2 doses of UB-612 vaccine in adolescent, younger and elderly adults. Around 3850 adult subjects will be randomized to be composed of the core group, while around 385 adolescents will be randomized to be the supplementary group. Subjects will be unblinded at Visit 5, and subjects in the vaccine group will be encouraged to have 3rd dose of vaccination.

This clinical study will be consisted of 7 clinical visits and one long-term follow-up visit. Subjects will also be unblinded at Visit 5, subjects in placebo group will withdraw from the study and subjects in vaccine group will be encouraged to have 3rd dose of vaccination (Day 197~Day 242) at Visit 6. Those who received 3rd dose will have Visit 7 (14 days after Visit 6) to check the booster effect. After Day 197, subjects will enter the long-term follow-up with a safety call bi-monthly.

Inclusion Criteria:

• Healthy male or non-pregnant female between the age of 12 to 85 years at time of enrolment.
• Women of childbearing potential and men must agree to practice medically effective contraception from first vaccination until 3 months after the last vaccination.
• Able to understand the content and possible risks of informed consent and willing to sign the Informed Consent Form (ICF).
• Able to understand and agrees to comply with all study procedures and be available for all study visits.
• Ear temperature ≤ 38.0°C.
• Healthy participants who are determined by medical history, physical examination, and clinical judgment of the investigator to be eligible for inclusion in the study. In the investigator's clinical judgement, participant may have a stable and well-controlled comorbidity associated with an increased risk of progression to severe COVID-19.

Exclusion Criteria:

• History of anaphylaxis, urticarial, or other significant adverse reaction requiring medical intervention after receipt of a vaccine.
• Female who is pregnant or positive in pregnancy test at screening or just prior to each vaccination administration.
• Female who is breast-feeding or plans to breastfeed from the time of the first vaccination through 60 days after the last vaccination.
• Any acute illness, as determined by the study investigator 3 days before first vaccination (these subjects can be re-scheduled).
• Any major surgery one month before first vaccination (these subjects can be -rescheduled).
• Known HIV antibody positive.
• Known active hepatitis B and hepatitis C disease.
• Previous exposure to SARS-CoV-2 or receipt of an investigational or licensed product for the prevention of COVID-19, MERS or SARS.
• Have history of Guillain-Barre syndrome.
• Subjects who take part in another clinical study within 12 weeks prior to the day of informed consent.
• Immune deficiency/disorder, whether due to genetic defect, immunodeficiency disease or immunosuppressive therapy.
• Subjects who plan to or are undergoing anti-cancer therapy.
• Platelet disorder or other bleeding disorder may cause injection contraindication.
• Prior chronic administration of immunosuppressant or corticosteroids, cytotoxic treatment in last 6 months before first vaccination.
• Prior administration of immunoglobulins and/or any blood products in last 4 months before first vaccination.
• Receipt of any seasonal or pandemic influenza vaccine within 14 days, or any other non-study vaccine within 28 days, before study intervention administration.
• Anticipated receipt of any seasonal or pandemic influenza vaccine within 14 days, or any other nonstudy vaccine within 28 days, after study intervention administration.
• Receipt of short-term (<14 days) systemic corticosteroids. Study intervention administration should be delayed until systemic corticosteroid use has been discontinued for at least 28 days. Inhaled/nebulized, intra-articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.
• Loss or donation of blood over 500 mL within 3 months prior to Screening Visit or intention to donate blood or blood products for transfusion during the study.
• Any medical disease or condition that, in the opinion of the study investigator, may confound the results of the study or pose an additional risk to the subjects by their participation in the study.
• Employees at the investigator's site, of the Sponsor or the contract research organization (CRO) who directly involved in the conduct of the study.