A Study to Evaluate the Ability of UB-612 COVID-19 Vaccine to Boost Immunity of Heterologous COVID-19 Vaccines.

Inclusion Criteria: * Healthy male or non-pregnant female between the age of 20 and 55 years at time of enrolment. * Fully vaccinated with two injections of AstraZeneca COVID-19 vaccine (ChAdOx1-S) with an 8-16 week interval between first and second doses. The second dose of AstraZeneca COVID-19 vaccine (ChAdOx1-S) must have been administered more than six (6) months from study Day 1. The CDC COVID-19 Vaccination Record Card or appropriate documentation (e.g., medical records) will be required for documentation. * Women of childbearing potential and men must agree to practice medically effective contraception from vaccination until 30 days after the vaccination. * Participant or the participant's legal representative must understand the procedures of the study and is 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. * Must be able to read, understand, and complete the protocol-required questionnaires and/or diary. * Negative serological test for Hepatitis B surface antigen (HBsAg), HCV RNA and HIV antibody * Negative results of SARS-CoV-2 N protein IgG ELISA * Negative result of RT-PCR screening of nasopharyngeal or throat swabs for SARS-CoV-2. * Ear temperature ≤ 38.0°C. * Indexes of hematology, biochemistry and immunology laboratory tests are within the normal ranges, or not clinically significant as judged by investigators 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 vaccination administration or plans to become pregnant from the time of study vaccination through 30 days after the administration of the study vaccine. * Female who is breast-feeding or plans to breastfeed from the time of the study vaccination through 30 days after the administration of the study vaccine. * Investigational non-coronavirus vaccines: previous receipt of an investigational vaccine (non-coronavirus) within 1 year before the planned administration of study vaccine. * Prior administration of attenuated, nucleic acid (mRNA or DNA) or vectored vaccines in last 1 month before the study vaccine or expectation of such vaccines in the month after the study vaccine. * Prior administration of subunit vaccine or inactivated vaccine in last 14 days before the study vaccine or expectation of receipt of such vaccines in the 14 days after the study vaccine. * Judged by the investigator on the basis of evidence or medical history, immunosuppressive or immunodeficient state, autoimmune diseases, chronic kidney disease (with dialysis), asplenia, or recurrent severe infections. * Prior chronic administration of immunosuppressant or corticosteroids, cytotoxic treatment in last 6 months before the study vaccination. * Receipt of short-term 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. * Has received systemic immunoglobulins or blood products within 4 months prior to enrollment. * 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. * Participants who received specific anti-SARS-CoV-2 monoclonal antibody products at any time. * Subjects who take part in another clinical study and are currently receiving or received any investigational intervention within 12 weeks prior to the day of informed consent. * Platelet disorder or other bleeding disorder may cause injection contraindication. * Any acute illness, as determined by the study investigator 3 days before vaccination. * Judged by the investigator on the basis of evidence or medical history, participants with evidence or history of clinically significant hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurologic, or allergic disease (including drug allergies) will be excluded. * History of malignancy within 5 years prior to screening visit, except basal cell carcinoma of the skin and cervical carcinoma in situ. * Known history of SARS, MERS or SARS-CoV-2 infection. * Alcoholism or substance abuser. * Behavioral, cognitive, or psychiatric disease that, in the opinion of investigators, affects the participant's ability to understand and cooperate with all study protocol requirements. * Employees at the investigator's site, of the Sponsor or the contract research organization (CRO) directly involved in the conduct of the study.