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 anaphylaxi…