Inclusion Criteria:  Healthy adults, age ≥ 18 years, inclusive, at time of enrollment, that have previously received an FDA-authorized COVID-19 vaccine (both prime and boost) ≥14 days and  ≤ 6 months before enrollment.  Able to understand and provide a signed informed consent that fulfills the relevant Institutional Review Board (IRB) or Independent Ethics Committee (IEC) guidelines. Agrees to the collection of biospecimens (eg, NP swabs and/or saliva sample) and venous blood per protocol. Ability to attend required study visits and return for adequate follow-up, as required by this protocol. Ability to swallow a capsule. Temperature < 38°C. Agreement to practice effective contraception for female subjects of childbearing potential and non-sterile males. Female subjects of childbearing potential must agree to use effective contraception while on study until at least 1 month after the last dose of vaccine. Non-sterile male subjects must agree to use a condom while on study until at least 1 month after the last dose of vaccine. Effective contraception includes surgical sterilization (eg, vasectomy, tubal ligation), two forms of barrier methods (eg, condom, diaphragm) used with spermicide, IUDs, oral contraceptives, and abstinence.  Exclusion Criteria:  Persistent grade ≥ 2 AEs related to previous COVID-19 vaccination at the time of enrollment. Allergy to any component of the investigational vaccine, or a more severe allergic reaction and history of allergies in the past. Pregnant and nursing women. A negative serum or urine pregnancy test during screening and on the day of and prior to each dose must be documented before the vaccine is administered to a female subject of childbearing potential. Chronic lung disease including chronic obstructive pulmonary disease (COPD) or moderate to severe asthma. Pulmonary fibrosis. Bone marrow or organ transplantation. Extreme obesity (defined as BMI of 35 kg/m2 or higher). Diabetes. Chronic kidney disease. Liver disease. Sickle cell disease. Thalassemia. Any disease associated with acute fever, or any infection. Self-reported history of SARS. History of hepatitis B or hepatitis C. HIV or other acquired or hereditary immunodeficiency. Serious cardiovascular diseases, such as heart failure, coronary artery disease, cardiomyopathies, arrhythmia, conduction block, myocardial infarction, pulmonary hypertension, severe hypertension without controllable drugs, etc. Cerebrovascular disease. Cystic fibrosis. Neurologic conditions, such as dementia. Hereditary or acquired angioneurotic edema. No spleen or functional asplenia. Platelet disorder or other bleeding disorder that may cause injection contraindication. Chronic use (more than 14 continuous days) of any medications that may be associated with impaired immune responsiveness within 3 months before administration of study vaccine. (Including, but not limited to, systemic corticosteroids exceeding 10 mg/day of prednisone equivalent, allergy injections, immunoglobulin, interferon, immunomodulators. The use of low dose topical, ophthalmic, inhaled and intranasal steroid preparations will be permitted.) Prior administration of blood products in last 4 months. Currently receiving treatment for cancer or history of cancer in the last five years (except basal cell carcinoma of the skin and cervical carcinoma in situ). According to the judgement of investigator, various medical, psychological, social or other conditions that could affect the subjects ability to sign informed consent. Assessed by the Investigator to be unable or unwilling to comply with the requirements of the protocol.