Individuals who: 1. Are able to provide consent 2. Agree to comply with all study visits and procedures 3. Are willing and able to adhere to study restrictions 4. Are sexually active and willing to adhere to contraceptive requirements 5. Are male, female, or transgender ≥21 to ≤80 years of age 6. For the previously vaccinated groups only, received 2 doses of SARS-CoV-2 vaccine 5 months or longer prior to study enrollment

Individuals who: 1. For the unvaccinated groups only, previously received any investigational or authorized MERS-CoV, SARS-CoV, and SARS-CoV-2 vaccines (including ARCT-021) 2. For the previously vaccinated groups only, previously received BNT162b2 but have not received 2 doses within at least 5 months prior to study enrollment 3. Are planning to receive other COVID-19 vaccines during the study period 4. Recently received other vaccines 5. Have a fever or are feeling sick close to the time of the first study vaccination 6. Have a known history of COVID-19 disease or asymptomatic SARS-CoV-2 infection 7. Are pregnant or breastfeeding 8. Have had a severe reaction to previous vaccines 9. Have a severe or uncontrolled disease(s) that may interfere with the interpretation of the study 10. Have some respiratory diseases 11. Have some significant heart diseases 12. Have some neurological conditions 13. Have sickle cell disease or some other blood disorders 14. Have had a major surgery within the past 6 months 15. Have a history of chronic liver disease 16. Have a history of autoimmune disease or immunodeficiency 17. Have received allergy injections, interferon, immunomodulators, cytotoxic drugs or other similar toxic drugs. 18. Have received blood products 19. Have a positive test for hepatitis B or C or human immunodeficiency virus 20. Have uncontrolled hypertension 21. Have had cancer except for cancers that were treated and that have low risk of returning 22. Are obese 23. Are Investigator site staff members, employees of Arcturus or the contract research organization (CRO) directly involved in the conduct of the study, or site staff members otherwise supervised by the Investigator or immediate family members of any of the previously mentioned individuals