Inclusion Criteria:  Individuals who:  are able to provide consent agree to comply with all study visits and procedures are sexually active and willing to adhere to contraceptive requirements are male or female ≥18 years of age (or, for Phase 1, 18 to < 60 years of age) are at higher risk of developing COVID-19 based on where they work or live  Exclusion Criteria:  Individuals who:  Significant infection or other acute illness, including body temperature >100.4°F (>38.0°C) on the day prior to or Day 1. Participants meeting this criterion may be rescheduled within the relevant window periods. Afebrile participants with minor illnesses can be enrolled at the discretion of the investigator. Pregnant or breastfeeding. Known history of COVID-19 (asymptomatic SARS-CoV-2 infection and/or nucleocapsid positive test is not exclusionary). Close contact with a person known to be SARS-CoV-2 positive or with a clinical diagnosis of COVID-19 within 7 days prior to enrollment. Participants meeting this criterion who remain asymptomatic for 7 days may be rescheduled for enrollment within the relevant windows. Known history of anaphylaxis, urticaria, or other significant adverse reaction to the vaccine or its excipients. Known history of anaphylaxis to other vaccines. Bleeding disorder considered a contraindication to intramuscular (IM) injection or phlebotomy. Immunosuppressive or immunodeficient state, asplenia, recurrent severe infections, or known to be HIV positive.  An underlying clinically significant acute or chronic medical condition or physical examination findings for which, in the opinion of the investigator, participation would not be in the best interest of the participant (eg, compromise the well-being) or that could prevent, limit, or confound the protocol-specified assessments.  Prior/Concomitant Therapy  Has previously received investigational or approved MERS-CoV, SARS-CoV, SARS-CoV-2 vaccines or who have plans to receive off-study COVID-19 vaccines. Has received a live replicating vaccine within 28 days prior to each study vaccination or a licensed inactivated or non-replicating vaccine within 14 days prior to first study vaccination. Has received treatment with immunosuppressive therapy, including cytotoxic agents or systemic corticosteroids, eg, for cancer or an autoimmune disease, within 6 months prior to Screening, or planned receipt throughout the study. If systemic corticosteroids have been administered short term (<14 days) for treatment of an acute illness, participants should not be enrolled into the study until corticosteroid therapy has been discontinued for at least 28 days prior to first study vaccine administration. Inhaled/nebulized, intra-articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted.  Has received systemic immunoglobulins or blood products within 3 months prior to first study vaccine administration or plans to receive such products during the study.  Other Exclusions  Demonstrated inability to comply with the study procedures. Investigator site staff members, employees of the Sponsor or the 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. Other restrictions apply to Phase 1 participants to ensure they are healthy.  Additional Exclusion Criteria for Phase 3c Participants Only:  No contraindications (as specified in the prescribing information) to receiving the ChAdOx1 vaccine.