Safety and Immunogenicity Study in Adults of AZD1222 and rAd26-S Administered as Heterologous Prime Boost Regimen for the Prevention of Coronavirus Disease 2019 (COVID-19)

Inclusion Criteria: Participants are eligible to be included in the study only if all of the following criteria apply: 1. Participants are: * Overtly healthy as determined by medical evaluation, or * Medically stable such that, according to the judgment of the investigator, hospitalisation within the study period is not anticipated and the participant appears likely to be able to remain in follow-up through the end of protocol-specified follow up. (A stable medical condition is defined as disease not requiring significant change in therapy or hospitalisation for worsening disease during the 3 months prior to enrolment.) 2. Able to understand and comply with study requirements/procedures based on the assessment of the investigator. 3. Female participants 1. Women of childbearing potential must: * Have a negative pregnancy test on the day of screening and on Day 1 * Use one highly effective form of birth control for at least 28 days prior to Day 1 and agree to continue using one highly effective form of birth control through 60 days following administration of the second dose of study vaccine. A highly effective method of contraception is defined as one that can achieve a failure rate of less than 1% per year when used consistently and correctly. Periodic abstinence, the rhythm method, and withdrawal are NOT acceptable methods of contraception. 2. Women are considered of childbearing potential unless they meet either of the following criteria: * Surgically sterilised (including bilateral tubal ligation, bilateral oophorectomy, or hysterectomy), or * Postmenopausal * For women aged \< 50 years, postmenopausal is defined as having both: * A history of ≥ 12 months amenorrhea prior to first dosing, without an alternative cause, following cessation of exogenous sex-hormonal treatment, and * A follicle-stimulating hormone level in the post-menopausal range (Until follicle-stimulating hormone is documented to be within menopausal range, the participant is to be considered of childbearing potential.) * For women aged ≥ 50 years, postmenopausal is defined as having a history of ≥ 12 months amenorrhea prior to first dosing, without an alternative cause, following cessation of exogenous sex-hormonal treatment. 4. Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in the protocol of the sudy. Exclusion Criteria: 1. Known past laboratory-confirmed SARS-CoV-2 infection less than 6 month prior to screening (the date of diagnosis must be confirmed by an official document). 2. Positive severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) reverse transcription-polymerase chain reaction (RT-PCR) test at screening. 3. Significant infection or other illness, including fever \> 37.8°C on the day prior to or day of randomisation. 4. Any confirmed or suspected immunosuppressive or immunodeficient state, including human immunodeficiency virus (HIV) infection; asplenia; recurrent severe infections and use of chronic immunosuppressant medication within the past 6 months (≥ 20 mg/day of prednisone or its equivalent, given daily or on alternate days for ≥ 15 days within 30 days prior to vaccination), except topical/inhaled steroids or short term oral steroids (course lasting ≤ 14 days). 1. Note: HIV-positive participants with CD4 counts \> 500 for ≥ 12 months and on a stable HIV antiretroviral regimen may be enrolled. 2. Note: Topical tacrolimus is allowed if not used within 14 days prior to the day of enrolment. 5. History of allergy to any component of the vaccine. 6. Any history of anaphylaxis or angioedema. 7. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the skin and uterine cervical carcinoma in situ). 8. History of serious psychiatric condition likely to affect participation in the study. 9. Bleeding disorder (eg, factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture. 10. Suspected or known current alcohol or drug dependency. 11. History of Guillain-Barré syndrome or any other demyelinating condition. 12. Any other significant disease, disorder or finding which may significantly increase the risk to the participant because of participation in the study, affect the ability of the participant to participate in the study or impair interpretation of the study data. (Note: The adverse events of special interest (AESIs) as outlined in Appendix F of the protocol should be considered when evaluating a participant for Exclusion Criterion 13, as the presence of these AESIs, especially if untreated or uncontrolled, may be a safety risk to the participant, affect the ability of the participant to participate in the study or impair interpretation of the study data. Investigators should review and consider the list of conditions in Appendix F. If any of these conditions is present in a participant, the Investigator is asked to utilise his/her clinical judgment in determining the participant's eligibility for the study. Should the participant have conditions as outlined in Appendix F and is enrolled, the Investigator is asked to document notes on site regarding the final rationale for enrolment.) 13. Severe and/or uncontrolled cardiovascular disease, respiratory disease, gastrointestinal disease, liver disease, renal disease, endocrine disorder and neurological illness (mild/moderate well controlled comorbidities are allowed). 14. Prior splenectomy. 15. History of cerebral venous sinus thrombosis or experienced major venous and/or arterial thrombosis. 16. Receipt of any vaccine (licensed or investigational) other than the study vaccine within 30 days before and after each study vaccination. 17. Prior (less than 12 months) or planned receipt of an investigational or licensed vaccine or product likely to impact on interpretation of the trial data (eg, adenovirus vectored vaccines, any coronavirus vaccines). (Note: Participant vaccinated against coronavirus 12 months or more prior to screening could be included.) 18. Administration of immunoglobulins and/or any blood products within the 3 months preceding the planned administration of the vaccine candidate. 19. Continuous use of anticoagulants, such as coumarins and related anticoagulants (ie, warfarin) or novel oral anticoagulants (ie, apixaban, rivaroxaban, dabigatran and edoxaban). 20. Participation in COVID-19 prophylactic drug trials for the duration of the study. 21. Involvement in the planning and/or conduct of the study (applies to both AstraZeneca/R-Pharm staff and/or staff at the study site). 22. Judgment by the investigator that the participant should not participate in the study if the participant is unlikely to comply with study procedures, restrictions, and requirements. 23. Previous randomisation in the present study. 24. For women only - currently pregnant (confirmed with positive pregnancy test) or breast feeding. 25. Unwilling to refrain from blood donation during the course of the study.