1. Participation in any other COVID-19 prophylactic drug trials for the duration of the study. Note: Participation in COVID-19 treatment trials is allowed in the event of hospitalization due to COVID-19. The study team should be informed as soon as possible, 2. Participation in SARS-CoV-2 serological surveys where participants are informed of their serostatus for the duration of the study. Note: Disclosure of serostatus post enrolment may accidentally unblind participants to group allocation. 3. Participation in this trial can only be allowed if volunteers are kept blinded to their serology results from local/national serological surveys, 4. Planned receipt of any vaccine (licensed or investigational), other than the study intervention, within 14 days before and after study vaccination, 5. Prior receipt of an investigational or licensed vaccine likely to impact on the interpretation of the trial data (e.g. Adenovirus vectored vaccines, any coronavirus or SARS vaccines), 6. Administration of immunoglobulins and/or any blood products within the three months prior to the planned administration of the vaccine candidate, 7. Any confirmed or suspected immunosuppressive or immunodeficient state, positive HIV status, asplenia, recurrent severe infections and chronic use (more than 14 days) of immunosuppressant medication within the past 6 months, 8. Topical steroids or short-term (course lasting <= 14 days) oral steroids are not an exclusion, 9. History of allergic disease or reactions likely to be exacerbated by any component of Ad5-nCoV, 10. Any history of angioedema, 11. Any history of anaphylaxis to any vaccine component, 12. Pregnancy, lactation or willingness/intention to become pregnant within 90 days after receiving study vaccine, 13. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the skin and cervical carcinoma in situ), 14. History of serious psychiatric condition likely to affect participation in the study, 15. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture, 16. Suspected or known current alcohol or drug dependency, 17. 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), 18. History of laboratory-confirmed COVID-19, 19. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban), 20. Any other significant disease, disorder or finding which may significantly increase the risk to the volunteer because of participation in the study, affect the ability of the volunteer to participate in the study or impair interpretation of the study data.

1. Participation in any other COVID-19 prophylactic drug trials for the duration of the study. Note: Participation in COVID-19 treatment trials is allowed in the event of hospitalization due to COVID-19. The study team should be informed as soon as possible, 2. Participation in SARS-CoV-2 serological surveys where participants are informed of their serostatus for the duration of the study. Note: Disclosure of serostatus post enrolment may accidentally unblind participants to group allocation. 3. Participation in this trial can only be allowed if volunteers are kept blinded to their serology results from local/national serological surveys, 4. Planned receipt of any vaccine (licensed or investigational), other than the study intervention, within 14 days before and after study vaccination, 5. Prior receipt of an investigational or licensed vaccine likely to impact on the interpretation of the trial data (e.g. Adenovirus vectored vaccines, any coronavirus or SARS vaccines), 6. Administration of immunoglobulins and/or any blood products within the three months prior to the planned administration of the vaccine candidate, 7. Any confirmed or suspected immunosuppressive or immunodeficient state, positive HIV status, asplenia, recurrent severe infections and chronic use (more than 14 days) of immunosuppressant medication within the past 6 months, 8. Topical steroids or short-term (course lasting <= 14 days) oral steroids are not an exclusion, 9. History of allergic disease or reactions likely to be exacerbated by any component of Ad5-nCoV, 10. Any history of angioedema, 11. Any history of anaphylaxis to any vaccine component, 12. Pregnancy, lactation or willingness/intention to become pregnant within 90 days after receiving study vaccine, 13. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the skin and cervical carcinoma in situ), 14. History of serious psychiatric condition likely to affect participation in the study, 15. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture, 16. Suspected or known current alcohol or drug dependency, 17. 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), 18. History of laboratory-confirmed COVID-19, 19. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban), 20. Any other significant disease, disorder or finding which may significantly increase the risk to the volunteer because of participation in the study, affect the ability of the volunteer to participate in the study or impair interpretation of the study data.

1. Participation in any other COVID-19 prophylactic drug trials for the duration of the study. Note: Participation in COVID-19 treatment trials is allowed in the event of hospitalization due to COVID-19. The study team should be informed as soon as possible, 2. Participation in SARS-CoV-2 serological surveys where participants are informed of their serostatus for the duration of the study. Note: Disclosure of serostatus post enrolment may accidentally unblind participants to group allocation. 3. Participation in this trial can only be allowed if volunteers are kept blinded to their serology results from local/national serological surveys, 4. Planned receipt of any vaccine (licensed or investigational), other than the study intervention, within 14 days before and after study vaccination, 5. Prior receipt of an investigational or licensed vaccine likely to impact on the interpretation of the trial data (e.g. Adenovirus vectored vaccines, any coronavirus or SARS vaccines), 6. Administration of immunoglobulins and/or any blood products within the three months prior to the planned administration of the vaccine candidate, 7. Any confirmed or suspected immunosuppressive or immunodeficient state, positive HIV status, asplenia, recurrent severe infections and chronic use (more than 14 days) of immunosuppressant medication within the past 6 months, 8. Topical steroids or short-term (course lasting =14 days) oral steroids are not an exclusion, 9. History of allergic disease or reactions likely to be exacerbated by any component of Ad5-nCoV, 10. Any history of angioedema, 11. Any history of anaphylaxis to any vaccine component, 12. Pregnancy, lactation or willingness/intention to become pregnant within 90 days after receiving study vaccine, 13. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the skin and cervical carcinoma in situ), 14. History of serious psychiatric condition likely to affect participation in the study, 15. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture, 16. Suspected or known current alcohol or drug dependency, 17. 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), 18. History of laboratory-confirmed COVID-19, 19. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban), 20. Any other significant disease, disorder or finding which may significantly increase the risk to the volunteer because of participation in the study, affect the ability of the volunteer to participate in the study or impair interpretation of the study data.

1. Participation in any other COVID-19 prophylactic drug trials for the duration of the study. Note: Participation in COVID-19 treatment trials is allowed in the event of hospitalization due to COVID-19. The study team should be informed as soon as possible, 2. Participation in SARS-CoV-2 serological surveys where participants are informed of their serostatus for the duration of the study. Note: Disclosure of serostatus post enrolment may accidentally unblind participants to group allocation. 3. Participation in this trial can only be allowed if volunteers are kept blinded to their serology results from local/national serological surveys, 4. Planned receipt of any vaccine (licensed or investigational), other than the study intervention, within 14 days before and after study vaccination, 5. Prior receipt of an investigational or licensed vaccine likely to impact on the interpretation of the trial data (e.g. Adenovirus vectored vaccines, any coronavirus or SARS vaccines), 6. Administration of immunoglobulins and/or any blood products within the three months prior to the planned administration of the vaccine candidate, 7. Any confirmed or suspected immunosuppressive or immunodeficient state, positive HIV status, asplenia, recurrent severe infections and chronic use (more than 14 days) of immunosuppressant medication within the past 6 months, 8. Topical steroids or short-term (course lasting =14 days) oral steroids are not an exclusion, 9. History of allergic disease or reactions likely to be exacerbated by any component of Ad5-nCoV, 10. Any history of angioedema, 11. Any history of anaphylaxis to any vaccine component, 12. Pregnancy, lactation or willingness/intention to become pregnant within 90 days after receiving study vaccine, 13. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the skin and cervical carcinoma in situ), 14. History of serious psychiatric condition likely to affect participation in the study, 15. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture, 16. Suspected or known current alcohol or drug dependency, 17. 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), 18. History of laboratory-confirmed COVID-19, 19. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban), 20. Any other significant disease, disorder or finding which may significantly increase the risk to the volunteer because of participation in the study, affect the ability of the volunteer to participate in the study or impair interpretation of the study data.