Phase III Trial of A COVID-19 Vaccine of Adenovirus Vector in Adults 18 Years Old and Above

Inclusion Criteria: * Adults of 18 years old and above. * Participants who are at high risk of SARS-CoV-2 infection. * Able and willing (in the Investigator's opinion) to comply with all study requirements. * Willing to allow the investigators to discuss the volunteer's medical history with their General Practitioner/personal doctor and access all medical records when relevant to study procedures. * Agreement to refrain from blood donation during the study. * provide written informed consent. Exclusion Criteria: * Participation in any other COVID-19 prophylactic drug trials for the duration of the study. * Participation in SARS-CoV-2 serological surveys where participants are informed of their serostatus for the duration of the study. * Planned receipt of any vaccine (licensed or investigational), other than the study intervention, within 14 days before and after study vaccination. * Prior receipt of an investigational or licensed vaccine likely to impact on the interpretation of the trial data. * Administration of immunoglobulins and/or any blood products within the three months prior to the planned administration of the vaccine candidate. * Planned receipt of any vaccine (licensed or investigational), other than the study intervention, within 14 days before and after study vaccination * 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) * Administration of immunoglobulins and/or any blood products within the three months prior to the planned administration of the vaccine candidate * Any confirmed or suspected immunosuppressive or immunodeficient state; positive HIV status; asplenia; recurrent severe infections and chronic use. * History of allergic disease or reactions likely to be exacerbated by any component of Ad5-nCoV * Any history of angioedema * Any history of anaphylaxis to any vaccine component * Pregnancy, lactation or willingness/intention to become pregnant during the study * Current diagnosis of or treatment for cancer (except basal cell carcinoma of the skin and cervical carcinoma in situ) * History of serious psychiatric condition likely to affect participation in the study * Suspected or known current alcohol or drug dependency * Severe and/or uncontrolled cardiovascular disease, respiratory disease, gastrointestinal disease, liver disease, renal disease, endocrine disorder and neurological illness * History of laboratory-confirmed COVID-19 * 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.