Heterologous Boost Immunization with Ad5-nCoV After Three-dose Priming with an Inactivated SARS-CoV-2 Vaccine

Inclusion Criteria: * Health subjects aged ≥18 years * The subject can provide with informed consent and sign informed consent form (ICF). * The subjects are able to and willing to comply with the requirements of the clinical trial program and could complete the 6-month follow-up of the study. * Participants who have received three-dose of inactivated SARS-CoV-2 vaccine more than 6 months ago. Exclusion Criteria: * Have the medical history or family history of convulsion, epilepsy, encephalopathy and psychosis. * Be allergic to any component of the research vaccines, or used to have a history of hypersensitivity or serious reactions to vaccination. * Vaccine-related SAE occurred after previous vaccination with COVID-19 vaccine. * Women with positive urine pregnancy test or in lactation. * Have acute febrile diseases or infectious diseases or have a history of SARS. * Axillary temperature\>37.0℃ * Have serious cardiovascular diseases, such as arrhythmia, conduction block, myocardial infarction, severe hypertension that cannot be controlled by medication (systolic blood pressure ≥180mmHg and/or diastolic blood pressure ≥110mmHg when measured in the field) * Have severe chronic diseases or condition is not stable, such as asthma, diabetes, thyroid disease * Congenital or acquired angioedema / neuroedema. * Have the history of urticaria 1 year before. * Have asplenia or functional asplenia. * Patients with chronic obstructive pulmonary disease, pulmonary fibrosis and other pulmonary abnormalities. * Have history of SARS-CoV-2 infection or COVID-19. * Have symptoms of upper respiratory tract infection. * Have traveled to medium or high risk areas or traveled abroad in the past 21 days, and epidemiologically contacted with SARS-CoV-2. * Any medical, psychological, social, or other conditions that, in the investigator's judgment, are inconsistent with the protocol or affect the subject's informed consent.