This is a multicenter, open-label, partially radomized, parallel-controlled clinical trial to evaluate the safety and immunogenicity of heterologous prime-boost immunization with an aerosolised adenovirus type-5 vector-based COVID-19 vaccine (Ad5-nCoV-IH) after two-dose priming with an inactivated SARS-CoV-2 vaccine (ICV) in adults aged 18 years and above. 10420 healthy subjects aged over or equal to 18 years whom have received two doses of ICV before 6 months or more, will be recruited from six provinces in China in this study.Of them, 10000 eligible participants in an open cohort will receive a booster dose of Ad5-nCoV-IH to evaluate the safety profile. Another 420 participants were involved in immunogenicity cohort and randomized in a ratio of 1:1 to receive a boost of Ad5-nCoV-IH or ICV. The ICV homologous to the priming series will be supplied as the booster. The occurrence of adverse reactions within 28 days and serious adverse events within 6 months after vaccination will be observed in all participants. In addition, blood and saliva samples will be collected from all participants in immunogenicity cohort on the day 0 before and 14, 28 and month 3 and 6 after the booster vaccination. Each subject will remain in this study for approximately 6 months.

Inclusion Criteria:

• Health subjects aged ≥18 years.
• Have received two-dose inactivated SARS-CoV-2 vaccine before 6 months or more.
• 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.

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.
• Women with positive urine pregnancy test.
• Have acute febrile diseases and infectious diseases.
• Axillary temperature>37.0℃.
• Have serious cardiovascular disease, 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 in progress cannot be smoothly controlled, such as asthma, diabetes, thyroid disease.
• Congenital or acquired angioedema / neuroedema.
• Have the history of urticaria 1 year before receiving the investigational vaccine.
• 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.