In this trial, a single-arm, open-label study design will be used to evaluate the safety and tolerability after vaccination with escalating doses of the investigational vaccine (Recombinant COVID-19 Vaccine (chimpanzee adenovirus vector) for Inhalation, RCVi) at low, medium, and high doses in healthy adults (previously primed with authorized vaccines).

Inclusion Criteria:

• People aged 21 to 65 years old, with body weight ≥ 50 kg for males and ≥ 45 kg for females, body mass index (BMI) within the range of 19.0-30.0 (including the boundary value), who can provide legal identification;
• Subjects who agree to participate in this study voluntarily and sign an Informed Consent Form.
• Subject who has the ability to understand the study procedures and be able to attend all scheduled follow-up;
• Individuals who completed basic vaccination of licensed vaccine or further received the first booster dose vaccination 4~12 months prior to recruitment in this study（including but not limited to mRNA or non-mRNA vaccine）.
• Female subjects who are not pregnant or breast-feeding;
• Women of childbearing age who agree to use effective contraception during the study; or have been using effective contraception within 2 weeks prior to enrollment.

Exclusion Criteria:

• Those who had fever (body temperature≥ 38.0 °C/100.4 °F), dry cough, fatigue, nasal congestion, runny nose, sore throat, myalgia, diarrhea, shortness of breath, and dyspnea in the past 14 days before vaccination;
• Subject whose SARS-CoV-2 nucleic acid test result is positive;
• Subject with a body temperature of ≥ 38.0 °C/100.4 °F (axillary temperature) on the day of enrollment;
• Subject who has oral ulcers, throat swelling and other oral and nasopharyngeal diseases;
• Subject with abnormal vital signs, physical examination and laboratory test indicators at screening that are judged by clinicians to be clinically significant;
• Subject who has a previous history of severe allergy to any drug, food or vaccination, such as urticaria, anaphylactic shock, allergic laryngeal edema, allergic dyspnea, skin eczema, Henoch-Schonlein purpura, thrombocytopenic purpura, local allergic necrosis reaction (Arthus reaction), etc.;
• Subject who has suffered from acute disease or in the acute attack stage of chronic disease within 3 days before vaccination, or has used antipyretic, analgesic and anti-allergy drugs;
• Subject within 6 months prior to vaccination participation in a coronavirus (MERS-CoV, SARS-CoV-2) vaccine and/or drug (small molecule) and/or antibody study; Participation in any clinical study within 3 months prior to vaccination or planned participation in other (drug or vaccine) clinical studies during the study;
• Subject who has received other vaccines within 1 month before vaccination;
• Subject who has used immunoenhancers or immunosuppressants in the past 3 months;
• Subject who was diagnosed with congenital or acquired immunodeficiency, or suspected to have systemic diseases that may interfere with the conduct or completion of the study, such as tuberculosis, hepatitis B, hepatitis C, human immunodeficiency virus (HIV), syphilis infection, etc.;
• Subject who was diagnosed with serious diseases, congenital anomalies or chronic disease that may interfere with the conduct or completion of the study (including but not limited to: allergy to vaccines, asthma and other respiratory diseases or chronic bronchitis, hypertension, hypotension, heart disease, kidney disease, diabetes mellitus (type 1 or type 2), autoimmune diseases, thalassemia, malignant tumor, atopy, existing skin diseases, etc.);
• Subject who has received blood or blood-related products (such as blood transfusion, use of human albumin, human immunoglobulin, etc.) within the past 6 months;
• Subject with a history or family history of convulsions, epilepsy, encephalopathy and psychosis;
• Subject with functional asplenia or splenectomy caused by any situation;
• Subject who has any condition that, in the opinion of the investigator, may interfere with the evaluation of the objectives of the study.