This is a randomized, observer-blind, placebo-controlled study, for evaluation of safety and immunogenicity of heterologous prime-boost immunization of recombinant COVID-19 vaccine (adenovirus type-5 vector) and RBD-based protein subunit vaccine (ZF2001) against COVID-19 in Chinese healthy population. 120 healthy subjects aged over 18 years of age who have been vaccinated with recombinant adenovirus type-5 vectored vaccine will be recruited in this study. Of them, 60 subjects will be enrolled in the "0-28 days" regimen and other 60 will be enrolled in "0-56 days" regimen. Subjects, 30 of them are 18-59 years old and 30 are 60 years old and above in each regimen will be randomly vaccinated with the second dose of subunit vaccine(ZF2001) against COVID-19 or a commercial influenza vaccine in a ratio of 2:1. They will then be vaccinated with the third dose of ZF2001 on month 4 after the second dose. The occurrence of adverse events within 28 days and serious adverse events within 6 months after the last vaccination will be observed. In addition, blood samples will be collected on day 0 before the second vaccination, day 14, 28 after the second vaccination and day 14, month 6 after third vaccination to test serum antibody levels and to profile the immune cells' subgroups and germlines. Each subject will remain in this study for approximately 12 months.

Inclusion Criteria:

• The subjects ≥ 18 years old who has completed one dose of recombinant Ad5 vectored COVID-19 vaccine.
• The subjects 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 can complete the 6-month follow-up of the study.
• Axillary temperature ≤ 37.0 ℃
• Individuals who are in good health condition at the time of entry into the trial as determined by medical history, physical examination and clinical judgment of the investigator and meet the requirements of these products immunization.

Exclusion Criteria:

• have a 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, pregnant or breast-feeding, or have a pregnancy plan within six months.
• have acute febrile diseases and infectious diseases.
• 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 trial vaccine.
• have asplenia or functional asplenia.
• have thrombocytopenia or other coagulation disorders (which may cause contraindications for intramuscular injection).
• have the history of immunosuppressive therapy, anti allergy therapy, cytotoxic therapy or inhaled corticosteroids (excluding corticosteroid spray therapy for allergic rhinitis, and acute corticosteroid therapy without dermatitis) over the past 6 months.
• have received blood products within 4 months before injection of trial vaccines.
• have received another investigational product within one month before injection of trial vaccine.
• have received attenuated vaccine within 1 month before injection of trial vaccine except the recombinant Ad5 vectored COVID-19 vaccine.
• have received subunit or inactivated vaccine within 14 days before the vaccination with trial vaccine.
• under anti tuberculosis treatment.
• not be able to follow the protocol, or not be able to understand the informed consent according to the researcher's judgment, due to various medical, psychological, social or other conditions.

Deidentified individual participant data will be available for request 1 month after the completion of the study

1 month to 1 year after the completion of the study

Researchers who provide a scientifically sound proposal will be allowed access to the individual participant data.These proposals will be reviewed and approved by the sponsor, investigator, and collaborators on the basis of scientific merit. To gain access, data requesters will need to sign a data access agreement.