This is an observer-blind, randomized study which aims to assess the immune response and the safety of two different approved vaccines for first and second dose in healthy adults.

This is a phase 2, observer-blind, randomized study to assess the safety and the immunogenicity of heterologous prime-boost COVID-19 vaccines regimens in healthy adults aged 18 to 65 years using two approved vaccines (Sinopharm / CNBG Vaccine (BBIBP-CorV) and Johnson & Johnson Vaccine (Ad26.COV2.S)).

The study will consist of 2 cohorts, one for main immunology endpoints (N=260, 65 per study arm) and one for more detailed immunological assessment (N=100, 25 per study arm). Two doses of vaccine will be administered intramuscularly 4 week apart. All the study participants will be follow-up for 12 months from the administration of first vaccine dose.

Inclusion Criteria:

• Individuals aged 18 to 65 years old at the time of consent.
• Residing within the area of the study and planning to stay for the study duration.
• HIV negative test result on the day of screening (for those who do not have a documented HIV test results in the last three months of screening).
• Female volunteers of childbearing potential with a negative pregnancy test on the day(s) of screening and vaccination, practicing/willing to practice continuous effective contraception* recommended by the National Health System up to 12 weeks after the booster vaccination..
• Agreement to refrain from blood donation during the course of the study.
• Able and willing to comply with all study requirements, based on the assessment of the investigator.
• Willingness to provide written informed consent before any trial procedure * Effective contraception is defined as follows: contraceptive medications delivered orally, intramuscularly, vaginally, or implanted underneath the skin, surgical methods (hysterectomy or bilateral tubal ligation), condoms, diaphragms, intrauterine device (IUD), and abstinence.

Exclusion Criteria:

• Pregnancy, lactation, or intention to become pregnant during the vaccination phase through three months after the booster dose.
• Prior receipt/ planned receipt of any vaccine other than the study intervention within 28 days before and after each study vaccination.
• Previous participation in any COVID-19 vaccination trial or vaccination campaign.
• Administration of immunoglobulins and/ or any blood products within the three months preceding the administration of the study vaccine.
• Known infection with hepatitis B, C virus.
• Known history of allergy or anaphylaxis to study vaccine components and/or excipients or other medications, or any other allergies deemed by the investigator to increase the risk of an adverse reaction.
• History of bleeding disorder (e.g., factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venipuncture.
• Continuous use of the anticoagulants, such as coumarins and related anticoagulants.
• Severe and/or uncontrolled cardiovascular disease, respiratory disease, gastrointestinal disease, renal disease, liver disease, endocrine disorders, and neurological illness (mild/moderate well controlled comorbidities are allowed).
• Any clinically significant abnormal finding on screening as judged by the investigator.
• Confirmed SARS-CoV-2 infection at enrollment.
• Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and chronic use (more than 14 days) immunosuppressant medication within 3 months prior to recruitment (topical steroids are allowed).
• Any other finding which in the opinion of the investigators would increase the risk of an adverse outcome from participation in the trial or result in incomplete or poor-quality data.

This is an observer-blind study. So in this study only the outcome assessors should be blinded namely the clinical staff in charge of the clinical outcomes assessment and the laboratory analysts.