Inclusion Criteria:  Potential participants must meet all inclusion criteria to be enrolled and participate in the study, as follows:  Adult male or female aged 18 years or more on the day of signing the ICF, confirmed by identification cards. Verified, documentation of past COVID-19 vaccination i. Study Part A: having completed the 2-dose homologous primary regimen (21 to 28 days apart) of inactivated COVID-19 vaccine of either Sinovac-Sinovac OR Sinopharm-Sinopharm ii. Study Part B: having received one dose of inactivated COVID-19 vaccine of either Sinovac or Sinopharm with the appropriate interval period Subject has provided written informed consent prior to performance of any study-specific procedures and is willing and has means to be contacted and to contact the investigator during the study. In the investigator's clinical judgment, the participant is in good health, or has stable and well-controlled medical conditions. Participant agrees to not donate bone marrow, blood, and blood products from the study vaccine administration until 3 months after receiving the study vaccine.  Exclusion Criteria:  Potential participants who meet any of the following exclusion criteria will be excluded from enrolment and participation in the study:  The participant has a clinically significant acute illness (this does not include minor illnesses such as diarrhea or mild upper respiratory tract infection), or is a patient under investigation (PUI) or has a body temperature ≥38.0ºC (100.4°F) within 24 hours prior to the planned study vaccination. Assignment may be made at a later date is permitted at the discretion of the investigator. Please notify the Sponsor (or medical monitor) of this decision. Contraindication to Ad26.COV2.S according to labelling of the product. For example, if the participant has a known or suspected allergy or history of anaphylaxis or other serious adverse reactions to vaccines or their excipients (including specifically the excipients of the study vaccine).  Participant has a history or current condition as follows  Known documented history of COVID-19 infection prior to enrollment Any confirmed or suspected immunosuppressive or immunodeficient state. Heparin-induced thrombocytopenia or thrombosis in combination with thrombocytopenia. Acute polyneuropathy (e.g. Guillain-Barré syndrome). Capillary leak syndrome Contraindication to IM injections and blood draws e.g., bleeding disorders. An underlying clinically significant acute or chronic medical condition or physical examination findings for which, in the opinion of the investigator, participation would not be in the best interest of the participant (e.g., compromise the well being) or that could prevent, limit, or confound the protocol-specified assessments. Major psychiatric illness which in the investigator's opinion would compromise the participant's safety or compliance with the study procedures.  If the participant received or plans to receive:  Licensed live attenuated vaccines - within 28 days before or after planned administration of study vaccine. Other licensed (not live) vaccines - within 14 days before or after planned administration of study vaccine. Treatment with immunoglobulins (Ig) in the 3 months or exogenous blood products (autologous blood transfusions are not exclusionary) in the 4 months before the planned administration of the study vaccine or has any plans to receive such treatment during the study. If the participant cannot communicate reliably with the investigator, or, in the opinion of the investigator, is unlikely to adhere to the requirements of the study or is unlikely to complete the full course of vaccination and observation. Employee of the study center directly involved with the proposed study or with study investigators.