Randomization and masking
The study epidemiologist created the random allocation sequence to assign the participants to the BBIBP-CorV or Fakhravac vaccine groups (in a 1:1 ratio) in the randomized arms using a variable-sized (4 and 6) block randomization method. Stata software was used for this purpose. We concealed the random allocation sequence using nine-character unique codes and embedded them into the study software. The software allocated the codes to the participants once their eligibility was confirmed. The person responsible for blinding delivered the assigned vaccine for administration by the vaccinator, who was unaware of the type of vaccine used. Participants and the rest of the research team remained masked to the type of vaccine. All vaccine doses had the same volume and color and were prepared in identical syringes.