Randomization and blinding
BLIND
In this study, the participants were randomly assigned in a ratio of 1:1 to receive either a heterologous booster dose of NVSI-06-09 or a homologous booster dose of BBIBP-CorV. For participant allocation, eligible persons were assigned to two booster groups using a stratified blocked randomization method by SAS 9.4 software, in which stratification was based on different doses (two or three doses) of BBIBP-CorV that they received prior to enrollment and the block size was set to 4. The vaccine randomization list was also generated by SAS 9.4 software using the block randomization method with a block size of 4. Both participant and vaccine randomization lists were generated by an unblinded statistician, and then imported into the Interactive Web Response System (IWRS). After enrollment, each participant was assigned a randomization number from IWRS. At the clinical site, a vaccine number was also obtained from IWRS for vaccination accordingly. If the vaccine vial is damaged, a new vaccine number will be assigned to the participant.Participants and clinical operation team involved in safety data collection and immunogenicity assessments were blind to treatment allocation during the trial. Vaccine preparation was done by independent personnel to ensure identical appearance between the studied and control vaccines. Individuals involved in randomization and blinding did not participate in other trial operations.