Randomization and blinding
The randomization scheme was generated using SAS version 9.4 (SAS Institute Inc, USA) for Interactive Response Technology (IRT) with 1:1 allocation to SII-NVX-CoV2373 or control vaccine separately for each prime cohort. The eligible study participants were enrolled and randomized in the study online through IRT. Each study participant was assigned a randomization number to allocate vaccine group after entry of eligibility data into the IRT system. The study participants, the study personnel responsible for the evaluation of any study endpoints and the laboratories involved in the immunogenicity testing were blinded to the treatment allocation. Personnel involved in getting randomization code by accessing IRT and vaccine administration were unblinded, and they did not conduct any study evaluations. The unblinded site personnel prepared the vaccine out of view of the study participant, and administered the vaccine with the final prepared syringe.