Randomization and masking
CRF
CRF
We used permuted block randomization to allocate eligible participants to the vaccine and placebo groups. Block sizes of four were used and each block contained an equal number of alternative interventions. Rand function in excel was used to generate the random sequence within each block. Randomly generated four-digit unique numbers called “randomization codes” were used to conceal the randomization sequence. An independent study epidemiologist created the random sequence and had access to it for emergency unblinding. The identical vaccine and placebo single-injection vials were labeled using the randomization codes. These codes were also embedded in the study software and successively assigned to the eligible participants, once they successfully pass all the stages before receiving the injection and their eligibility for enrollment was confirmed by the research physician. Therefore all participants and the research team were blinded to the study group allocation.We used a pair of unique codes to label every blood specimen collected for immunogenicity assessment. One of the pairs was put on the paper Case Report Form (CRF) in its relevant field, indicating the type and timing of the immunogenicity test, and the other was on the collection tube. The label used on the collection tube is concealed behind a scratch label and should be revealed by lab offices. The key to the pair was created and kept by the study epidemiologist.