Randomization and blinding
-11, BLIND
Eligible participants were enrolled into three different age cohorts (3-5, 6-11 and 12-17 years old). Within each cohort, participants were randomly assigned, in a ratio of 1:1:1:1:1:1:1, to receive two doses of CoronaVac from one of the seven lots from three workshops on a commercial scale, with four weeks apart. Randomization codes were generated by the statistician using the method of age-stratified block randomization, and were then allocated to each participant in the sequence of enrolment order. Participants received the vaccines labelled with the same code number. Investigators, participants, and laboratory staff were all blinded to the vaccine lot allocation.The randomized statistician and other blind coding personnel, who were not allowed to participate in other works of this trial, were responsible for blinding the vaccines. The blind code file was then sealed and maintained by the randomized statistician and was not to be opened until the unblinding procedure of the study.