Immunogenicity assessment
For all participants, each time 3 ml of blood samples were collected before the first dose vaccination (Day 0) and at 28 days after the second dose vaccination (Day 56). Neutralizing antibody titers against the ancestral strain were then tested using a micro-cytopathogenic effect assay, which was done by Sinovac (The immunogenicity objectives were to evaluate the consistency of neutralizing antibody response among different lots within workshop 2 or 3, as well as the consistency between different workshops. The primary endpoint was geometric mean titer (GMT) of neutralizing antibody at 28 days after full course of vaccination. The second endpoints included seropositive rate, seroconversion rate, and geometric mean increase (GMI) of neutralizing antibody. The immunogenicity assessment was conducted in the per-protocol set (PPS), which included all participants who completed two doses of vaccination, had available neutralizing antibody results above and complied with the protocol. The time windows for Day 28 and Day 56 were +10 days.