Statistical analysis
The sample size of trial subjects was determined using the Power Analysis and Sample Size (PASS15.0) software. It was assumed that the anti-Omicron nAb GMT in the experimental group was superior to that in the control group on day 14 after vaccination, with a superiority margin after log10 transformation of 0.0212. The other assumption parameters include standard deviation of the antibody GMT after log10 transformation of 0.55, the type I error (one-sided) probability of 0.025, the expected power of 0.9, and a 20% dropout rate. As such, a total of 398 participants were required. It was further assumed that the nAb level in the experimental group was non-inferior to that in the control group on day 14 post-vaccination. When using the non-inferiority threshold after log10 transformation of −0.17609 and the other parameters remained the same as above, a total of 516 participants were required to achieve 0.9 power to detect non-inferiority. Taken together, the sample size was determined to be 516, with 258 in the experimental group and the other 258 in the control group.Baseline characteristics were assessed based on the FAS, which includes all randomized participants whose baseline data were available and valid. Safety analysis was carried out on the SS that includes all participants who received the booster vaccination. Immunogenicity was analyzed based on the PPS including all participants who received the booster vaccination and had valid pre- and post-vaccination immunogenicity data.For baseline characterization, continuous and categorical characteristics between groups were compared using two-sided Student’s 