Fig. 1

Screening, randomization, and analysis populations. 522 healthy adults were screened and enrolled, of which 6 persons withdrew before vaccination and 516 participated in the study. The participants were randomly assigned to receive a booster injection of either NVSI-06-09 (260 participants) or BBIBP-CorV (256 participants). All these participants that received the booster vaccination were contained in the safety set (SS) for the analysis of vaccine safety. 255 participants in NVSI-06-09 booster group and 249 in BBIBP-CorV booster group who had valid immunogenicity data were included in the per-protocol set (PPS) for the analysis of vaccine immunogenicity