Limitations
subvariants
This study has limitations, including its short period of time (median surveillance time: 64 [IQR, 57–69] days) (Notably, the protective efficacy was assessed during the predominant circulation of the Delta variant, which was later replaced by the Omicron variant. However, the post hoc analyses of immune responses against the more recent Omicron subvariants support potential effectiveness against a broad distribution of future variants (Although the study was not powered for the assessment of vaccine efficacy, another limitation of the study was the low number of cases that were accrued in each group due to the implementation of the blinded crossover, which made the 95% CIs appear wide. However, the lower bound of the vaccine efficacy 95% CI was over the threshold of 30% established by the US FDA to grant emergency use authorization to COVID-19 vaccines.