<qa>  What was the first dose of the study drug?This was a randomized, open-label, active-controlled clinical study. After providing voluntary written informed consent, subjects underwent protocol-specific assessments and tests within the 4 weeks prior to administration of the study vaccine, and those who met all of the inclusionexclusion criteria were randomized sequentially to the test group or the comparator group according to the pre-generated randomization code. To investigate the safety of the study drug with a two-dose regimen in children, the test group received another dose 28 days after the first dose. 