Fig. 1: Flow Chart.

# Participants in placebo group withdrew from the study and received COVID-19 vaccines since the inactivated COVID-19 vaccine was approved for emergency use among children and adolescents aged 3–17 years. *Participants in the 1.5 μg group were excluded from the per-protocol analysis because they did not have a blood sample taken 28 days after the third dose. †Participants in the 1.5 μg group were excluded from the per-protocol analysis because they received other licensed inactivated COVID-19 vaccine before the third dose, and one did not have a blood sample taken 28 days after the third dose. ‡Participants in the 3.0 μg group were excluded from the per-protocol analysis because they received other licensed inactivated COVID-19 vaccine before the third dose, and five did not have a blood sample taken 28 days after the third dose.