Statistical analysis
Because there were no previous studies relevant to the primary safety outcome of the three vaccines used in this trial, no statistical methods were used to calculate the sample size. Based on practical considerations and guidelines in China that the minimum sample size for each group in a phase I clinical trial should be 20–30 individuals, 90 participants per CoronaVac age subgroup and 45 participants per control age subgroup (2:1) were determined. Data were entered into EpiData software (version 3.1, EpiData Association). Analysis methods were described in previous trials (