This study was a randomized, double-blinded, placebo-controlled, phase $1$ and phase $2$ trial conducted in Shangqiu City, China.

• In phase $1$ of the trial, healthy individuals who tested negative for antibodies against SARS-CoV-2 were placed into two age categories: one group for $18$ to $59$ year olds and another group for $60$ years and older. Each individual was then randomly administered a placebo or the vaccine (BBIBP-CorV). The placebo and vaccine were administered through a two-dose schedule on days $0$ and $28$ of the trial in doses of either $2$ μg, $4$ μg, or $8$ μg.

• In phase $2$ of the trial, healthy individuals ranging from $18$ to $59$ years of age were randomly administered either the vaccine or placebo. The placebo and vaccine were administered through a single-dose schedule on day $0$ with $8$ μg, or through a two-dose schedule on either days $0$ and $14$, days $0$ and $21$, or days $0$ and $28$ in doses of $4$ μg.

• The primary outcomes for the trial were tolerability and safety. The secondary outcome was established as immunogenicity against SARS-CoV-2.