Abstract Background Control of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic needs effective vaccines. Methods In a phase 2 randomized, double-blind, placebo-controlled trial, 500 adults aged 18–59 years or ≥60 years were randomized in 2:2:1 ratio to receive 3 doses of 5 μg or 10 μg of a SARS-CoV-2 inactivated vaccine, or placebo separated by 28 days. Adverse events (AEs) were recorded through day 28 after each dosing. Live virus or pseudovirus neutralizing antibodies, and receptor binding domain immunoglobulin G (RBD-IgG) antibody were tested after the second and third doses. Results Two doses of the vaccine elicited geometric mean titers (GMTs) of 102–119, 170–176, and 1449–1617 for the 3 antibodies in younger adults. Pseudovirus neutralizing and RBD-IgG GMTs were similar between older and younger adults. The third dose slightly (<1.5 fold) increased GMTs. Seroconversion percentages were 94% or more after 2 doses, which were generally similar after 3 doses. The predominant AEs were injection-site pain. All the AEs were grade 1 or 2 in intensity. No serious AE was deemed related to study vaccination. Conclusions Two doses of this vaccine induced robust immune response and had good safety profile. A third dose given 28 days after the second dose elicited limited boosting antibody response.