The repeated COVID-19 outbreaks, despite global vaccination, highlights the need for booster doses. Here, we present the outcomes, up until day 90 post vaccination, of a randomized, double-blind, placebo-controlled phase 1 clinical trial of the mRNA-based vaccine candidate; COReNAPCIN®, as a booster dose in adults aged 18-50 who had previously received three doses of inactivated vaccines. In the study, 30 participants randomly (2:2:1) received 25 μg, or 50 μg of COReNAPCIN®, or placebo. The results indicated that COReNAPCIN®was well tolerated in vaccinated individuals in both groups with no life-threatening or other serious adverse events. The most noticeable solicited adverse events were pain at the site of injection, fatigue and myalgia. Regarding the immunogenicity, given the seroprevalence of SARS-CoV-2 antibodies due to the vaccination history for all, and previous SARS-CoV-2 infection for some participants, the recipients of COReNAPCIN®, two weeks post vaccination, showed significant fold increases in the level of anti-RBD (6.6 and 8.1 folds) or anti-spike (11.5 and 21.7 folds), and potent neutralizing antibodies (10.2 and 8.4 folds) in 25 and 50 μg groups, respectively, while no meaningful changes were observed in the placebo group. Additionally, the significant increase in the spike-specific IFN-γ T-cell response upon vaccination, underscores the activation of cellular immunity. Altogether, the favorable safety, tolerability, and immunogenicity profile of COReNAPCIN®support its further clinical development.Trial registration numberIRCT20230131057293N1