Design: This is a phase 1, dose-escalation open-label trial to evaluate the safety and immunogenicity of MVC-COV1901, a recombinant stabilized prefusion SARS-CoV-2 spike (S-2P) protein vaccine with adjuvant of aluminum hydroxide and CpG 1018. Methods: We enrolled 45 healthy adults from 20 to 49 years of age to be administered with two vaccinations of MVC-COV1901 in a low dose (LD), middle dose (MD), and high dose (HD) of spike protein at 28 days apart. There were 15 participants in each dose

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... up, and all of them were followed up for 28 days after the second vaccination at the time of interim analysis. Adverse events (AEs) and laboratory data were recorded for safety evaluation. Blood samples were collected for wild-type SARS-CoV-2 and pseudovirus neutralization assays as well as SARS-CoV-2 spike-specific immunoglobulin G (IgG) at various times. Overall, the study duration will be 7 months. Results: Solicited events were mostly mild and similar in the participants of all three dose groups. No subject experienced fever. There were no serious nor adverse events of special interest at the time point of this interim report. After the second vaccination, the SARS-CoV-2 spike specific IgG titers increased with peak geometric mean titers at 7178.245 (LD), 7746.086 (MD), and 11220.58 (HD), respectively. Serum neutralizing activity was detected by two methods in all participants of MD and HD groups, with geometric mean values generally comparable to those of a panel of control convalescent serum specimens. All of the participants in the MD and HD groups were seroconverted after the second vaccination. Conclusions: The MVC-COV1901 vaccine is safe and elicits remarkable immune responses especially in the MD and HD groups.