The study reports interim findings from a phase 1 clinical trial of the mRNA-1273 vaccine, which encodes a stabilized prefusion spike trimer of SARS-CoV-2. The vaccine was administered to 45 healthy adults in a dose-escalation, open-label trial, with doses of 25 μg, 100 μg, and 250 μg. After the first vaccination, antibody responses were higher with higher doses, and after the second vaccination, titers increased further. All participants developed serum-neutralizing activity after the second vaccination, with values similar to those in convalescent serum specimens. Solicited adverse events were mild to moderate, and systemic adverse events were more common after the second vaccination, particularly with the highest dose. No serious adverse events were reported. The vaccine induced robust immune responses, supporting further development. The 100 μg dose showed high neutralization responses and Th1-skewed CD4 T cell responses, making it a promising candidate for advanced clinical trials.The study reports interim findings from a phase 1 clinical trial of the mRNA-1273 vaccine, which encodes a stabilized prefusion spike trimer of SARS-CoV-2. The vaccine was administered to 45 healthy adults in a dose-escalation, open-label trial, with doses of 25 μg, 100 μg, and 250 μg. After the first vaccination, antibody responses were higher with higher doses, and after the second vaccination, titers increased further. All participants developed serum-neutralizing activity after the second vaccination, with values similar to those in convalescent serum specimens. Solicited adverse events were mild to moderate, and systemic adverse events were more common after the second vaccination, particularly with the highest dose. No serious adverse events were reported. The vaccine induced robust immune responses, supporting further development. The 100 μg dose showed high neutralization responses and Th1-skewed CD4 T cell responses, making it a promising candidate for advanced clinical trials.