This article reports on a phase 1 clinical trial of the mRNA-1273 vaccine, which encodes a stabilized prefusion SARS-CoV-2 spike protein. The trial involved 45 healthy adults aged 18 to 55 years who received two vaccinations, 28 days apart, at doses of 25, 100, or 250 micrograms. The vaccine was well tolerated, with most adverse events being mild or moderate. After the first vaccination, antibody responses increased with higher doses, and after the second vaccination, titers increased further. Neutralizing activity was detected in all participants, with values similar to those in convalescent serum specimens. The 250-microgram dose group experienced more systemic adverse events, with three participants reporting severe events. The vaccine induced strong immune responses, including binding antibodies and neutralizing antibodies, with neutralizing activity comparable to that of convalescent serum specimens. T-cell responses were also observed, with CD4 T-cell responses being strongly biased toward Th1 cytokines. The vaccine showed a robust immunogenicity profile, likely due to its innovative antigen design and lipid nanoparticle delivery system. The results support further development of the vaccine, with a phase 2 trial planned for 600 healthy adults. The study highlights the potential of mRNA vaccines in rapidly developing effective immunizations against SARS-CoV-2.This article reports on a phase 1 clinical trial of the mRNA-1273 vaccine, which encodes a stabilized prefusion SARS-CoV-2 spike protein. The trial involved 45 healthy adults aged 18 to 55 years who received two vaccinations, 28 days apart, at doses of 25, 100, or 250 micrograms. The vaccine was well tolerated, with most adverse events being mild or moderate. After the first vaccination, antibody responses increased with higher doses, and after the second vaccination, titers increased further. Neutralizing activity was detected in all participants, with values similar to those in convalescent serum specimens. The 250-microgram dose group experienced more systemic adverse events, with three participants reporting severe events. The vaccine induced strong immune responses, including binding antibodies and neutralizing antibodies, with neutralizing activity comparable to that of convalescent serum specimens. T-cell responses were also observed, with CD4 T-cell responses being strongly biased toward Th1 cytokines. The vaccine showed a robust immunogenicity profile, likely due to its innovative antigen design and lipid nanoparticle delivery system. The results support further development of the vaccine, with a phase 2 trial planned for 600 healthy adults. The study highlights the potential of mRNA vaccines in rapidly developing effective immunizations against SARS-CoV-2.