A phase 1, dose-escalation, open-label trial of the mRNA-1273 vaccine, which encodes the stabilized prefusion SARS-CoV-2 spike protein, was conducted in healthy adults. The study was expanded to include 40 older adults (56-70 years and ≥71 years). Participants received two doses of either 25 μg or 100 μg of the vaccine, 28 days apart. The vaccine was well tolerated, with mostly mild or moderate adverse events, predominantly after the second dose. Binding-antibody responses increased rapidly after the first dose, with higher titers observed in the 100 μg group. Neutralizing antibody responses were detected in all participants, with responses similar to those in younger adults. The 100 μg dose induced higher binding and neutralizing antibody titers than the 25 μg dose. The vaccine also elicited a strong CD4 cytokine response involving type 1 helper T cells. The study suggests that the 100 μg dose is more effective in inducing immune responses in older adults. The results support the use of the 100 μg dose in further trials. The study was funded by the National Institute of Allergy and Infectious Diseases and others.A phase 1, dose-escalation, open-label trial of the mRNA-1273 vaccine, which encodes the stabilized prefusion SARS-CoV-2 spike protein, was conducted in healthy adults. The study was expanded to include 40 older adults (56-70 years and ≥71 years). Participants received two doses of either 25 μg or 100 μg of the vaccine, 28 days apart. The vaccine was well tolerated, with mostly mild or moderate adverse events, predominantly after the second dose. Binding-antibody responses increased rapidly after the first dose, with higher titers observed in the 100 μg group. Neutralizing antibody responses were detected in all participants, with responses similar to those in younger adults. The 100 μg dose induced higher binding and neutralizing antibody titers than the 25 μg dose. The vaccine also elicited a strong CD4 cytokine response involving type 1 helper T cells. The study suggests that the 100 μg dose is more effective in inducing immune responses in older adults. The results support the use of the 100 μg dose in further trials. The study was funded by the National Institute of Allergy and Infectious Diseases and others.