The mRNA-1273 vaccine, an mRNA-based vaccine encoding the prefusion-stabilized full-length spike protein of SARS-CoV-2, demonstrated 94.1% efficacy in preventing symptomatic Covid-19 illness in a phase 3 trial involving 30,420 participants. The trial, conducted across 99 U.S. sites, randomly assigned participants to receive either the vaccine or a placebo. The primary endpoint was the prevention of Covid-19 illness 14 days after the second dose in participants who had not previously been infected. The vaccine showed similar efficacy across key secondary analyses, including assessments 14 days after the first dose and in participants with a history of SARS-CoV-2 infection. Severe Covid-19 occurred in 30 participants, all in the placebo group. The vaccine was well-tolerated, with more frequent moderate, transient reactogenicity in the mRNA-1273 group. Serious adverse events were rare and similar in both groups. The vaccine showed no safety concerns aside from transient local and systemic reactions. The study was funded by the Biomedical Advanced Research and Development Authority and the National Institute of Allergy and Infectious Diseases. The results indicate that the mRNA-1273 vaccine is effective in preventing symptomatic SARS-CoV-2 infection and is safe for use. The trial is ongoing, and further follow-up is needed to assess long-term efficacy and safety.The mRNA-1273 vaccine, an mRNA-based vaccine encoding the prefusion-stabilized full-length spike protein of SARS-CoV-2, demonstrated 94.1% efficacy in preventing symptomatic Covid-19 illness in a phase 3 trial involving 30,420 participants. The trial, conducted across 99 U.S. sites, randomly assigned participants to receive either the vaccine or a placebo. The primary endpoint was the prevention of Covid-19 illness 14 days after the second dose in participants who had not previously been infected. The vaccine showed similar efficacy across key secondary analyses, including assessments 14 days after the first dose and in participants with a history of SARS-CoV-2 infection. Severe Covid-19 occurred in 30 participants, all in the placebo group. The vaccine was well-tolerated, with more frequent moderate, transient reactogenicity in the mRNA-1273 group. Serious adverse events were rare and similar in both groups. The vaccine showed no safety concerns aside from transient local and systemic reactions. The study was funded by the Biomedical Advanced Research and Development Authority and the National Institute of Allergy and Infectious Diseases. The results indicate that the mRNA-1273 vaccine is effective in preventing symptomatic SARS-CoV-2 infection and is safe for use. The trial is ongoing, and further follow-up is needed to assess long-term efficacy and safety.