The 95 cases included 15 people 65 or older and 20 people who were Hispanic, Black, Asian or multiracial. The company said the vaccine appeared equally safe and effective in all the subgroups.
The results were analyzed by an independent data safety monitoring board, appointed by the National Institutes of Health.
Moderna, based in Cambridge, Mass., developed its vaccine in collaboration with researchers from the Vaccine Research Center, part of the National Institute of Allergy and Infectious Diseases.
Dr. Anthony S. Fauci, director of the institute, said in an interview: “I had been saying I would be satisfied with a 75 percent effective vaccine. Aspirationally, you would like to see 90, 95 percent, but I wasn’t expecting it. I thought we’d be good, but 94.5 percent is very impressive.”
At a news briefing on Monday, Dr. Fauci and Dr. Francis Collins, director of the National Institutes of Health, emphasized that the hopeful news did not mean people could let down their guard. On the contrary, they implored the public to “double down” on mask-wearing, distancing, hand-washing and avoiding crowds, and to stay that course until vaccine becomes available.
Stéphane Bancel, the chief executive of Moderna, said in a statement that the results had provided “the first clinical validation that our vaccine can prevent Covid-19 disease, including severe disease.”
Pfizer’s chief executive, Dr. Albert Boula, tweeted congratulations to Moderna.
Pfizer and Moderna each announced the findings in news releases, not in peer-reviewed scientific journals, and the companies have not yet disclosed the detailed data that would allow outside experts to evaluate their claims. Therefore, the results cannot be considered conclusive. The studies are continuing, and the figures on effectiveness may change.