Moderna’s vaccine has become a symbol of the triumphs of government scientists during the pandemic. After China released the genetic sequence of the new virus in early January, scientists at Moderna and the National Institutes of Health were able to zero in on the design for a vaccine in just two days. Unlike Pfizer, Moderna has worked closely with Operation Warp Speed, the federal program aimed at accelerating vaccine development. Nearly $2.5 billion in federal funds helped Moderna buy raw materials, expand its factory and enlarge its work force by 50 percent.

Moderna’s vaccine effort stands in contrast to two other high-profile projects that the United States had hoped would build up its supply: one from the pharmaceutical companies Sanofi of France and GlaxoSmithKline of Britain, and another from the British-Swedish drug maker AstraZeneca and the University of Oxford.

AstraZeneca and Oxford ended up using two different doses in clinical trials in Britain and Brazil. At one level, the efficacy was 62 percent, and at the other it was 90 percent. The muddled results have left it unclear when AstraZeneca will have enough data to secure an emergency authorization.

Separately, Sanofi and GlaxoSmithKline had disappointing results in early clinical trials. While their vaccine produced a promising immune response in volunteers under 50, it failed to do so in older ones. The companies are planning a new set of trials with a different version, making it unlikely they can produce a vaccine before the end of 2021.

Moderna’s vaccine has had much smoother sailing. The F.D.A. review showed that it worked equally well in white, Black and Hispanic volunteers, men and women, healthy participants and those at risk of severe Covid-19 with conditions like obesity and diabetes. For people 65 and older, the trial provided an estimated efficacy of 86.4 percent, lower than the overall estimate of 94.1 percent. But the apparent difference was not statistically significant. And 86.4 percent is still very high.

So far, two potential differences between the Pfizer-BioNTech and Moderna vaccines have emerged in the F.D.A.’s reviews, but the findings may reflect a shortage of data rather than genuine distinctions.