In the 29,960-person trial, the vaccine demonstrated an overall efficacy of 90.4 percent, on par with the vaccines made by Pfizer-BioNTech and Moderna, and higher than the one-shot vaccine from Johnson & Johnson. The Novavax vaccine showed an efficacy of 100 percent at preventing moderate or severe disease. Despite these impressive results, the vaccine’s future in the United States is uncertain and it might be needed more in other countries. Novavax says it may not seek emergency authorization from the Food and Drug Administration until the end of September. And with a plentiful supply of three other authorized vaccines, it’s possible that the agency may tell Novavax to apply instead for a full license — a process that could require several extra months... By the time Novavax gets the green light from the U.S. government, it may be too late to contribute to the country’s first wave of vaccinations. But many vaccine experts expect that, with waning immunity and emerging variants, the country will need booster shots at some point. And the protein-based technology used in the Novavax vaccine may do a particularly good job at amplifying protection, even if people have previously been vaccinated with a different formulation. “They may be really the right ones for boosters,” said Dr. Luciana Borio, who was the acting chief scientist at the F.D.A. from 2015 to 2017.