Novavax, a small American company buoyed by lavish support from the U.S. government, announced on Monday the results of a clinical trial of its Covid-19 vaccine in the United States and Mexico, finding that its two-shot inoculation provides potent protection against the coronavirus.

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.

The company’s chief executive, Stanley Erck, acknowledged in an interview that Novavax would probably win its first authorization elsewhere. The company is also applying in Britain, the European Union, India and South Korea.

“I think the good news is that the data are so compelling that it gives everybody an incentive to pay attention to our filings,” Mr. Erck said.

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.

Last year, the Trump administration’s Operation Warp Speed program awarded Novavax a $1.6 billion contract for 100 million future doses. The company won this tremendous support despite never having brought a vaccine to market in over three decades.

In January, Novavax announced that its 15,000-person trial in Britain found that the vaccine had an efficacy of 96 percent against the original coronavirus. Against Alpha, a virus variant first identified in Britain, the efficacy fell slightly to 86 percent. In South Africa, where Novavax ran a smaller trial on 2,900 people and the Beta variant was dominant, the company found an efficacy of just 49 percent.

But the South Africa trial was complicated by the fact that a number of the volunteers had H.I.V., which is known to hamper vaccines. In addition, the study was so small that it was difficult to estimate how much protection the vaccine provided H.I.V.-negative volunteers.

With the support of Operation Warp Speed, Novavax drew up plans for an even larger late-stage trial in the United States and Mexico. But difficulties with manufacturing delayed its launch until December.

By then, the United States had authorized the Pfizer-BioNTech and Moderna vaccines. In February, with the Novavax trial still underway, the government authorized Johnson & Johnson’s.

As it waited for trial results, Novavax partnered with other companies to start making massive quantities of its vaccine. In India, it joined forces with the Serum Institute, and in South Korea, SK Biosciences. Novavax reached an agreement with Gavi, the Vaccine Alliance, to supply 1.1 billion doses to middle- and low-income countries.

But the company’s difficulties with scaling continued, and it needed more time to develop special tests used to confirm the quality of its product.

The new results were based on 77 trial volunteers who came down with Covid-19. The volunteers who received placebo shots were far more likely than the vaccinated ones to get sick, a statistical difference that translated to an efficacy of 90.4 percent.

“It’s a strong result,” said Natalie Dean, a biostatistician at the University of Florida. “It puts them up in that high tier.”

The vaccine showed the same efficacy in a group of high-risk volunteers — people who were over the age of 65, had medical risk factors or had jobs that exposed them to the virus.

Novavax sequenced the genomes of 54 of the 77 viral samples, and found that half were Alpha, the variant that became dominant in the United States this spring.

The vaccine’s side effects were relatively mild. Some volunteers reported fatigue, headaches and other minor symptoms. “This vaccine seems easier on the arms,” said John Moore, a virologist at Weill Cornell Medicine who was a volunteer in the Novavax trial.

Novavax will apply for authorization in the United States after it finishes developing a quality control test, according to its chief executive. “You have to test them every way from Sunday to show that under any conditions you get the same answer,” Mr. Erck said. “And that takes time.”

Mr. Erck said that the company plans on making 100 million doses per month by the end of the third quarter, and 150 million doses per month by the end of the fourth quarter.

With each passing week, the United States is building a bigger supply of authorized vaccines from other companies, raising the question of whether the nation needs to give any more emergency use authorizations, or EUAs.

“The law says that once you have sufficient doses, there’s no need for additional E.U.A.’s,” Dr. Borio said.

One sign that the F.D.A. is changing its approach to Covid-19 vaccines came last week. An American company called Ocugen had been seeking emergency authorization for Covaxin, a Covid-19 vaccine now in use in India. But on Thursday, the company announced that the F.D.A. had recommended they instead take the standard path to a full approval, known as a biologics license application, which takes many additional months.

But because Novavax has been consulting with the F.D.A. since last year about its trials, Mr. Erck said the company might be able to continue with its plans for seeking emergency use authorization.

“So far, they’ve indicated that if you’re in the process for an E.U.A, you can continue for an E.U.A.,” Mr. Erck said. “Anybody could tell you that could change, but I don’t know how to predict that.”

Dr. Paul Offit, a professor at the University of Pennsylvania and a member of the F.D.A.’s vaccine advisory panel, said that Novavax’s highly effective vaccine would be most welcome. “The more the merrier,” he said. “I think there’s room for many more vaccines, because we’re going to be dealing with this virus for years, if not decades.”

Novavax is preparing for that future by investigating how its vaccine could work as a booster. A new version of the vaccine contains the proteins from the Beta variant first identified in South Africa.

Researchers gave Beta boosters to baboons that had been vaccinated with the original version of the Novavax vaccine in experiments a year ago. The researchers found that the baboon’s immunity against Covid-19 shot up after this booster, protecting them against Beta, Alpha and the original version of the coronavirus.

“When you boost, you see a very high recall response,” said Matthew Frieman, a virologist at the University of Maryland School of Medicine and a co-author of the new study. The study has not yet been published in a scientific journal.

Dr. Frieman said the new study offered encouraging evidence that Novavax vaccines might work well as boosters. It also suggested that people getting vaccines for the first time might do well to get a blend of the original and Beta versions to widen their protection against new variants, he said.

“Novavax may be used as a booster in the U.S., but it certainly will be the first vaccine a lot of people are going to see around the world,” he said.