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

The vaccine demonstrated an overall efficacy of 90.4 percent in the 29,960-person trial, comparable to Pfizer-BioNTech and Moderna vaccines and higher than Johnson & Johnson’s one-shot vaccine. The Novavax vaccine was found to be 100 percent effective in preventing moderate or severe disease.

Despite these promising results, the vaccine’s future in the United States is uncertain, and it may be more needed in other countries. According to Novavax, it may not seek emergency approval from the Food and Drug Administration until the end of September. With a plentiful supply of three other authorized vaccines, the agency may instruct Novavax to apply for a full license instead — a process that could take several months longer.

In an interview, Novavax’s CEO, Stanley Erck, admitted that the company’s first authorization would most likely come from somewhere else. The company is also submitting applications in the United Kingdom, the European Union, India, and South Korea.

By the time Novavax receives approval from the US government, it may be too late to contribute to the country’s initial round of vaccinations. However, due to waning immunity and emerging variants, many vaccine experts believe the country will require booster shots at some point. And the protein-based technology used in the Novavax vaccine may be particularly effective at boosting immunity, even if people have previously been immunized with a different formulation.

Novavax announced in January that a 15,000-person trial in the United Kingdom found that the vaccine had a 96 percent efficacy against the original coronavirus. Against Alpha, a virus variant discovered in the United Kingdom, the efficacy dropped slightly to 86%. In South Africa, where Novavax conducted a smaller trial on 2,900 people and the Beta variant predominated, the company discovered efficacy of only 49 percent.

While waiting for trial results, Novavax collaborated with other companies to begin mass production of its vaccine. It collaborated with the Serum Institute in India and SK Biosciences in South Korea. Novavax and Gavi, the Vaccine Alliance, agreed to supply 1.1 billion doses to middle- and low-income countries.

However, the company’s scaling issues persisted, and it required more time to develop special tests used to confirm the quality of its product.

The new findings are based on 77 trial volunteers who were infected with Covid-19. Volunteers who received placebo shots were far more likely to become ill than those who were vaccinated, a statistical difference that translated to an efficacy of 90.4 percent. The vaccine had the same efficacy in a group of high-risk volunteers — people over the age of 65, with medical risk factors, or who worked in jobs where they were exposed to the virus. Novavax sequenced the genomes of 54 of the 77 viral samples and discovered that half of them were Alpha, the dominant variant in the United States this spring.

Last week, the FDA signaled a shift in its approach to Covid-19 vaccines. Ocugen, an American company, had sought emergency approval for Covaxin, a Covid-19 vaccine now in use in India. However, the company announced on Thursday that the FDA had recommended that they instead take the standard path to full approval, known as a biologics license application, which takes many more months.

Novavax is preparing for this future by researching how its vaccine can function as a booster. A new vaccine version contains the proteins from the Beta variant discovered in South Africa.

Researchers gave Beta boosters to baboons that had been vaccinated with the original version of the Novavax vaccine in previous experiments. The researchers discovered that the baboons’ immunity to Covid-19 increased following the booster, protecting them against Beta, Alpha, and the original version of the coronavirus.