“If they’re in a clinical trial with an experimental vaccine, they may get a vaccine sooner than usual,” he said.

If Novavax is successful, the new vaccine could add to an expanded portfolio of vaccinations in the US by late spring. Moderna and Pfizer have agreements with the United States to deliver 400 million doses by the middle of the year, enough to fully vaccinate 200 million people, and both companies are in talks to deliver an additional 100 million doses each after that.

Johnson & Johnson, who recently reported that its single-dose vaccine was effective in a large US study, could get approval this month but may not be able to ship any significant quantities to the US until April. AstraZeneca’s U.S. study is also ongoing, and the company has signed a deal to provide Americans with 300 million doses of its two-shot vaccine.

But any number of obstacles could affect Novavax’s progress. As other vaccines became more widespread, participants in the Novavax study could drop out. Although the results in the UK were promising, the US study could produce different results. Or the company might fail to demonstrate to regulators that it can reliably manufacture its vaccine on a large scale. With the US likely to have three approved vaccines available to the public soon, the company is under pressure to move forward or at risk of losing ground to competitors.

Novavax, based in Gaithersburg, Md., Struggled for years to get a successful product to market, and in 2019 its stock traded so low it was in danger of being delisted from the Nasdaq. Then, last spring and summer, two big deals saved it from doom. In May, the Coalition for Epidemic Preparation Innovations awarded the company up to $ 388 million to make its Covid vaccine available worldwide. In July, it was selected by the federal government’s Operation Warp Speed ​​program to develop and sell its vaccine to the United States. Later in the summer, the company reported encouraging results in preliminary studies.

However, the company struggled last fall to begin the U.S. process with its previous October target. Novavax has put great emphasis on manufacturing and has plants around the world. However, making these vaccines is a delicate and unpredictable process even for seasoned drug makers, and Novavax has struggled to outgrow the smaller batches required for early studies.

“Things happen – little things happen,” said Stanley C. Erck, president and chief executive of Novavax. “And so little things happen and you lose weeks.”