the company was on the verge of closing after a major trial failure for another vaccine, and it was forced to sell off its manufacturing facility to raise money.

Last year, the Trump administration placed a major bet on the tiny company as part of its Operation Warp Speed project, signing a $1.6 billion contract for delivery of 110 million doses by early this year. In April, the total amount of the deal was increased to $1.75 billion, according to Novavax’s financial filings. The company’s large trial in the United States and Mexico has still not been completed, although executives said on Monday that they expected results from that study “in a few weeks.”

Novavax officials said they now did not expect to deliver those doses until the end of this year or early 2022. A spokeswoman for Novavax said there was no penalty for later delivery in its contract with the U.S. government.

Novavax’s spotty track record does not offer confidence that it can rise to the challenge of producing billions of doses, said Les Funtleyder, a health care portfolio manager at E Squared Capital Management who invests in domestic and emerging markets. “It seems they were really unprepared for a challenge of this magnitude,” he said.

Recent news of internal turnover — such as the departure last month of Novavax’s chief financial officer, five months after taking the role, for personal reasons — does not help, Mr. Funtleyder said. “It’s a bad look,” he said.

children older than 12, in an effort to catch up to Moderna and Pfizer, which have already tested their products in that age group.

The vaccine can also be stored at normal refrigeration temperatures, without the freezing temperatures required by Pfizer and Moderna’s vaccines.

“By the end of 2021 there will still be a great need for safe and effective vaccines that can travel well,” said Ms. Taylor, of Duke University. “Novavax looks like it can fit that description.”

Dr. Saad B. Omer, the director of the Yale Institute for Global Health, noted that when concerns were raised over the Johnson & Johnson and AstraZeneca vaccines because of links to blood clots, countries with multiple vaccines available were able to switch to other options.

“It’s good to hedge our bets,” he said. “If we want to avoid, for example, body blow after body blow to low-income countries in many parts of the world that has an impact on everyone, we need to vaccinate a huge chunk of the world.”

Benjamin Mueller and Noah Weiland contributed reporting.

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