The chief executive of Pfizer said on Friday that the company would not apply for emergency authorization of its coronavirus vaccine before the third week of November, ruling out President Trump’s assertion that a vaccine would be ready before Election Day on Nov. 3.
In a statement posted to the company website, the chief executive, Dr. Albert Bourla, said that although Pfizer could have preliminary numbers by the end of October about whether the vaccine works, it would still need to collect safety and manufacturing data that will stretch the timeline to at least the third week of November.
Close watchers of the vaccine race had already known that Pfizer wouldn’t be able to meet the requirements of the Food and Drug Administration by the end of this month. But Friday’s announcement represents a shift in tone for the company and its leader, who has repeatedly emphasized the month of October in interviews and public appearances.
In doing so, the company had aligned its messaging with that of the president, who has made no secret of his desire for an approved vaccine before the election. He has even singled out the company by name and said he had talked to Dr. Bourla, whom he called a “great guy.”
Some scientists applauded Pfizer’s announcement.
“This is good, really good,” said Dr. Eric Topol, a clinical trial expert at Scripps Research in San Diego who was one of 60 public health officials and others in the medical community who signed a letter to Pfizer urging it not to rush its vaccine.
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He said company officials had assured him that a vaccine would most likely not be authorized before the election, but the letter Friday is “even more solid about their not being part of any political machinations.”
Dr. Bourla has pushed back against any suggestion that Pfizer’s vaccine timeline was politically motivated. In September, Pfizer was the driving force behind a pledge by nine vaccine companies to “stand with science” and not put forward anything that had not been properly vetted. Earlier this month, he published an open letter to employees that said he “would never succumb to political pressure” and expressing disappointment that “we find ourselves in the crucible of the U.S. presidential election.”
Pfizer is one of four companies testing a coronavirus vaccine in late-stage clinical trials in the United States, and it has been the most aggressive in its timeline estimates. Moderna, AstraZeneca and Johnson & Johnson have said that later in the year is more likely, matching the predictions of federal health officials. (AstraZeneca and Johnson & Johnson’s trials have been paused for potential safety concerns, which could further delay their outcomes.)
In interviews, Mr. Bourla has said that he expects a “conclusive readout” by late October, with an application for emergency authorization that could be filed “immediately.”
Pfizer’s trial of 44,000 volunteers tests the vaccine by giving one group the vaccine, another group the placebo, and waiting until a certain number of people become infected with the virus. If significantly more people who received the placebo got infected, then the vaccine is considered to be effective.
A company spokeswoman said last month that Pfizer would not be anywhere near completion of its trial by the end of October and that when Dr. Bourla had referred to a “conclusive readout,” he meant it was possible the outside board of experts monitoring the trial would have by that date found promising signs that the vaccine works.
In his statement on Friday, Dr. Bourla acknowledged those timelines were uncertain. “Since we must wait for a certain number of cases to occur, this data may come earlier or later based on changes in the infection rates.”
He also said the company would release the results of any decision by the outside panel — good or bad — within a few days of its decision.
Dr. Bourla’s statement arrived soon after the F.D.A. published new guidelines detailing how the agency would evaluate a vaccine for emergency authorization, a document published after weeks of stalling by the White House. The guidelines, which do not carry the force of law, call for gathering comprehensive safety data in the final stage of clinical trials before an emergency authorization can be granted.
In a tweet on Oct. 6, Mr. Trump accused the F.D.A. of having a political agenda with the recommendations, which he said “make it more difficult for them to speed up vaccines for approval before Election Day.” Mr. Trump called it a “political hit job.”
In a video he posted from the White House a day later, Mr. Trump said that a vaccine should be available before the election, “but frankly, the politics gets involved.”
“They want to play their games,” he said. “It’s going to be right after the election.”
Jonathan Martin contributed reporting.