“Right now, our model — our best case — predicts that we will have an answer by the end of October,” the chief executive, Dr. Albert Bourla, told the “Today” show earlier this month. In other interviews, he has said he expected a “conclusive readout” by then, with an application for emergency authorization that could be filed “immediately.”…

And yet by all other accounts, the idea that it will be ready in October is far-fetched. Even if the vaccine shows promising signs in clinical trials — still a big if — the company will not have collected enough data by then to say with any statistical confidence that it is safe and effective.

By repeating a date that flies in the face of most scientific predictions, Dr. Bourla is making a high-stakes gamble. If Pfizer puts out a vaccine before it has been thoroughly tested — something the company has pledged it will not do — it could pose a major threat to public safety. The perception matters, too: If Americans see the vaccine as having been rushed in order to placate Mr. Trump, many may refuse to get the shot.