Federal health officials have repeatedly pointed out that his timetable is off — that it will take at least a year — but his single-minded focus on warp-speed production of a new vaccine represents a striking philosophical shift. For years, Mr. Trump was an extreme vaccine skeptic who not only blamed childhood immunizations for autism — a position that scientists have forcefully repudiated — but once boasted he had never had a flu shot.

At least a decade before Mr. Trump was elected president, with responsibilities that would include nominating experts to lead the nation’s health centers, the hotelier and commercial developer was holding forth with great confidence about medical topics. When an interviewer would note that physicians disagreed with the dim view he took of vaccines, Mr. Trump remained ever ebullient, impervious and dismissive of scientific authority.

Perhaps that bravado was infectious: As a private citizen and celebrity, Mr. Trump was, somewhat mystifyingly, often sought by the media for his thoughts on vaccines.