In a message on social media that baffled many scientists, President Trump questioned the effectiveness of the Covid vaccines and demanded that the makers prove that they work.

“Many people think they are a miracle that saved Millions of lives,” Mr. Trump wrote on Truth Social. “Others disagree!”

“I want the answer, and I want it NOW,” he wrote. The president went on to complain that while one manufacturer, Pfizer, had shown impressive figures to him, “they never seem to show those results to the public. Why not???”

It is unclear what data Mr. Trump was referring to. Hundreds of reports have tracked the efficacy of the vaccines since they first debuted in 2021. The shots have saved millions of lives in the United States and elsewhere, dozens of studies have estimated.

Still, in some ways, Mr. Trump’s demand for data is a welcome change from what administration officials have been saying recently about vaccines, said John Wherry, director of the Institute for Immunology and Immune Health at the University of Pennsylvania.

Health Secretary Robert F. Kennedy Jr. has been “blatantly misrepresenting the data,” Dr. Wherry said. As for other administration officials, “they’ve been sort of ignoring data, or claiming the data is false, or talking about anecdotes.”