Outside scientists said it was dangerous to set public expectations that a vaccine could be available by any deadline, given the many scientific unknowns and the fact that the first candidates are just now being injected into humans. It was unclear which vaccine trial Slaoui was referring to, and the initial small safety trials underway are not designed to show whether a vaccine actually works.

“It’s unfair to the American public to have science by news release — or in this case, ‘I’ve taken a look behind the curtain, and I’ve seen something good, and you have to trust me.’ If there are data, let’s see them,” said Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “We have a fragile confidence in vaccines in this country. And we need to manage expectations.”…

Peter Jay Hotez, dean for the National School of Tropical Medicine at Baylor College of Medicine, said that with some large trials likely to begin this summer, the most reasonable timeline for proving a vaccine safe and effective would be the second half of next year — and even that would be “an absolute record.”

A number of senior administration officials said on the condition of anonymity that they are also skeptical of the president’s timeline but that Trump wanted to provide “hope.” Birx has gently told the president it will be more difficult than he thinks to create a vaccine quickly.