Already, Trump has muddied the waters on the difference between a vaccine entering various trial phases and it being ready for widespread public use. While vaccines are able to be quickly developed by researchers, it takes time to get the vaccine on the market. Trump has also raised questions about whether a vaccine for the flu could work on this completely different virus.

“You take a solid flu vaccine, you don’t think that could have an impact, or much of an impact, on corona?” Trump asked.

“No,” a pharmaceutical researcher quickly replied.

And he's raised doubts about other aspects of coronavirus using language that scientists find troubling.

Appearing Wednesday night on Sean Hannity's Fox News show, Trump described the World Health Organization’s official 3.4 percent coronavirus death rate as a “false number” based on a “hunch.” While the figure may eventually come down, as the current data may not include some undetected cases, it is not "false."

Trump explained it this way: “If we have thousands or hundreds of thousands of people that get better, just by, you know, sitting around and even going to work, some of them go to work, but they get better."

On vaccines specifically, though, the president has long been a source of dubious information. As a presidential candidate, Trump tweeted discredited theories that vaccines cause autism and also spouted the theory during a September 2015 debate.

Andrew Wakefield, the British doctor disciplined for fraud over his claim that the MMR vaccine caused autism, attended Trump’s inauguration. A short time later, Robert F. Kennedy Jr., who also blames vaccines for many ills, said Trump had promised to establish a commission to investigate such theories — but nothing came of it.

Yet during last year’s measles epidemic — the worst in 27 years — Trump did urge people to vaccinate their children.

A senior White House official pushed back on the notion that the president was confusing important information, arguing that Trump was trying to clarify medical vernacular that might be unclear to the public.

“It’s not confusion, he’s just asking questions,” the official said. “There’s just a lot of back and forth when you’re talking about vaccines versus treatment.”

Vaccine scientists have been amazed at the speed with which the National Institutes of Health and vaccine makers are assembling candidate vaccines to prevent coronavirus. While most vaccines take years or decades to develop, the 12-18 month scenario that Fauci has laid out is realistic, according to medical experts. Although it’s far from assured.

“The concern we have is, 12-18 months may be too late,” said Kathryn Edwards, a pediatric infectious disease expert, also at Vanderbilt. “The bulk of the outbreaks could be over by then.”

If conditions are desperate enough a year from now, the FDA might authorize use of a vaccine without incontrovertible evidence that it protects against disease.

But putting out a vaccine before it’s been tested in epidemic conditions could be risky, Edwards said. For instance, during tests of some earlier vaccines against SARS and MERS, which are similar to the recent coronavirus, some animals developed so-called immune enhancement, meaning they got sicker when exposed to the virus than unvaccinated animals. A similar phenomenon may have occurred in some children who were injected with dengue virus vaccines in the Philippines in 2018.

Several vaccines are being studied, but the one moving the fastest is produced by Moderna Therapeutics of Cambridge, Mass., with scientists from Fauci’s institute.

The first phase of the trial is set to be hosted at a Kaiser-Permanente research institute located in Seattle, but the outbreak of disease there has complicated the plan. The 50 or so study subjects will have to be carefully monitored to assure none of them is infected with the virus during the trial, which is aimed solely at confirming that the vaccine is safe , one scientist familiar with the trial said.

Trump’s allies said the president’s rhetoric on a shortened vaccine timeline is helping accelerate the process.

“If President Trump went by the old messaging playbook, we wouldn’t see a coronavirus vaccine for years. Instead he’s aggressively pushing the NIH, [the Centers for Disease Control], our top researchers and even private drug companies to get it done immediately,” said Jason Miller, senior communications adviser on the 2016 Trump campaign.

“And if President Trump ruffles a few feathers with his hyperbole in the pursuit of a cure to keep people safe, there will be zero complaints,” he added. “Good for him.”