(Bloomberg) -- President Donald Trump once again promised that a Covid-19 vaccine for Americans could be ready within weeks, a timeline faster than has been deemed possible or likely by the scientific agencies overseeing the effort and the companies developing the products.

Trump’s optimistic promises have left him in a battle with the realities of time, the deadline of the election, and the increasing skepticism of the stock market that he frequently cites as a barometer of his success. On Thursday, stocks continued a two-day slide, underscoring the skepticism on Wall Street that a vaccine is near.

On Tuesday, Trump said at an event hosted by ABC that it “could be three, four weeks, but we think we have it.”

On Wednesday, Trump said that vaccine distribution could start sometime in October. “That'll be from mid-October on. It may be a little bit later than that, but we'll be all set,” he said.

And on Thursday morning, Trump said a vaccine will be ready "either before or just shortly after" election day on Nov. 3. Taken together, Trump is promising a vaccine for some part of the American public between Oct. 6 and early November.

But Trump’s own health experts have said they expect a vaccine at the earliest by November or December, if everything goes perfectly. For most Americans it will take until well into next year to get the shots, according to Centers for Disease Control and Prevention Director Robert Redfield and National Institute of Allergy and Infectious Diseases Director Anthony Fauci. In addition, the head of Trump’s vaccine effort, Moncef Slaoui, has said recently that it’s “extremely unlikely” that a vaccine will be ready by Nov. 3.

The result is a strange moment in which the part of the government with the most to gain politically is openly arguing with the federal scientific agencies that have the most to lose reputationally. The administration seems unable to agree on a basic set of facts about the vaccines. At stake is the U.S. economy, the education of millions of American children, the resumption of normal life for the country, political control of the White House, and tens or hundreds of thousands of lives.

Read more: Why to be skeptical that everything will go perfectly with a vaccine

With each pushback by the health agencies, Trump or his deputies have questioned their expertise. On Wednesday night at a press conference, Trump said Redfield had spoken in error when he said a vaccine would take until the middle of next year for much of the American public.

“I think he misunderstood the questions,” Trump said.

And on Thursday morning, White House Chief of Staff Mark Meadows also questioned Redfield’s knowledge.

“The CDC is not developing the vaccines,” Meadows said. “While Dr. Redfield may have a timeline in mind, to my knowledge he hasn't had intimate discussions with those processes.”

The CDC, at the behest of the Health and Human Services Department, has contributed $700 million to Operation Warp Speed, all from money allocated to the CDC in this year’s stimulus legislation, Bloomberg reported Wednesday, citing people familiar with the matter.

Vaccine Makers

Whether Redfield or Fauci’s timelines are correct or not, the companies developing the shots have likewise been more cautious than the president. 

Pfizer Inc., the U.S. drugmaker furthest along in its tests, said in an email Thursday that it may have an answer on whether its vaccine is effective by the end of October. That’s consistent with what the company has previously said is its likely best case. 

Moderna Inc., the other U.S. company in late-stage trials, is slightly behind Pfizer. In an interview Wednesday, Moderna Chief Executive Officer Stephane Bancel said it was most likely that it could have an early answer on whether its shot works by November. It is “technically possible” but unlikely to get results in October, he said.

AstraZeneca Plc, the U.K. drugmaker also in late-stage trials, has had its U.S. tests paused while it investigates a potentially serious side effect in one patient. The University of Oxford, AstraZeneca’s partner, has said the symptoms likely weren’t related to the shot itself and trials have resumed in the U.K.

No other companies that are part of the U.S. effort are far enough along to have a vaccine ready in the near term.

In addition, drugmakers developing vaccines have promised not to submit a product for review by the Food and Drug Administration until they have conclusive safety and efficacy data, not just a first answer on effectiveness, which will take more time. The results they produce will have to be reviewed by the FDA, a process that could take additional weeks or months, especially if the agency convenes a public meeting to review the data. 

All of this, of course, assumes that the vaccines under development work and are safe. That’s far from a guarantee — most pharmaceutical products fail in development, and even those that make it to late-stage testing can reveal surprise safety worries or disappointing efficacy results. While the ongoing trials are the final stage of testing, they’re also the first time the vaccine has been tested against the virus in the real world. 

All of that takes time, and likely more time than Trump has allowed.

“FDA will need time to very carefully evaluate the whole package and they’ll want to consult their advisory committee. That doesn’t need to take months but it will take several weeks at the least,” Trump’s former FDA commissioner Scott Gottlieb, who now sits on Pfizer’s board, said in a text message.

“In a best case, if the initial data shows one or more vaccines are safe and effective, it’s very unlikely even a limited authorization for emergency use could be issued before the end of November or early December,” Gottlieb said.

©2020 Bloomberg L.P.