ap photo In this July 27 photo, Nurse Kathe Olmstead gives volunteer Melissa Harting, of Harpersville, N.Y., an injection as a study of a possible COVID-19 vaccine, developed by the National Institutes of Health and Moderna Inc., gets underway in Binghamton, N.Y.

Could the U.S. really see a coronavirus vaccine before Election Day?

A letter from federal health officials instructing states to be ready to begin distributing a vaccine by Nov. 1 — two days before the election — has been met, not with exhilaration, but with suspicion among some public health experts, who wonder whether the Trump administration is hyping the possibility or intends to rush approval for political gain.

The skepticism comes amid growing questions about the scientific credibility of the Food and Drug Administration and the Centers for Disease Control and Prevention, and their vulnerability to political pressure from President Donald Trump.

Dr. Anthony Fauci, the government’s top infectious-disease expert and a member of Trump’s coronavirus task force, told CNN on Thursday that it is unlikely but “not impossible” that a vaccine could win approval in October, instead of November or December, as many experts believe.

“And I would assume, and I’m pretty sure, it’s going to be the case that a vaccine would not be approved for the American public unless it was indeed both safe and effective,” he said.

Similarly, Dr. Moncef Slaoui, chief adviser to Operation Warp Speed, the administration’s project to hasten the development of a vaccine, told NPR that it is possible but “extremely unlikely” a vaccine will be ready before the election.

White House spokeswoman Kayleigh McEnany, meanwhile, gave assurances that Trump “will not in any way sacrifice safety” when it comes to a vaccine. And executives of five top pharmaceutical companies pledged that no COVID-19 vaccines or treatments will be approved, even for emergency use, without proof they are safe and effective.

The concerns were set off by a letter dated Aug. 27 in which CDC Director Robert Redfield asked the nation’s governors to help government contractor McKesson Corp. set up vaccine distribution facilities so that they are up and running by Nov. 1. Redfield did not say a vaccine would be ready by then.

Some longtime scientific advisers to the government said the CDC’s preparations for a possible early vaccine do not necessarily mean that an answer will come sooner or that there will be a rush to judgment about whether one works.

“Being prepared for early success … is actually prudent,” said Dr. Steven Nissen, a Cleveland Clinic cardiologist. If a vaccine is working, “you want to be prepared to act on the information.”

Still, to some public health experts, the timing of the letter smacked of a political stunt by a president facing a tough reelection.

“I think it’s almost a certainty,” said Ashish Jha, dean of the Brown University School of Public Health. He said November “feels awfully early.”

Jha noted the FDA’s recent move to approve emergency use of convalescent plasma against COVID-19 following threats and complaints from Trump about the slow pace of the agency’s review.

Last week, FDA Commissioner Stephen Hahn was forced to correct himself after overstating the lifesaving benefits of the plasma treatment. The CDC also came under fire from scientists in recent days for quietly updating its guidelines to suggest fewer Americans need to get tested for the virus.

Jha said the stakes here are much higher.

“You want to have pretty good safety data on vaccines,” he said. “So I am deeply worried that that’s not what is going to happen.”

Senate Democratic leader Chuck Schumer questioned the Trump administration’s motives.