WASHINGTON — President Trump said on Wednesday that the White House “may or may not” approve new Food and Drug Administration guidelines that would toughen the process for approving a coronavirus vaccine, and suggested the plan “sounds like a political move.”
The pronouncement once again undercut government scientists who had spent the day trying to bolster public faith in the promised vaccine. Just hours earlier, four senior physicians leading the federal coronavirus response strongly endorsed the tighter safety procedures, which would involve getting outside expert approval before a vaccine could be declared safe and effective by the F.D.A.
The president’s comments, to reporters in the White House briefing room, came after the doctors told a Senate panel that they had complete faith in the F.D.A., and that science and data — not politics — were guiding its decisions. Last week, Mr. Trump used the same setting to declare that the director of the Centers for Disease Control and Prevention had “made a mistake” when he said most Americans would not complete the vaccination process until next summer and that masks were at least as important as a vaccine to control the virus’s spread.
The F.D.A. had planned to issue stricter guidelines for the emergency authorization of any new coronavirus vaccine, which would add a new layer of caution to the vetting process, even as the president has insisted a vaccine will be ready as early as next month. Mr. Trump, though, cast doubt on the F.D.A. plan.
“That has to be approved by the White House,” he said, adding, “We may or may not approve it.” Raising questions about why vaccine makers would want to delay the process, he said, “We are looking at that, but I think that was a political move more than anything else.”
He pointedly said he had “tremendous trust in these massive companies” that are testing the vaccines, adding, “I don’t know that a government as big as” the federal government could do as well.
At Wednesday’s Senate hearing, the doctors — Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, Dr. Stephen M. Hahn, the commissioner of the Food and Drug Administration, Dr. Robert R. Redfield, the C.D.C. director, and Admiral Brett P. Giroir, the coronavirus testing czar — defended their scientific integrity amid mounting evidence that Mr. Trump and his administration have interfered with their agencies’ decision-making and growing public doubts about a vaccine.
In a show of public support that would have been unnecessary in the pre-Trump era, all four officials pledged to take any vaccine approved by the F.D.A. and said they would encourage their families to do the same.
Six weeks before Election Day and a day after the United States surpassed 200,000 virus-related deaths, the doctors spent much of the hearing fending off misinformation spread by conservatives in the highest reaches of government and criticism from liberals that could be undermining faith in the government’s work.
At one point, Senator Rand Paul, Republican of Kentucky, upbraided Dr. Fauci for favoring an “economic lockdown” — Dr. Fauci insisted he had said no such thing — and suggested, without evidence, that spread of the virus had slowed in New York because “they have enough immunity to actually stop it.”
Dr. Fauci shot back that the senator was wrong. “This happens with Senator Rand all the time,” he said, mistakenly using Mr. Paul’s first name. About 22 percent of New Yorkers are immune to the virus, Dr. Fauci said, adding sharply, “If you believe 22 percent is herd immunity, I believe you’re alone in that.”
But Democrats played their own part in questioning government science. Senator Tim Kaine, Democrat of Virginia, pressed Dr. Redfield on why his agency took down guidance acknowledging for the first time that the coronavirus was spread mainly by air, which the C.D.C. said had been “posted in error” on its website. Dr. Redfield conceded that some information in the guidance was accurate, but said the document was only a “first draft” that had not adequately vetted by agency scientists.
Mr. Kaine insinuated, without evidence, that political pressure was at work.
“When you put up a document at the C.D.C. that you have just testified is accurate, and then it’s changed to suggest that the risk is more minimal by someone for some reason, it contributes to the massive confusion that is so troubling,” he said, adding, “And that leads to, ‘Gosh, well is the vaccine going to be safe?’”
Polls show a troubling drop in the number of Americans who would be willing to take a coronavirus vaccine. A survey published last week by the Pew Research Center found that 51 percent of Americans would either probably or definitely take a vaccine, down from 72 percent in May.
Each of the officials who testified Wednesday acknowledged that public faith in his agency had been shaken.
“Every one of the decisions we have reached has been made by career F.D.A. scientists based on science and data, not politics,” Dr. Hahn told the panel, adding, “F.D.A. will not permit any pressure from anyone to change that. I will fight for science.”
Mr. Trump has insisted a vaccine will be ready as early as next month. That seems increasingly unlikely, and Dr. Redfield said again on Wednesday that immunizing every American could take until July 2021, a timeline Mr. Trump said last week was wrong.
The four officials returned to Capitol Hill after a recent period of turmoil inside the Department of Health and Human Services, the parent agency that oversees their work. First, there were revelations that Trump loyalists inside the department, including Michael R. Caputo, its top spokesman, tried to meddle with the C.D.C.’s weekly scientific reports.
Then Mr. Caputo took medical leave last week after a rant on Facebook in which he accused C.D.C. scientists of engaging in “sedition.” After that came a report in The New York Times that C.D.C. guidance about coronavirus testing, which suggested certain people exposed to the virus did not need to be screened, was not written by agency scientists and posted to its website despite their serious objections. The agency reversed the recommendation last week after widespread criticism.
On Wednesday, Dr. Redfield insisted to the senators that agency scientists were indeed involved, though he acknowledged that the guidance was the product of the White House coronavirus task force, including Dr. Giroir.
Without naming Mr. Caputo, he told senators that remarks from the Department of Health and Human Services suggesting that there was a “deep state” inside the agency were “offensive to me.” And he defended the agency’s scientists, likening them to military people who never disclose their political leanings at work.
Dr. Redfield said that preliminary results from a study on the prevalence of Covid-19, the disease caused by the coronavirus, “show that more than 90 percent of population remains susceptible” to the illness. And Senator Lamar Alexander of Tennessee, the chairman of the committee, urged Congress to start planning for the next pandemic, warning that experts say that one could come as soon as next year. “We must act now to stop the cycle of panic, neglect, panic,” he said.
Federal scientists, Dr. Fauci said, are beginning to study so-called long-haulers — people who experience long-term side effects of coronavirus infection, including fatigue, fever, neurological problems and cognitive abnormalities. In addition, scientists have found “a disturbing number of individuals” who appeared to have recovered have inflammation of the heart, he said.
“These are the kinds of things that tell us we must be humble and we do not completely understand the nature of this illness,” Dr. Fauci said.