WASHINGTON – Trump administration officials Wednesday defended a new recommendation that people without COVID-19 symptoms abstain from testing, even as scientists warned that the policy could hobble an already weak federal response as schools reopen and a potential autumn wave looms.

The day after the CDC issued the revised guidance, there were conflicting reports on who was responsible. Two federal health officials said the shift came as a directive to the Atlanta-based CDC from higher-ups in Washington at the White House and the Department of Health and Human Services.

Adm. Brett Giroir, the administration’s coronavirus testing czar, called it a “CDC action,” written with input from the agency’s director, Dr. Robert Redfield. But he acknowledged that the revision came after a vigorous debate among members of the White House coronavirus task force — including its newest member, Dr. Scott Atlas, a frequent Fox News guest and a special adviser to President Donald Trump.

“There was no weight on the scales by the president or the vice president or Secretary Azar,” Giroir said, referring to Alex Azar, secretary of Health and Human Services.

Regardless of who is responsible, the shift is highly significant, running counter to scientific evidence that people without symptoms could be the most prolific spreaders of the coronavirus. And it comes at a very precarious moment. Hundreds of thousands of college and K-12 students are heading back to campus, and broad testing regimens are central to many of their schools’ plans.

Democratic governors who were weighing how to keep the virus contained as their economies and schools come to life said limiting testing for asymptomatic citizens would make the task impossible.

“The only plausible rationale,” Gov. Andrew Cuomo of New York told reporters in a conference call, “is that they want fewer people taking tests because, as the president has said, if we don’t take tests, you won’t know the number of people who are COVID-positive.”

Over the weekend, the Food and Drug Administration, under pressure from Trump, gave emergency approval to expand the use of antibody-rich blood plasma to treat COVID-19 patients. The move came just days after scientists, including Dr. Anthony Fauci, the nation’s top infectious disease expert, and Dr. Francis Collins, director of the National Institutes of Health, intervened to stop the practice because of lack of evidence that it worked.

The move echoed a decision by the FDA to grant an emergency use waiver for hydroxychloroquine, a malaria drug repeatedly sold by Trump as a treatment for COVID-19. The agency revoked the waiver in June when clinical trials suggested the drugs’ risks outweighed any possible benefits.

The testing shift, experts said, was a far more puzzling reversal. Giroir said the move was “discussed extensively by” members of the White House coronavirus task force, and he named Redfield, Atlas, Fauci and Dr. Stephen Hahn, commissioner of food and drugs. Notably, he did not name Dr. Deborah Birx, the White House coronavirus response coordinator. But he said Fauci was among those who had “signed off.”

In a brief interview, Fauci said he had seen an early iteration of the guidelines and did not object. But the final debate over the revisions took place at a task force meeting Thursday, when Fauci was having surgery under general anesthesia to remove a polyp on his vocal cord. In retrospect, he said, he now had “some concerns” about advising people against getting tested, because the virus could be spread through asymptomatic contact.

“My concern is that it will be misinterpreted,” Fauci said.

The newest version of the CDC guidelines, posted Monday, amended the agency’s guidance to say that people who had been in close contact with an infected individual — typically defined as being within 6 feet of a person with the coronavirus and for at least 15 minutes — “do not necessarily need a test” if they do not have symptoms.

Exceptions might be made for “vulnerable” individuals, the agency noted, or if health care providers or state or local public health officials recommended testing.