But inside the health department, officials have complained that Redfield and CDC have been slow to resolve essential problems, like clarifying whether dozens of public health labs around the nation will soon have diagnostics capable of testing for coronavirus. Until then, the labs must currently mail samples to CDC’s Atlanta headquarters, delaying test results. Former Food and Drug Administration Commissioner Scott Gottlieb and some White House officials have warned that without sufficient screening capacity, possible cases of coronavirus might be going undetected.

Azar — Redfield’s boss — told a Senate panel Tuesday that the problem is being investigated. “The diagnostic works at CDC,” Azar stressed, adding that 12 labs around the nation are now using the tests, too. But dozens still are not.

Redfield on Wednesday told a House panel that his agency is working with FDA to release a new version of its test as soon as this week. But the CDC director initially told the panel that he did not know off-hand how many coronavirus samples CDC could screen per day. He clarified later in the hearing that the number is between 350 and 500.

Meanwhile, Trump officials across the administration have been frustrated by CDC staff’s caginess on revising preparedness plans that were supposed to be years in the making or at CDC staff who have produced conflicting data, such as different case counts over how many Americans have been infected. CDC officials also made the decision to support the Japanese health department’s quarantine of thousands of passengers aboard the Diamond Princess cruise ship, which was marooned for days off the coast of Japan, and disagreed with efforts to evacuate the ship even after it became clear that coronavirus infections were spreading aboard the vessel.

CDC also has repeatedly made unilateral decisions without informing other health officials, leaving the rest of HHS scrambling to catch up, such as holding one recent coronavirus briefing with outside groups before the slides had even been vetted. “HHS learned about that information at the same time that outside hospitals did,” said one official.

Nancy Messonnier, who leads CDC’s National Center for Immunization and Respiratory Diseases, on Tuesday also startled HHS and White House officials by elevating the perceived threat of coronavirus in a news briefing with reporters. “We are asking the American public to work with us to prepare for the expectation that this could be bad,” Mesonnier said in the briefing, before walking through possible steps such as encouraging schools to consider closing. Top Trump advisers have worried that panic over a still-unseen U.S. outbreak could threaten the president’s reelection, and Azar and other officials have spent the past two days working to calm the public reaction to Messonnier’s comments.

Nancy Messonnier, who leads CDC’s National Center for Immunization and Respiratory Diseases. | Samuel Corum/Getty Images

But Messonnier’s remarks, and her high profile during the coronavirus fight, speak to a reality at CDC: Unlike other parts of the Trump administration, where career civil servants have fled and political appointees have made over operations, the sprawling CDC operation and its global staff have largely retained their independence.

More than 80 percent of CDC scientists polled in 2018 said the agency continued to uphold its scientific principles under Trump, compared with less than 40 percent of scientists at the Environmental Protection Agency. Meanwhile, many longtime CDC staffers have played a key role in fighting the coronavirus outbreak, like principal deputy director Anne Schuchat and Messonnier, who’s become a target for right-wing talk radio because she’s the sister of former Deputy Attorney General Rod Rosenstein.

In front of a Senate appropriations panel on Tuesday, Azar defended the public-health agency for its round-the-clock response to a possible outbreak, arguing that it’s played an essential role in staving off an outbreak that remains largely theoretical in the United States.

“We are now, what, 50 days into it? This is historic,” Azar said. “No administration, no CDC in American history has delivered like this.”

Redfield on Wednesday also sought to quell concerns about an imminent outbreak, telling lawmakers that the risk to Americans remains low and people should continue with their daily lives. “We are encouraging people again to just think about being prepared,” Redfield said.

Adam Cancryn contributed to this report.