TWO BASTIONS of public health and scientific expertise, the Centers for Disease Control and Prevention and the Food and Drug Administration, should be at the forefront of public trust in battling the coronavirus pandemic. But in the past week, both agencies, and their leaders, allowed themselves to be undermined and humiliated by President Trump and his reckless management of the pandemic response.

In both cases, the core difficulty is that Mr. Trump remains in denial about the toll of a pandemic that has taken the lives of more than 178,000 Americans. He clings to wishful notions that the virus will vanish or a miracle cure will appear, and these fallacies generate negligence and errors by people who should know better.

This was on display on Sunday evening when Mr. Trump called a White House news conference about the FDA’s emergency-use authorization for convalescent plasma. The FDA commissioner, Stephen Hahn, declared that those treated had experienced “a 35 percent improvement in survival,” which he called “a pretty substantial clinical benefit.” He added, “What that means is — and if the data continue to pan out — 100 people who are sick with covid-19, 35 would have been saved because of the administration of plasma.” The president crowed, “It’s a tremendous number” and “a very historic breakthrough.” But Dr. Hahn later admitted he had overstated the benefits. He had confused relative risk reduction, between two different groups of patients, with absolute survival improvement. Such embarrassing mistakes can sink public confidence at a time when trust is absolutely essential for the success of a vaccine that will face FDA scrutiny and approval. Dr. Hahn’s error was strike two for the FDA after the White House earlier demanded and won an emergency-use authorization for the antimalarial drug hydroxychloroquine, which has proved useless in fighting the virus. The FDA later withdrew it.

Later this week came another political lightning bolt from Mr. Trump’s White House coronavirus task force, pressing the CDC to publish guidance saying that asymptomatic individuals — those without symptoms — who have been in close contact with an infected person “do not necessarily need a test.” Previous guidance said everyone in such situations should be tested for the coronavirus. Asymptomatic individuals may account for 40 percent of those infected, and can spread the disease. Experts agree that tests need to be thoughtfully prioritized, given that they remain in woefully short supply — another consequence of Mr. Trump’s failed response. But the new guidance sent a confusing signal about testing, suggesting that it may be unnecessary. This runs counter to the bedrock public health advice that widespread testing, contact tracing and isolation are key to virus control. However, less testing dovetails with Mr. Trump’s dreamy tweets that the high U.S. caseload is caused by so much testing. The CDC director, Robert Redfield, should have protested this ill-advised change. On Friday, he modified the guidance again, saying “testing may be considered for all close contacts” of an infected person. This is extremely poor risk communication, which is an essential part of maintaining public confidence.

It is a sorry day when the CDC and FDA become campaign stage props. These guardians of public health and well-being should be resisting Mr. Trump’s magical thinking, not allowing him to tarnish their credibility.

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