Dr. Anthony S. Fauci, the nation’s leading expert on infectious diseases, said that President Trump might be right that the experimental treatment he received and promoted has helped him in his fight with Covid-19 — but that his case alone doesn’t prove it.
“I think it’s a reasonably good chance that the antibody that he received, the Regeneron antibody, made a significant difference in a positive way in his course,” Dr. Fauci, who is not involved in the president’s care, said on Thursday during an interview on MSNBC.
He pushed back against Mr. Trump’s claim that the treatment has now been shown to be a “cure” for the disease, which has killed more than 210,000 Americans so far.
“When you have only one, you can’t make the determination that that’s a cure,” he said. “You have to do a clinical trial involving a large number of individuals, compared either to a placebo or another intervention.”
The manufacturer, Regeneron, has applied to the Food and Drug Administration for emergency approval of the experimental treatment, a cocktail of monoclonal antibodies.
Mr. Trumped received the Regeneron cocktail on Friday after he announced that he had tested positive for coronavirus. He has also received other treatments that are used for patients with severe cases of Covid-19, including remdesivir and dexamethasone.
In a five-minute video on Wednesday, Mr. Trump said that it was a “blessing from God” that he had been infected with the coronavirus and that the Regeneron cocktail had suddenly made him feel better. “I felt unbelievable,” he said. “I felt good immediately.”
The president said he would make sure that hundreds of thousands of doses would be available to Americans soon, free of charge, and he gave the impression that he would push the F.D.A. to approve the treatment. The company, however, has said that access to the treatment would be extremely limited at first, with only enough doses for 50,000 patients.
The president’s true condition is unclear. The White House doctor overseeing his care, Dr. Sean P. Conley, has admitted that he misled the public when Mr. Trump was hospitalized at Walter Reed National Military Medical Center.
In the televised interview, Dr. Fauci said it was known that a Covid-19 patient can feel good one day and go downhill quickly the next.
“The chances of that happening? I don’t know,” Dr. Fauci said of Mr. Trump’s case. “As good as he looks, I don’t think that’s going to happen, but I don’t know.”