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.