Dr. Walensky said the agency was finalizing guidance to deal with the problem.
At one point on Tuesday, Senator Christopher S. Murphy, Democrat of Connecticut, stepped in to defend the officials testifying.
We suffered through four years with a president who literally made things up about this virus, who simplified the story over and over and over again, he said. We still have a lot to learn. And so I, frankly, appreciate the fact that we have leaders today who recognize that we still have gaps in information who occasionally may err on the side of caution in order to save lives.
The hearing took several heated detours to address accusations that the National Institutes of Health, where Dr. Fauci is a top official, had supported research in a laboratory in Wuhan, China, where some top Trump administration officials maintained the novel coronavirus might have leaked from. Most scientists agree that the coronavirus most likely emerged in the natural world and spread to humans from animals.
Senator Rand Paul, Republican of Kentucky, who is known for sparring with Dr. Fauci, accused the governments top infectious disease expert of backing an American scientists collaboration with the Wuhan Institute of Virology. The institute houses a state-of-the-art laboratory known for its research on coronaviruses.
Dr. Fauci quickly shot down the suggestion about so-called gain of function research, saying the N.I.H. had never supported such work there.
Katie Rogers contributed reporting.