In response to angry, sometimes mocking questions from Republican senators, Fauci declined to answer more than 100 times.
Fauci also said he was surprised when Trump was booed by some of his supporters after he revealed he'd gotten his booster shot.
Fauci said shortening that recommended COVID isolation time is under consideration for those who are fully vaccinated or even boosted.
He said omicron appears to have taken over all other variants.
"If you want to be optimally protected, absolutely get a booster," he said.
"The question is, will we be prepared for it?" Fauci said.