Dr. Anthony Fauci on Sunday defended himself and the National Institutes of Health against charges from Sen. Rand Paul, R-Ky., that he misled the public about U.S. gain-of-function research at the Wuhan lab in China.

Speaking with George Stephanopoulos of ABC News' "This Week," Fauci once again denied that the NIH knowingly funded research on viruses that could have created COVID-19. Fauci said it was "molecularly impossible" for bat virus research in China to have produced COVID-19.

"There’s all of this concern about what's gain of function or what's not, with the implication that that research led to SARS-CoV-2, and COVID-19," Fauci said.

"Unequivocally, anyone that knows anything about viral biology and phylogeny of viruses knows that it is molecularly impossible for those viruses that were worked on to turn into SARS-CoV-2 because they were distant enough molecularly that no matter what you did to them, they could never, ever become SARS-CoV-2," he said.

Paul had called for Fauci to be fired and has consistently conflicted with Fauci and other health experts about COVID-19.

In the interview on Sunday, Fauci also defended the safeness of “mix and matching” the different vaccines when getting the booster shot.

“Would it be appropriate and safe and effective to get boosted in the third shot for the mRNA and the second shot for J&J by another product?” Fauci said.

“And the answer is, it's perfectly fine. We would hope that people, if available, would get the boost from the original product. But, if not, there's the flexibility of what we're calling mixing and matching, in other words, getting something other than the time of the first shot,” he added.