Sen. Rand Paul and Dr. Anthony Fauci had the latest in their series of tense exchanges Thursday, when the director of the National Institute of Allergy and Infectious Diseases appeared before the Senate Health, Education, Labor & Pensions Committee.

Paul has been a fierce critic of Fauci, accusing him of falsely denying that the government has funded "gain-of-function" research at the Wuhan Institute of Virology, where Paul believes COVID-19 likely originated. Fauci has denied this and continued to do so in no uncertain terms.

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"Dr. Fauci, I don't expect you today to admit that you approved of NIH funding for a gain of function research in Wuhan, but your repeated denials have worn thin," Paul told Fauci as a precursor to his questioning, stating that the National Institutes of Health admitted that it funded a grant to EcoHealth Alliance with a sub-award to the Wuhan lab. Paul said that as part of this work, it engaged in experiments in Wuhan that led to the creation of viruses that do not occur in nature and increased in deadliness.

"The facts are clear. The NIH did fund gain of function research in Wuhan despite your protestations," continued Paul, a Republican from Kentucky, claiming that Fauci’s "persistent denials … are not simply a stain on your reputation but are clear and present danger to the country and to the world."

Sen. Rand Paul, a Republican from Kentucky, speaks during a Senate Health, Education, Labor, and Pensions Committee hearing in Washington, D.C., on Thursday, Nov. 4, 2021.  (Al Drago/Bloomberg via Getty Images)

Fauci has argued, and did so again later in the hearing, that the viruses studied at Wuhan that the NIH funded "could not possibly have turned into SARS-CoV-2," but Paul noted that this argument was misleading because "no one is alleging that."

What Paul did say was that the research in Wuhan "could cause a pandemic even worse the next time." He also claimed that COVID-19 could have been created from a virus that Chinese officials have not disclosed.

"Will you today finally take some responsibility for funding gain of function research in Wuhan?" Paul asked.

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"Senator, with all due respect, I disagree with so many of the things that you've said," Fauci said to begin his response. He went on to say that "gain of function" is "a very nebulous term" that outside parties have worked on assigning "a more precise definition."

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, testifies during the Senate Health, Education, Labor and Pensions Committee hearing titled Next Steps: The Road Ahead for the COVID-19 Response, in Dirksen Building in Washington, D.C., on Thursday, Nov. 4, 2021.  (Tom Williams/CQ-Roll Call, Inc via Getty Images)

Up until recently, the NIH website had a section that discussed gain of function research, providing a broad definition of "a type of research that modifies a biological agent so that it confers new or enhanced activity to that agent."

On Oct. 20, the NIH removed that section from its website, replacing it with one that discusses "enhanced potential pandemic pathogen" research, which it defined as "research that may be reasonably anticipated to create, transfer or use potential pandemic pathogens resulting from the enhancement of a pathogen’s transmissibility and/or virulence in humans."

Paul accused Fauci and the NIH of "defining away gain of function … saying it doesn't exist because you change the definition" on the site.

"There's the preponderance of evidence now points towards this coming from the lab and what you've done is change the definition on your website to try to cover your a--," Paul asserted.

Fauci again said the current definition of gain of function research was not created by the NIH but by outside bodies, but Paul continued to charge that Fauci was evading responsibility and failing to acknowledge that the government was participating in risky activity.

"You won't admit that it's dangerous. And for that lack of judgment, I think it's time that you resign," Paul said.

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Fauci fired back that Paul had made an "egregious misrepresentation" with what he said, including his claim that COVID-19 likely leaked from the Wuhan lab. He said that while "we leave open all possibilities, it's much more likely that this was a natural occurrence."

Fauci also responded to Paul’s assertion that the NIH continued to support research at the Wuhan lab. He insisted that this was not the case, although Paul claimed that the NIH approved it in August 2020 and that Chinese officials said it was still true a month ago. Fauci stated that it is no longer the case.