A heated exchange between Dr. Anthony Fauci and Sen. Rand Paul emerged Thursday during a Senate committee hearing focused on the response to the coronavirus pandemic in the United States.

The clash came when Republican senator from Kentucky pressed Fauci, the director of the National Institute of Allergy and Infectious Diseases at the NIH and top adviser to President Joe Biden, over the use of masks and face coverings by individuals who have recovered from COVID-19 or received their doses of the vaccine already.

Guidance from the Centers for Disease Control and Prevention states individuals who have been fully vaccinated should still wear face coverings in public, in gatherings with others from outside their household, and when visiting another person who is unvaccinated and at high risk for severe illness from COVID-19.

During the meeting of the Senate Committee on Health, Education, Labor and Pensions, Paul repeatedly interrupted Fauci in his responses, accusing the doctor of engaging in “theater” by wearing two masks even though he has been vaccinated.

“You want people to wear a mask for another couple of years,” the senator said. “You’ve been vaccinated and you parade around in two masks for show. You can’t get it again, there’s virtually zero percent chance you’re getting it.”

“Let me just state for the record that masks are not theater — masks are protective,” Fauci said at one point after several interruptions, before being cut off again by the senator.

The leading infectious disease expert said when talking about the risks of reinfection, as Paul was, it is important not to neglect the presence of variants which are an “entirely different ball game” to the “wild-type” of the coronavirus — the background strain that contains no major mutations.

“If I vaccinate you or me against the wild-type you get a certain level of antibody that’s specific for a particular viral strain,” Fauci said. “If there’s a circulating variant, you don’t necessarily have it. You have some spillover immunity to be sure, but you diminish, by anywhere from two- to eight-fold, the protection. So the point I’m saying is that there are variants now circulating.”

The United States is now facing dominance of the B.1.1.7 variant, which was first detected in the United Kingdom, but there are others that remain concerning, too, he explained to the committee.

“We’re not dealing with a static situation of the same virus,” Fauci said.

Watch the full exchange, below:


Get Boston.com’s e-mail alerts:

Sign up and receive coronavirus news and breaking updates, from our newsroom to your inbox.