White House chief medical advisor Dr. Anthony Fauci pushed back on Thursday against Republican Sen. Rand Paul's claim that people are not at risk of Covid after they have recovered or have been vaccinated.

In a fiery exchange during a Senate hearing examining the nation's coronavirus response efforts, Paul told Fauci that Americans shouldn't have to wear masks after getting vaccinated because there is "virtually 0% chance" they are going to get Covid-19.

"Isn't it just theater?" the Kentucky junior senator, an ophthalmologist, asked during a Senate Health, Education, Labor and Pensions Committee hearing.

"You've been vaccinated and you parade around in two masks for show. You can't get it again," Paul said. "There's virtually 0% chance you're going to get it and you're telling people that have had the vaccine who have immunity — you're defying everything we know about immunity by telling people to wear masks who have been vaccinated."

In response, Fauci said, "Here we go again with the theater."

"Can I just state for the record that masks are not theater," Fauci said. "I totally disagree with you."

The emergence of new, highly contagious variants poses a threat to people who have recovered from Covid or who have been vaccinated, he said.

New variants, particularly the B.1.351 strain first identified in South Africa, have been shown to evade the protection of vaccines.

"In the South African study conducted by [Johnson & Johnson], they found that people who were infected with wild type and were exposed to the variant in South Africa, the 351, it was as if they had never been infected before, they had no protection," Fauci said.

Fauci agreed it was unlikely someone would get infected with the original strain for at least six months, "But we in our country now have variants."

The exchange came a little over a week after the Centers for Disease Control and Prevention released new guidance that said people who are fully immunized can safely visit other vaccinated people indoors without wearing a mask or social distancing.

But the CDC also recommended that vaccinated people should still wear masks in public settings, gatherings with unvaccinated people from more than one other household and with people who are at increased risk for severe illness.

While a growing body of evidence suggests that people who are inoculated against Covid are less likely to transmit the disease to others, it's still not known how long someone's protection might last or the effectiveness of the shots against emerging variants, the CDC said on March 8.