President Joe Biden's top health advisor—Dr. Anthony Fauci—and Kentucky Republican Senator Rand Paul got into a heated exchange again during a Thursday Senate committee hearing on the pandemic that's killed over 500 thousand Americans.

Paul—one of many Republicans who's railed against basic safety precautions to slow the spread of the virus—accused Fauci of forcing the public to wear masks, dismissing the effective measure as little more than "theatre."

As vaccinations for the virus are steadily administered, Fauci and other health experts have warned that masks are still necessary as the majority of the public remains unvaccinated and variant strains of the virus grow increasingly dominant.

Fauci quickly rebuked Paul's accusations.

After Paul oversimplified "everything we know about immunity," Fauci said:

"Here we go again with the theatre, let's get down to the facts."

The doctor went on to correct Paul's assertions, using the studies Paul himself cited and emphasizing the difference between infections of the variant strains and the original strain.

Fauci went on to say:

"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 that are now circulating."

While data so far indicates that the three FDA approved vaccines in the U.S. protect against variants, the scope of that protection is yet to be fully revealed and new strains will continue to emerge.

In a study from The New York Blood Center published last week in Science Magazine, Doctors Christopher D. Hillyer and Larry Luchsinger wrote:

"As variant strains emerge, we will need to reevaluate vaccine efficacy by testing the inhibition of viral infection in vivo rather than by quantifying the antibodies produced after in vitro exposure. Reliable proof of immunity through vaccination may only come through reinfection challenge experiments or through longitudinal studies of postvaccination subjects."

The vast majority of people were quicker to believe epidemiology expert Dr. Fauci over Senator Paul, an eye doctor.

For many, Paul's credibility was shot even before his latest confrontation with Fauci.

Paul has repeatedly refused to wear a mask since being infected with the virus last March, falsely believing he has permanent, blanket immunity to the virus and its variants.