Rand Paul forgoes mask, claims 'immunity' to COVID-19. Experts say he can't be so sure.

Rand Paul, the only senator known to have contracted COVID-19, defended his decision not to wear a mask on Capitol Hill on Tuesday, citing his "immunity" to a disease that top scientists are still trying to understand.

"I have immunity. I've already had the virus, so I can't get it again and I can't give it to anybody," Paul, R-Ky., told reporters, referring to his March diagnosis. "I can't get it again, nor can I transmit. So of all the people you'll meet here, I'm about the only safe person in Washington."

Full coverage of the coronavirus outbreak

Paul, who is a doctor, can't be so sure of any of those claims, according to experts and guidance provided by the Trump administration. Medical professionals, including Dr. Anthony Fauci, have said that while some type of immunity for recovered patients is probably likely, the extent — including the duration of the immunity and whether a recovered person can be reinfected — isn't known.

Paul's claim "flies in the face of expert guidance from organizations like the WHO and CDC ... who both very clearly said we don't know," said Dr. Vin Gupta, a pulmonologist and global health policy expertwho is an NBC News and MSNBC contributor. "It's the pinnacle of misinformation. It's not true. We have no evidence."

The Centers for Disease Control and Prevention says "the immune response, including duration of immunity, to SARS-CoV-2 infection is not yet understood." The CDC recommended in April that a face covering should be worn in public settings where social distancing is difficult to maintain.

Broadly speaking, some infections result in lifelong immunity (think chickenpox) and others will produce short-term immunity in recovered patients. Many experts believe some kind of immunity will come with recovery, but there are reports of recovered COVID-19 patients who have tested positive again after having tested negative.

Fauci, the nation's top infectious disease expert, said on Comedy Central's "The Daily Show" in March that some immunity is likely.

"We don't know that for 100 percent certain because we haven't done the study," he said. "But I feel really confident that if this virus acts like every other virus that we know, once you get infected, get better, clear the virus, then you will have immunity that will protect you against reinfection."

Despite uncertainties around immunity, Gupta said "the science is unequivocal on masks" because they protect the wearer and the people around them.

Download the NBC News app for full coverage and alerts about the coronavirus outbreak

Gupta said he was disheartened to see that Paul chose not to wear a mask upon the Senate's return to Washington this week. Paul owned an ophthalmology practice and performed eye surgery for 18 years, according to his Senate biography.

"If that isn't brash and arrogant enough, he then says he's immune when Dr. Fauci, WHO, CDC says hold on, wait a minute? Somehow he has information that no one else has privileged access?" he told NBC News. "It's really irresponsible."