"Natural immunity" has been a hot topic since the start of the pandemic, and it's coming up again as Republicans challenge President Biden's vaccine mandate for businesses with at least 100 employees.

In Florida, for example, Gov. Ron DeSantis signed legislation last week that bans businesses from imposing Covid vaccine mandates — including an exemption specifically for people who had Covid and recovered.

"We recognize people who have natural immunity," DeSantis said in a press conference Thursday. "Whatever a private employer wants to do, you're automatically exempt because of natural immunity."

If you've recovered from Covid, you do have a degree of immunity against the virus — but the amount can vary significantly, based on the severity of your illness and how long ago you recovered.

Currently, there's no known way to test your immunity levels. The U.S. Food and Drug Administration says antibody tests, which can determine whether or not you've had Covid, are unreliable judges of your immunity — because antibodies alone don't fight against Covid. Memory B cells and T cells also play a significant role in helping your immune system fight against infections.

And even if you have lots of natural immunity, getting vaccinated significantly boosts your chances of staying healthy, according to data published last month by the Centers for Disease Control and Prevention. The agency studied more than 7,000 people, and found that unvaccinated people who'd recovered from Covid were five times as likely to catch it again, compared to people who got two doses of an mRNA vaccine from Pfizer or Moderna.

To date, only 59% of the U.S. population is fully vaccinated against Covid, according to the CDC.

The development of Covid variants makes vaccination particularly important for people who've recovered from the virus. Currently, the delta variant is dominating cases in the U.S. — but "most of the people who were infected in 2020, and all the way through until the middle of 2021 were infected with earlier variants of the coronavirus," Dr. Chris Beyrer, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, tells CNBC Make It.

"It may be that you had coronavirus last year and you feel like, therefore I don't need a vaccine," he adds. "But that natural immunity is unlikely to protect you against delta."

Combining vaccination with natural immunity gives you a "hybrid immunity," which provides even more protection than vaccination alone — sending your antibody levels to "astronomical levels that outpace what you get from two doses of vaccine alone," Rishi Goel, an immunologist at the University of Pennsylvania, told Nature last month.

Experts also say all three of the country's approved Covid vaccines are safe for people who've already recovered from the virus. "There's been no evidence at all of any harm associated with Covid vaccination if you've already had, or you suspect that you've had Covid," Beyrer says.

If you fall into that category, you may experience more side effects from the Covid vaccines — like fever, chills and tiredness — because your immune system has already been exposed to the virus. That's actually a good thing: It means your body is mounting a faster response to the vaccine.

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