Florida Governor Ron DeSantis signed into law last week what he called “the strongest piece of legislation that's been enacted anywhere in the country” against COVID-19 vaccine mandates. Businesses that violate the law by requiring workers to be vaccinated without allowing exemptions face fines of up to $50,000.

The Sunshine State is now the first in the US with such a law targeting private businesses, and yes, even hospitals, that introduce vaccine mandates for workers. And for no particular reason at all, let’s just remember that DeSantis opposed President Biden’s federal vaccine mandate back in September specifically on the grounds that issuing such orders to businesses and individuals was “fundamentally wrong” and “coercive”, saying that “threatened mandates and firing and business consequences ... alienates [people].”

Floridians wishing to work without getting the incredibly effective and safe vaccines have a few options: they can apply for a medical or religious exemption (and hope their employer doesn’t require them to swear off the Tylenol as well) or agree to undergo regular testing for COVID-19 infection – a get-out clause that was actually included in President Biden’s original vaccine mandate back in September.

But there’s another alternative – one that DeSantis claimed was “science-based”: the new law allows workers to avoid vaccination if they can offer their employer “proof of natural immunity.”

What this means in practice is not particularly clear – the law says that it’s up to the state’s Department of Health to “establish standards for competent medical evidence that the employee has immunity to COVID-19.” Testing for immunity using scientific methods is notoriously difficult – but given that said Department of Health is led by surgeon general Joseph Ladapo, the controversial physician known for spreading anti-vaccine misinformation and touting the supposed benefits of COVID-19 infection, it’s likely that a prior infection will be all it takes to duck the mandate.

DeSantis isn’t the only politician seeking to cast “natural immunity” as a pandemic panacea: in late September, a group of 15 Republican doctors, dentists, and pharmacists in Congress sent a widely-criticized letter to the Centers for Disease Control and Prevention (CDC) urging them to take immunity gained from prior infection into account when setting vaccine policies. More recently, President Jair Bolsonaro of Brazil used the same idea to once again avoid taking his medicine, declaring his “immunity levels … through the roof” thanks to a previous infection.

Here's the problem. 

“Natural immunity” is certainly a thing, but it’s highly unpredictable. The amount of protection gained from a prior infection can vary depending on many things: how sick you got, how long ago it was, and which particular variant you happened to contract.

“[M]ost 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,” Johns Hopkins epidemiologist Chris Beyrer told CNBC Make It.

“It may be that you had coronavirus last year and you feel like, therefore I don’t need a vaccine,” he explained. “But that natural immunity is unlikely to protect you against Delta.”

More than that, experts widely agree “natural immunity” simply isn’t that good compared to vaccination. A recent CDC study found that people relying on a prior COVID-19 infection to protect them were five times more likely to end up hospitalized with COVID-19 symptoms than those who got vaccinated. On the other hand, those who survived COVID-19 and then got vaccinated have been found to develop “hybrid immunity” – a super-potent immunity, capable of mounting antibody levels 100 times higher than infection alone.

Despite nearly two-thirds of Americans being in favor of vaccine mandates, they have been met with hostility by Republican lawmakers across the country. Like Florida, Utah recently signed into law an exemption from vaccination mandates for those with previous COVID-19 infection, with GOP politicians in New Hampshire and Idaho signaling their intentions to follow suit.

Along with vaccine mandates, Florida’s new law prohibits mask mandates in schools and quarantining students exposed to COVID-19. The state is already one of the 26 suing the White House over the federal vaccine mandate, and DeSantis announced last week that Florida would be joining another lawsuit aiming to block vaccine mandates on healthcare workers.

“Many nurses could lose their job,” DeSantis said of the mandate. “So you're actually making the hospitals short-staffed as a result of the mandate.”

“So how does this,” — he said, referring to the act of removing the people most likely to spread a deadly infection from a place full of people most likely to die of it — “make any sense?”