In Florida, children have to be vaccinated to attend public or private schools. There are two exceptions: parents can get a doctor to say a vaccine would be medically dangerous, or they can opt out of vaccines on religious grounds.

That religious exemption has been granted with greater and greater frequency in the past decade, according to state data reviewed by the Tampa Bay Times.

Between 2011 and 2018, religious exemptions shot up each year from about 6,500 students to almost 25,000 — an increase of about 375 percent over those years.

The single biggest year-to-year jump took place last year. In 2017, the state granted 19,729 religious exemptions. In 2018, that number rose by 25 percent to 24,768. (Exemptions can apply to students who received some, but not all, required vaccinations.)

University of Florida medical ethicist Bill Allen said the religious carve-out is not being used for its intended purpose. Instead, he believes it's being abused.

"If parents don't want to vaccinate because they think vaccines cause Autism, they'll claim a religious exemption," he said.

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Of course, there's no way to know the intention of every parent seeking the exemption. And although no major religion prohibits vaccinations, some smaller religious sects, such as the Church of Christ, Scientist, do.

But that almost certainly doesn't explain a 25 percent year-over-year spike in exemptions granted, experts say.

Much more likely, said University of South Florida Health assistant professor Jill Roberts, parents are claiming the exemptions after being persuaded by sophisticated online anti-vaccination misinformation groups.

"They put out books. They do speaking engagements. They sell videos, and it looks legit," Roberts said.

Those groups, with official-sounding names like the National Vaccine Information Center, propagate conspiracies like the debunked link between vaccinations and autism.

Numerous credible medical organizations, including the World Health Organization, the American Medical Association, the American Academy of Pediatrics and the Center for Disease Control and Prevention, agree that no such link has ever been proven.

The overwhelming medical consensus is that vaccines are safe, and that not vaccinating children creates a public health risk to them and to other children.

In the mid-20th century, measles infected millions of Americans every year. By 2000, vaccinations had entirely eliminated the disease from America.

But in recent years, as anti-vaccination sentiment has hardened, particularly in the Pacific Northwest, measles has seen an American resurgence. The Center for Disease Control has already reported 465 cases this year, putting America on pace for its highest level of cases in decades.

In late 2018, measles hit Pinellas County for the first time in 20 years when three unvaccinated people caught the disease.

Roberts said the growing frequency of measles outbreaks in America should cause Florida to reconsider the religious exemption. In 2015, California did just that.

"You really need to tend towards a no tolerance policy," Roberts said. "You really need to put the public health above these issues."

The religious carve-out for schools was first put into Florida law in 1971, long before any of the modern misinformation campaigns around vaccines took off on social media. Allen said the law was initially intended to be an affirmation of the First Amendment, which grants freedom of religion. For years, few took advantage of it: prior to 2011, the state granted a total of about 32,000 exemptions, data shows.

But a 1998 decision by the First District Court of Appeal in Tallahassee gave the religious exemption even greater weight: the court ruled that Florida officials are not allowed to question the sincerity of a requested religious exemption.

Florida could always get rid of the exemption entirely. However, that idea appears to have no traction in the state Legislature. Katherine Betta, a spokeswoman for Senate President Bill Galvano, R-Brandenton, said although Galvano "personally supports vaccinations," no bill has been filed this session that would end the religious exemption.

A spokesman for Florida House Speaker José Oliva, R-Miami Lakes, did not respond to requests for comment.

The good news from a public health perspective: even if the religious exemption serves as nothing more than a loophole for parents skeptical of vaccines, vaccination levels do remain high overall. Almost 94 percent of the state's kindergartners, and over 96 percent of seventh-graders, were vaccinated in the 2017-2018 school year, state data shows.

But it will likely prove difficult to sway the few anti-vaccine holdouts in Florida. California's recent history shows that even if the religious carve-out went away tomorrow, some parents would still look to get their children medically exempt from immunizations.

The only thing that could truly convince hardcore anti-vaxxers?

A huge outbreak of a deadly, infectious disease, Allen said.

"Until there's a crisis and something bad happens," he said "it's going to be hard to convince a lot of these people."

Contact Kirby Wilson at kwilson@tampabay.com or 727-893-8793. Follow @kirbywtweets.