Months after a virulent measles outbreak spread from one Disneyland visitor to more than 100 California residents, the state that’s been described as an “anti-vaccination hotbed” is poised to pass one of the nation’s toughest laws requiring immunizations.

The bill approved by the California Assembly Thursday strikes down the state’s personal belief exemption, which allows parents to opt their children out of vaccinations for religious reasons. Under the new rules, only children with serious health problems (such as a compromised immune system) would be exempt from mandatory vaccination schedules, and those who opt out will have to be homeschooled.

The legislation now goes to California Gov. Jerry Brown (D). According to the Los Angeles Times, it’s unclear if Brown will sign it.

“The governor believes that vaccinations are profoundly important and a major public health benefit, and any bill that reaches his desk will be closely considered,” Evan Westrup, Brown’s spokesperson, told the Times Thursday.

If the bill becomes law, California will become only the third state in the country to deny exemptions based on religious convictions. The other two are Mississippi and West Virginia.

But in California, many if not most of those who opt out of the state’s mandatory vaccination schedules do so on the basis of “personal belief exemptions,” rather than religious ones. Though decades of studies have found vaccines to be safe, strains of vaccine skepticism are particularly high in some California communities. In January, the Post’s Wonkblog found that in some school districts the fraction of kindergartners with PBES is more than half. At a handful of private schools it topped 75 percent.

Thirty-two other states bar vaccination exemptions based on personal or moral beliefs.

The debate about mandated vaccinations has the political world talking. A spike in measles cases nationwide has President Obama, lawmakers and even potential 2016 candidates weighing in on the vaccine controversy. (Pamela Kirkland/The Washington Post)

The debate about the vaccination bill has been vitriolic in recent weeks. According to the LA Times, Sen. Richard Pan (D-Sacramento), a pediatrician and an author of the bill, has received death threats. Protesters at the California Democratic Party’s convention in Sacramento last month called the right to refuse vaccinations “a civil rights issue” and compared the state to Nazi Germany.

Hundreds of parents who converged on the Assembly this week said that the legislation would take away their right to make decisions about what’s best for their child’s health. And Assemblyman Mike Gatto (D-Glendale) told the LA Times that the bill was likely unconstitutional because it “infringes on the rights of children to attend school.”

For their part, proponents of the legislation were not above using tear-jerker tactics to support their cause. On Wednesday, advocates for stricter requirements stood beside 7-year-old leukemia survivor Rhett Krawitt as he delivered a petition bearing 30,000 signatures to the governor. Earlier this year, Krawitt gave a rousing speech at a local school board meeting in which the first grader said “I give a damn” about mandatory vaccinations. Because his own immune system has been compromised by chemotherapy, Krawitt’s parents say, he can’t be vaccinated, leaving him vulnerable to measles and other infectious diseases.

The issue of vaccinations is so charged because it pits personal choices against an epidemiology principle known as “herd immunity.” As the Department of Health and Human Services explains it:

When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease because there is little opportunity for an outbreak. Even those who are not eligible for certain vaccines—such as infants, pregnant women, or immunocompromised individuals—get some protection because the spread of contagious disease is contained.

When the level of immunization drops low enough, diseases like measles, which is highly contagious, can spread rampantly among the unprotected population. While healthy unvaccinated kids usually survive a bout of measles (and it’s worth noting that there have been no fatalities reported so far from the Disneyland outbreak), people with weakened immune systems — the elderly, children under five, immunocompromised people like Krawitt — are vulnerable to severe and sometimes fatal complications.

“Measles is not a benign disease,” Tara C. Smith, an epidemiologist at Kent State University, wrote in Slate earlier this year.

Ariel Loop, a Pasadena mother whose 4-month-old son Mobius was infected during the Disneyland measles outbreak, told the LA Times she was relieved that the bill got approved.

“I’m hoping Jerry Brown does the right thing and signs it once it gets through the last Senate [vote],” Loop said.