SACRAMENTO, Calif. » California lawmakers on Tuesday approved a hotly contested bill that would impose one of the strictest vaccination laws in the country, after five hours of highly emotional testimony that brought hundreds of opponents to the Capitol.

SB277 is intended to boost vaccination rates after a measles outbreak at Disneyland that sickened more than 100 in the U.S. and Mexico. It has prompted the most contentious legislative debate of the year with thousands of opponents taking to social media and legislative hearings to protest the legislation.

The Assembly Health Committee approved the legislation 12-6 Tuesday evening with one lawmaker abstaining, sending it to the full Assembly for its final legislative hurdle.

If the bill becomes law, California would join Mississippi and West Virginia as the only states with such strict requirements.

Yet despite impassioned, ongoing pleas from parents seeking to maintain medical choice, a large portion of those who obtain personal belief exemptions are not fundamentally opposed to vaccination, said Dr. Mark Schleiss, a pediatrician specializing in infectious disease at the University of Minnesota.

Schleiss, who hasn’t taken a position on the California bill, said most parents of unvaccinated children want to learn more and better understand the issues. Some parents, he said, simply find it more convenient to sign the back of a form or only partially vaccinate their children.

"I don’t see the majority of parents being so committed to withholding vaccinations that their minds wouldn’t be changed," he said. "I think this will have an impact."

The bill, sponsored by Democratic Sens. Richard Pan of Sacramento and Ben Allen of Santa Monica, would only allow children with serious health problems to opt out of school-mandated vaccinations. School-age children who remain unvaccinated would need to be home-schooled.

It would apply to elementary schools, secondary schools and day care centers.

Hundreds of passionate opponents descended on the Capitol Tuesday, and parents spilled into the hallways as they waited hours to testify. One woman was removed from the committee hearing after shouting at lawmakers.

The bill’s supporters sought to dispel claims that measles no longer poses a threat, while a larger number of critics focused on potential risks associated with vaccines and told lawmakers the proposal was an unnecessary government overreach.

Santa Monica pediatrician Dr. Jay Gordon, testifying against the bill, sought to separate the bill from the December measles outbreak, noting that all those cases of measles occurred outside a school environment.

"If SB277 had been in place last year, this outbreak would have proceeded in much the same manner," he said.

Dr. John Swartzberg, a clinical professor specializing in immunology at University of California, Berkeley, who also hasn’t taken a position on the measure, said in an interview that eliminating the personal belief exemption would be a silver bullet for California to strengthen its defenses against the spread of communicable disease.

Data from the California Department of Public Health show that the number of personal belief exemptions for incoming kindergarteners rose every year between 2010 until 2014.

But the tide turned in 2014 after the state overhauled its vaccine requirement. That measure, requiring parents to obtain a signed waiver from their child’s physician before claiming a personal belief exemption, was inconvenient enough to sway those who were "vaccine hesitant," Swartzberg said.

The percentage of claimed personal belief exemptions fell by 19 percent, settling about 2.5 percent overall.

"That was the first evidence that using laws to impact behavior would actually be effective," Swartzberg said.

Researchers believe SB277 would help the state reach what immunologists call "herd immunity," the percentage at which enough people are vaccinated to protect the whole community.

The California Department of Public Health has been hesitant to draw any conclusions from the 2014 immunizations rate increase.

According to the Centers for Disease Control and Prevention, herd immunity for measles is between 92 and 94 percent. In 2014, 92.6 percent of California kindergarteners received the measles vaccine, but a number of suburban pockets outside major cities have vaccination rates far lower.

Opposition has been especially passionate.

Echoing previous remarks likening the bill’s authors to Nazis, Assemblyman Jim Patterson, R-Fresno, at the rally compared keeping unvaccinated children at home to putting them in internment camps.

He later backed off that comparison. "Obviously a poor choice of words on my part today," he wrote on Twitter.