California Gov. Jerry Brown just signed into law a controversial bill that will require almost all schoolchildren in the state to be vaccinated — unless they have a medical reason for opting out.

The law means parents can no longer refuse to vaccinate their children for religious or philosophical reasons.

The bill, SB277, was a response to a large measles outbreak that originated at the Disneyland theme park in the state in December. Since then, more than 160 people across the United States have been diagnosed with the disease (the majority of them in California). The mandate has faced fierce opposition from anti-vaccine groups, which have been campaigning against the bill since it was introduced in February.They say a vaccine mandate is unconstitutional and overrides parents' rights to make decisions for their kids.

Gov. Brown said he signed the bill in the interest of public health, making California the third and biggest state with such a mandate, after Mississippi and West Virginia.

California's law requires kids to get vaccinated unless they have a medical exemption

The story of SB277 started last February: Following the Disneyland measles outbreak, California lawmakers introduced legislation that would require children to receive mandatory vaccines before starting school. Under the law, the only people who can opt out are those who have a medical reason, such as an allergy or a disease that would make them unfit for the shots. And unvaccinated kids can only be homeschooled.



In April, the bill passed the state Senate health committee by a 6-2 vote. "I've personally witnessed the suffering caused by vaccine-preventable diseases, and all children deserve to be safe at school," Democratic Sen. Richard Pan, the bill's co-author and a pediatrician, said in a statement. (Benjamin Allen, another democratic senator, was the bill's other author.)

On May 14, SB277 made it through a final vote on the Senate floor. Last Thursday, the general assembly approved the mandate on a bipartisan vote of 46-31, and today Gov. Jerry Brown signed the bill — its final step.

"The science is clear that vaccines dramatically protect children against a number of infectious and dangerous diseases," Brown wrote in a statement. "While it's true that no medical intervention is without risk, the evidence shows that immunization powerfully benefits and protects the community."

California will now be the biggest state with such a mandate, and only the third — after Mississippi and West Virginia — without a religious and personal-belief vaccine exemption.

There's fierce opposition to California's bill

The bill wasn't guaranteed to make it past the governor's desk. Similar efforts to end the personal-belief exemption failed in Oregon and Washington. And the California measure has been the subject of emotionally charged debate.

Ahead of last week's assembly vote, one opposition group paid for robocalls from a woman claiming to be a mother and saying the bill would "take away our right to make important medical decisions for our children," according to local news.



Another group, A Voice for Choice, ran highly charged advertisements in the Sacramento Bee newspaper depicting the tombstones of children who died from "lethal vaccination."



As one of the group's members said in the Los Angeles Times, "We are pulling out all the stops. This bill is unconstitutional." On the bill's signing today, another opponent said in the Times, "I'm heartbroken... It's so coercive. It's so punitive."

Supporters of SB277, including Gov. Brown, argued the measure is necessary to protect public health.

There is no evidence-based reason to refuse or delay vaccines

Vaccines are overwhelmingly safe, and documented harms from them are infinitesimally remote. The reason for this push for better vaccine uptake has to do with herd immunity: you need to have a certain level of vaccine coverage in a population in order to stop a disease from spreading. Vaccinated people help prevent diseases from moving through a population and protect those who can't get vaccinated, such as infants or people with medical exemptions.



Researchers have pointed out that the government-approved schedule is appropriate, based on when kids are most at risk for diseases and when their immune systems are most receptive to them.



Even so, there has been a growing "vaccine delayer" movement, whereby parents are increasingly creating their own vaccine schedules — delaying some vaccines and refusing others based on beliefs about the harms of shots, particularly that kids get "too many, too soon."



"There is just no science to [delaying vaccines]," Dr. Doug Opel, a Seattle pediatrician who studies vaccines, told me. "What gets lost a lot of the time is that there's an incredible amount of data underlying the recommended schedule."

To read more about the parents who are opting to create their own vaccine schedules, see our feature "The vaccine delayers."



In California, the state-level vaccination rates are over 90 percent — but there are pockets of resistance in some schools and communities, where the opt-out rate can be as high as 50 percent.



The people who oppose these measures invoke their own facts and a personal-choice narrative. As one of the bill's detractors, Republican Sen. Jim Nielsen, told the Sacramento Bee: "I have very profound feelings about parental rights and responsibilities and great dismay in American society over the decades how much that parental right, that parental responsibility, has diminished."



The good news, however, is that these parents are still in the minority in the United States. Though vaccine delayers outnumber outright vaccine deniers (a 2011 survey of parents of young kids found that more than one in 10 used an alternative vaccine schedule), the overwhelming majority of parents chose to vaccinate their kids right on time.