Disney likes to remind visitors that its theme parks are where “dreams come true.” But events there this past December sparked a serious wake-up call. Lurking among the fantastical floats and rides of Disneyland was the measles virus, which ultimately infected 111 visitors. To prevent such infections in the future, California State Sen. Richard Pan (D), a pediatrician, has proposed a bill that would eliminate vaccine exemptions based on personal beliefs.

The bill, SB 277, would bar children who are unvaccinated for nonmedical reasons from attending public or private schools. Currently, the personal belief exemption serves as a loophole that allows parents to choose not to vaccinate their children for essentially any reason, including ones founded on misinformation. Fewer than half of all states in the U.S. grant such exemptions, and several of those that do have recently engaged in efforts to tighten their opt-out requirements. These efforts have been met with formidable resistance, particularly from parents, as the failures of similar bills in Washington State and Oregon demonstrate. In California, however, the Disneyland measles event and other recent outbreaks of vaccine-preventable diseases may provide the edge needed to bring about change in the state.

According to public health experts, at least 92 percent of a population must be vaccinated in order to maintain the “herd immunity” needed to protect those who cannot be inoculated, including very young children and people with compromised immune systems due to illnesses such as leukemia. But over the past 12 years the percentage of families using the personal belief exemption in California has increased greatly, resulting in the loss of herd immunity in more than a quarter of schools in the state as of 2014.

After stalling last Wednesday when the Senate Education Committee raised concerns that the bill may prevent some children from receiving an adequate education, the committee passed it 7–2 yesterday. Next, the bill heads to the Senate Judiciary Committee. Scientific American spoke with Pan about the problems of dropping vaccination rates, the bill he authored as part of the solution and how he is addressing parental resistance.

[An edited transcript of the interview follows.]

What motivated you to create this bill, and what does it mandate? 

Every child deserves to get educated, and every child deserves to be safe at school. When you have groups of children together at a school where not enough are vaccinated, that becomes a danger to those children who can’t be vaccinated. My bill is not a mandatory vaccination bill. It basically says that you decide whether you want to vaccinate your children or not. But if you choose not to, there is this consequence [of being barred from schools], because your right to choose cannot harm someone else. There is an option to home-school your child.

The U.S. declared measles eradicated in 2000, thanks to vaccination, but the outbreak shows that it persists. What is the scope of the problems caused by declining vaccination rates?

What we’re seeing is that these outbreaks are getting larger and larger because there are larger pockets of unvaccinated people. We’re starting to get close to a tipping point—now we’re seeing outbreaks that are continuing to spread. This was not the first time measles has showed up in Disneyland. But what happened in past outbreaks is that someone would show up who had measles, maybe a couple of other people might catch it, and then that was it. But this one didn’t just stop at Disneyland. Instead, a bunch of people got measles, and when they brought it home, there were too many people who were unvaccinated around them. Other people got it and it spread across California, across the U.S.—even into Canada and Mexico.

Why did you choose to go about mitigating this problem by eliminating the personal belief exemption rather than, for example, increasing vaccine education efforts?

One factor is that we saw increasing outbreaks due to personal belief exemptions. In 2010, for example, we had a pertussis (whooping cough) outbreak across California. There were over 9,000 cases, including 10 infant deaths and hundreds of hospitalizations. When researchers looked at the epidemiological data, one of the factors related to the spread of disease was areas were there were high rates of personal belief exemptions, according to a paper published in Pediatrics.

The other factor is that studies indicate that even when people are educated with the facts they sometimes became even more entrenched in terms of their behavior. I authored a bill in 2012 that said that in order for you to get a personal belief exemption, you have to talk to a licensed health care professional to get educated. Since that bill passed—though this was the first year that it’s actually been implemented—new research has come out that indicates that unfortunately because of vaccine misinformation being spread across the Internet and other sources, many families were more entrenched in not getting their children vaccinated

What are the arguments against this bill, and how have you addressed them?

The opposition’s arguments fall into three main categories: One is that they claim vaccines are dangerous and unfortunately bring up a lot of debunked ideas, such as that vaccines cause autism. Research shows that’s not true, that vaccines are very safe, and certainly much safer than the diseases that they prevent.

The second part of the argument is that these diseases aren’t that serious. Of course, they haven’t had direct experience with these diseases. They haven’t seen children in the ICU [intensive care unit] with measles encephalitis or seen babies cough to death.

The third argument is that the bill imposes on personal choice. Generally, of course, we want to give people as much freedom as possible. The question is, should you have the right to do something that will harm others? The Supreme Court has said that you cannot. Children who have exemptions due to chronic illness need the protection of all the other [vaccinated] children. Do those families choose not to send their children to school in order to keep them safe? That’s not fair to those families because they didn’t have a choice. They didn’t choose for their child to have leukemia, for example.

Given the failure of similar efforts in Washington State and Oregon, how likely do you think it is that this bill will be passed?

I feel confident that once my colleagues understand the science and the truth and the importance of this bill in protecting public safety, we’re going to get it passed. When I worked on a prior bill on immunizations, which was passed into law in 2012, requiring families to be counseled by a licensed health care professional about their vaccinations, the same people opposing my bill now also opposed that one. And most of the people who were coming out to speak in support were the medical and public health folks.

The difference now is that we have parents who are speaking out to support this effort because they have received notices from their schools that someone had brought measles or whooping cough into the school. They have seen notices going to stores and restaurants that if you were there on particular days, you need to look out for signs of measles. People don’t want to live in a place where every time they turn around they have to worry that their kid got exposed to a deadly disease. People are calling and saying “Do something about this.”

Has California tried to get similar bills passed before?

[In 2010] there was a bill that was passed into law requiring children to be immunized against pertussis when they were entering seventh grade. But I would say every vaccine that’s required for school entry has been passed by the legislature, so there have obviously been previous efforts where people have successfully passed bills to add vaccines to the requirement.

If the bill passes, how might schools be affected by parents pulling their children out, as some have threatened?

Given the way we finance schools here in California, there will not be a significant impact on schools if people decide not to vaccinate their children—which is still a small minority—and end up home-schooling their children. There are so many school districts that are supporting this bill, including the Los Angeles and San Francisco unified school districts, along with the California School Boards Association. These are organizations that are acutely sensitive to funding. They support this bill because they know how important it is to keep children safe and that the real financial burden is these outbreaks happening in school.