In December 2014 something unusual happened at Disneyland. People came to visit Mickey Mouse, and some of them left with measles. At least 159 people contracted the disease during an outbreak lasting several months. This is more than the typical number in a whole year in the United States.
The leading theory is that measles was introduced in Disneyland by a foreign tourist. That could happen anywhere. Medical experts generally agree that the fact that it took off was probably a result of California’s low vaccination rates, which in turn was a result of an inability to persuade a significant share of Californians that vaccines were important.
The episode made national news, but in the next few years, another development was striking but attracted less national attention: Because of a policy change, California was able to turn it around. Data from a county-by county analysis shows that in many schools with the lowest vaccination rates, there was an increase of 20 to 30 percentage points in the share of kindergartners vaccinated between 2014 and 2016. One law changed the behavior of impassioned resisters more effectively than a thousand public service announcements might have.
Limiting outbreaks of vaccine-preventable diseases relies on “herd immunity.” Essentially, if enough people are vaccinated, a disease cannot get a foothold.For measles, this number is around 90 percent to 95 percent. In other words, if 95 percent of people in an area are vaccinated for measles, an outbreak is unlikely even if the disease is introduced.
Our best data on vaccination rates, in California and elsewhere, relies on records collected from schools at kindergarten entry. California requires these records from all schools, public and private, so they provide a comprehensive measure.
In 2014, for California over all, about 93 percent of entering kindergartners were vaccinated for measles. This wasn’t bad. It could have been better — a place like North Carolina is at about 98 percent — but this was a high enough rate to be in the range of herd immunity.
The trouble is that herd immunity is about the vaccination rate among the people you interact with, and you’re not interacting with the entire state of California. Local vaccination rates matter. If the overall state vaccination rate of 93 percent was because each area had a vaccination rate of 93 percent, that would be one thing. But if it’s because a bunch of areas had very high rates, and a bunch had lower ones, that’s quite another. And this second case was California in 2014.
The accompanying chart gives a sense of the distribution of vaccination rates across counties in California in 2014.
In 2014, there were a lot of areas of California with very low vaccination rates. If we take the herd immunity rate to be 95 percent, 70 percent of children were in counties below that rate. Even taking the bottom of the herd immunity range — 90 percent — found 36 percent of children in counties below that rate.
A focus on individual schools was even more striking. At the Berkeley Rose School, in Alameda County, only 13 percent of kindergarten students were up to date on vaccinations in 2014. George De La Torre Jr. Elementary, in Los Angeles, was at 14 percent.The Community Outreach Academy, a large public school in Sacramento, was at 46 percent.These were on the lower end — but they were not the lowest rates.
There were two ways a student could be unvaccinated in 2014 in the California public schools. Some students were admitted “conditionally” — that is, not fully vaccinated but planning to be soon. Other students had a formal “personal belief exemption.”That is, for religious or other reasons — often misplaced fears of vaccine injury — the parents could choose not to vaccinate their children at all.
These varied greatly across schools. In the Berkeley Rose School, a private Waldorf school, all of the unvaccinated students (87 percent of the kindergartners) had personal belief exemptions. In elementary schools in poorer parts of Los Angeles, the lack of up-to-date vaccination was due mostly to conditional enrollment. In practice, in this period there was little follow-up on the vaccination of conditionally enrolled students, so conditional non-vaccination could easily turn into long-term non-vaccination.
In the end, the result was the same: many schools with many unvaccinated children, and they were at risk. Measles is extremely contagious.If you introduce it into a school where only 13 percent of students are vaccinated, a lot of people will become ill.
In response to the Disneyland outbreak, California suddenly went from a state with quite lax school vaccination standards to one with extremely strict requirements. The state passed Senate Bill 277, which went into effect in 2016 and eliminated all personal belief exemptions and tightened the approach to conditionally enrolled students. No longer could a parent say, “I’ll do it later”; there had to be a plan for vaccine completion over a period of about six months.
The only remaining exemptions were for medical reasons.And since all schools, public and private, have to report the vaccination status of enrolled children, including documentation, the state has a way to monitor this. Without seeing your vaccination records, a school simply is not allowed to enroll you. And children have to be enrolled in school.
Public health researchers have studied the relationship between state vaccination rules and vaccination rates, and have generally found that stricter vaccination laws generate higher vaccination rates on average. But these studies tend to focus on state levels over all, rather than on the distribution. In a place like California, with so many low-vaccination schools, we had a chance to ask: What would actually happen?
What happened was that people got vaccinated.
In 2016, 97 percent of children lived in counties with a kindergarten vaccination rate above 95 percent, and a full 99.5 percent in places over 90 percent.
Looking at the school level, we can see which schools contributed to this change. We took schools in 2014 and divided them into 10 groups based on their vaccination rates. For each group of schools, we calculated their vaccination rates in 2014 and 2016. This provided a way to summarize which group of schools accounted for the changes over time.
In the accompanying chart, we see the results. Schools in the bottom group had about 60 percent of their students up to date on vaccines in 2014. This is pretty abysmal, and this is an average, so many places were even lower. By 2016, this group had close to a 90 percent vaccination rate. It was an astonishing 25-percentage-point increase in vaccinations over a period of just two years.
When we look at what drives this, one big factor is a huge decline in the conditional enrollment numbers. In George De La Torre Jr. Elementary, where 86 percent of children were conditionally enrolled with no measles vaccine in 2014, the vaccination rate in 2016 was 99 percent.
But perhaps more striking are the changes for places where personal belief exemptions were high, places where there was concern that people were really committed to no vaccinations. In the Community Outreach Academy, the vaccination rate increased to 83 percent from 46 percent over this period. This was almost entirely a result of reductions in personal belief exemptions.
And what about the Berkeley Rose School, with its 87 percent personal belief exemption rate? By 2016, 57 percent of entering students were vaccinated —a huge change, and that was only in the first year of the law.
When SB 277 was passed, people worried about the possible effects: Would children be pulled out of school? This concern was misplaced. Over all, there has been no change in enrollment, even in schools with the lowest vaccination rates in 2014. People worried that parents would substitute (fake) medical exemptions for belief exemptions. This did happen, a little, but not nearly enough to offset the increases.
In the end, the effect of the law was simple: More children were vaccinated, and the risk of disease outbreaks has gone down.
Under-vaccination is a significant policy problem. As earlier generations knew, people die of measles, and of whooping cough, and of other diseases that vaccines can prevent. Figuring out how to increase vaccination is a challenge. We often rely on education, but it is hard to change people’s minds on this topic, as doctors and policymakers — as well as any parents who have engaged on an internet message board — know all too well.
From a policy standpoint, these findings offer a ray of hope for vaccine proponents. Maybe changing minds isn’t so important. People may not have altered their attitudes about vaccination, but the fact is that these laws actually changed behavior.
In Oregon, parents can opt out of getting their children immunized by completing a 15-minute online “education” module. Many of them do: The share of people in Oregon counties with kindergarten vaccination rates over 95 percent was close to 100 percent in 2000; in 2015, it was about 30 percent. Perhaps lawmakers there and in other states should consider a more stringent exemption policy before, not after, they have their own measles outbreak.
Emily Oster is a Professor of Economics at Brown University. She is the author of “Expecting Better: Why the Conventional Pregnancy Wisdom Is Wrong and What You Really Need to Know.” Geoffrey Kocks is an Economics and Applied Math student at Brown.
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