After a much-publicized measles outbreak at Disneyland, the state of California passed a new law requiring that all children be vaccinated—and it worked "more effectively than a thousand public service announcements might have," Emily Oster and Geoffrey Kocks write for the New York Times' "The Upshot."
California's measles outbreak
At least 159 people contracted measles during a months-long outbreak at Disneyland in 2014, more than the typical number in an entire year in the United States, Oster, a professor of economics at Brown University, and Kocks, a student in economics and applied math at Brown, write. And while public health officials say the disease was likely introduced by a foreign tourist, experts believe that the reason the outbreak was so severe was because of California's low vaccination rates.
Why California had such low vaccination rates
Generally speaking, Oster and Kocks write, it's best to have an immunization rate between 90% and 95% to prevent measles outbreaks. In 2014, when the Disneyland outbreak happened, California reported around 93% of all kindergartners entering school were vaccinated for measles.
While that rate "wasn't bad," the authors write that it varied throughout the state. For instance, the authors write, "if we take the herd immunity rate to be 95%, 70% of children were in counties below that rate."
The rates were even more striking when broken down by individual schools, Oster and Kocks write. At the Berkeley Rose School in Alameda County, for example, just 13% of kindergartners were fully vaccinated in 2014, while at George De La Torre Jr. Elementary in Los Angeles, that rate was just 14%.
At the time, Oster and Kocks write there were two ways a student without a medical exemption for vaccination could "could be unvaccinated … in the California public schools": They could have conditional acceptance, meaning they were not fully vaccinated but were planning to be soon, or they could have a "'personal belief exemption,'" a waiver from vaccination requirements based on religious or other personal beliefs. There also was "little follow-up on the vaccination of conditionally enrolled students, so conditional non-vaccination could easily turn into long-term, non-vaccination," the authors write.
How the state increased vaccination rates
However, after the Disneyland outbreak, the state Legislature passed Senate Bill 277, and "California suddenly went from a state with quite lax school vaccination standards to one with extremely strict requirements," the authors write. The law, which went into effect in 2016, "eliminated all personal belief exemptions and tightened the approach to conditionally enrolled students" so that such students had to have "a plan for vaccine completion over a period of about six months," according to Oster and Kocks. As a result, the only lingering exemption was for medical reasons—and that exemption, given that all schools required vaccination records for admittance, could be closely monitored.
The law worked, Oster and Kocks write: After California enacted the law, 97% of children were living in counties with a kindergarten vaccination rate over 95% in 2016 and 99.5% resided in counties with rates over 90%.
While prior research has suggested stricter rules correlate with higher rates of vaccination, Oster and Kocks write that those studies often focused on "state levels overall, rather than on the distribution." In turn, Oster and Kocks broke down the rates by individual schools, comparing batches of schools in 2014 against the same batches in 2016 to identify which schools drove the overall increase in vaccination rates.
They found that schools in the lowest-performing batch had just 60% of their students fully vaccinated in 2014, but by 2016, that same batch of schools had a vaccination rate of 90%. "It was an astonishing 25-percentage-point increase in vaccinations over a period of just two years," Oster and Kocks write.
And according to the authors, while sharp decreases in conditional enrollment accounted for part of the change, "perhaps more striking [were] the changes for places where personal belief exemptions were high, places where there was concern that people were really committed to no vaccinations." For instance, the vaccination rate at the Community Outreach Academy increased from 46% in 2014 to 83% in 2016—almost entirely because of a reduction in exemptions for personal beliefs, Oster and Kocks write.
An example for other states
According to Oster and Kocks, "under-vaccination is a significant policy problem," and these findings suggest that, "from a policy standpoint," there is "a ray of hope for vaccine proponents," demonstrating that changing minds "isn't so important." They conclude, "People may not have altered their attitudes about vaccination, but the fact is that these laws actually changed behavior" (Oster/Kocks, "The Upshot," New York Times, 1/16).
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