How California became a role model on measles
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 overall, 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.
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.
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