We need to know who’s driving this epidemic of non-vaccination so we can decide what to do about it. One key element of the pattern is that it’s a practice of groups, not (just) individuals. And one way such groups are organized and maintained may be by interacting in and around schools — hotspots for parenting fads and identity performance, as well as (one hopes) useful information.

Kieran Healy the other day posted some visualizations of California vaccine exemption rates across schools and counties — then dug deeper on school type here. While he was writing his second post I followed his links to the data and did some more descriptive work, adding some more information on school type and now poverty levels.

I used this California Department of Education data source for the free-lunch eligibility rates (2013-2014), and this California Department of Public Health data source for the vaccine exemption rates for kindergartners (2014-2015). The measure of vaccine exemption is the “Personal Belief Exemption (PBE), whereby a parent requests exemption from the immunization requirements for school entry.” Some of them got supposedly got counseling before making the request, while others further requested a religious exemption from the counseling — those two groups are combined here in the PBE rate. I weighted the analyses by the number of kindergartners enrolled in each school, so the rates shown are for students, not schools (this also helps with the outliers, which are mostly very small schools).

1. Runaway vaccine exemptions are problems of the private and charter schools

I don’t know what percentage of kids need to be immunized, for which diseases, for the proper level of community protection. But these distributions are very skewed, so it makes sense to look at the extremes. Here are three measures, for just under 7,000 schools: the mean percent PBE (the average percent exempt among each kids’ classmates), the percentage of kids in a school with no exempt kindergartners, and the percentage of kids attending schools with more than 5% exempt.

The average charter school kindergartner goes to school with classmates almost 5-times more likely to be non-vaccinated; and charter school kids are more than 3-times as likely to be in class with 5% or more kids exempt.

2. Schools with lots of poor kids have much lower exemption rates

The relationship between exemption rates and percentage of children eligible for free lunch is negative and very strong. Because there are so many schools with no PBEs, I used a tobit regression to predict exemption rates (I also excluded the top 1% outliers). Note the private schools are excluded here because the free lunch data was missing for them. Here is the output:

Just in case the zeros are data errors, I reran the regression excluding the zero cases (logged and not logged), and got weaker but still very strong results.

I illustrate the relationship in the next section.

3. The poverty-exemption relationship is stronger in charter schools

Charter schools have fewer kids eligible for free-lunch than regular public schools (43% versus 55%). However, although the relationship between poverty and exemptions is strong in both charter and regular public schools, it is stronger in the charter schools (the interaction term is almost 7-times its standard error). Here they are, showing the steeper free-lunch slope for charter schools (with a logged y-axis):

Rich charter schools on average have the highest exemption rates, while poor schools — charter or not — are heavily clustered around zero (note it’s jittered so you can see how many cases are at zero).

Update: Here is the same data, for all public schools, presented more simply:

One interpretation of this pattern goes like this: Charter schools do not per se promote vaccine exemption. But because they are more parent-driven, or targeted at certain types of parents, charter schools are more ideologically homogeneous. And because anti-vaccine ideology is concentrated among richer parents, charter schools provide them with a fertile breeding ground in which to generate and transmit anti-vaccine ideas. That’s why, although richer parents in general are driving vaccine denial, it’s especially concentrated in charter schools. This seems consistent with the general echo-chamber nature of information sharing in cultural niches, and the clusering/contagious nature of parenting fads. (The same may or may not hold for private schools.) This could be totally wrong — I’m open to other interpretations.

I’m also happy to share my data on request, but I’m not posting it yet because when I tinker with it more I might make corrections.