Unexplained Variation in CA Child Vaccination Rates?

The idea for this post came when thinking about wired.com’s recent posts about Silicon Valley child vaccination rates. Basically wired looked at vaccination rates at several tech company sponsored day cares and found some low (as well as high) vaccination rates. Some of the companies with reportedly low vaccination rates blamed inaccuracies in the data. For more information see my post at


The data used for California child care and kindergarten vaccination rates is located at


A little background. The personal belief exemption (PBE) in California allows parents to not vaccinate their children if it violates their personal belief. On January 1st 2014, the law changed slightly so that parents could still claim a PBE, but only with the signature of a pediatrician. The permanent medical exemption (PME) exempts children for medical reasons.

The explanation given by the Silicon Valley companies of misreporting data got me thinking. What fraction of the unvaccination rate is unexplained? PBE’s and PME’s allow for unvaccination, but how about what’s leftover? I used kindergarten rather than child care facility data because the former had a longer time horizon. Children are required by law to vaccinate upon entrance of kindergarten if they do not hold a PME or PBE. Perhaps then the unexplained unvaccination could be a proxy for mismeasurement? On the other hand. Perhaps the regulation is somewhat lax and children without PME’s and PBE’s are slipping through the cracks? I created the following graphic in R.


The r code to clean the data and generate the plot is located at my github account, napairolero


There are several interesting things to note about the plot. First, the PME rate is relatively flat over time. Children don’t seem to be exempted for medical reasons any more or less over time. Second, the PBE rate does drop sharply with the altered law in 2014. The doctor’s signature for a PBE seems to be deterring its use. Next, take a look at the unexplained unvaccination rate. Recall that this is the part of unvaccination not explained by PME and PBE. It jumps sharply between 2011 and 2012, which the unvaccination rate mirrors. Why? Secondly, it increases in 2014. If the unexplained unvaccination just gives measurement error then this is not that interesting. However, if students are allowed to slip through the cracks because of lax regulation then perhaps some unvaccinated children filtered out of PBE status to entering kindergarten without a PBE? If this is the case then perhaps CA has more of an enforcement issue and needs to do more than just amending the PBE law.

On the other hand, if the unexplained unvaccination rate is just measurement error then the accused silicon valley companies could point to it and show that their vaccination levels are much higher than reported. Rather than looking at the unvaccination rate for these companies, wired.com should be looking at PBE and PME status at tech company sponsored child care facilities.


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