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Home » California’s vaccine exemption laws – clustering effects

California’s vaccine exemption laws – clustering effects

Last updated on August 24th, 2019 at 04:32 pm

All 50 US states (along with several territories and DC) require mandatory vaccination for children entering public (and frequently, private) schools. This system has essentially ended most vaccine preventable diseases in the USA, including measles, polio, chickenpox, and many others.

Broad vaccination is considered one of the 10 greatest achievements in public health. Vaccines should probably be number 1 on the list. Overall, the immunization mandate has established a strong herd effect, which has generally ended transmission of these diseases.

Even though vaccinating children before they enter school is mandatory, there are ways around it, if you choose. Every state allows medical exemptions, which is based on a proven risk for a child to not be vaccinated with one or more vaccines. For example, some vaccines are produced in chicken eggs, and a tiny percentage of children are allergic. Medical exemptions are absolutely critical to the well being of the child, and no pro-science (pro-vaccine) writer or researcher would be opposed to those types of exemptions.

Furthermore, most states have vaccine exemption laws which allows personal belief exemptions (PBE). These PBEs fall into one of two groups–religious exemptions, that is, the parent “claims” that their religion is opposed to vaccines; or personal exemptions, which are simply based on the fact that the parents are opposed to vaccination for whatever reason that hits their brain after 20 minutes of Googling “facts.”

Most states allow both types of exemptions, some only allow religious exemptions, and one state, Mississippi, allows only medical exemptions. As a progressive, there is little positive I can say about Mississippi, but this is a major positive. So congrats Mississippi for caring about children, at least in this one important way.

Regulation of PBEs

Some states carefully regulate PBEs through their vaccine exemption laws – while others basically want a signature on a piece of paper. There have been several court cases in New York (here, here and here), for example, that have stated that the state does have the right to regulate religious based PBEs in order to protect the safety of the vast majority of students. New York has show little patience in “fake” religious exemptions based on invented religious beliefs. Good for them.

New Mexico has decided to more closely regulate religious exemptions, requiring definitive proof that the applicant for the exemption was legitimately a member of the church and that the church’s policy was opposed to vaccination (a rare situation indeed). Federal courts in other states have blocked parental demands for vaccine exemptions as not being a “free exercise of religion.”

There are various weaknesses in the regulation of religious PBEs and are frequently abused by antivaccination cultists who have no problem with lying on the forms and putting their own and other children at risk of disease. Ironically, hardly any mainstream religion (that is the majority of religions practiced by the US population) is opposed to vaccination.

California PBE situation


In California, the PBE rate has increased 380% from 0.5% in 1996 to 2.3% in 2010. In response to the increase in rates of personal belief exemptions, in 2012 the California legislature passed Assembly Bill 2109 (AB2109) in September 2012. Previously, California’s law provided for a very simple process that allowed a parent to obtain a PBE by signing an affidavit that stated that any vaccination was contrary to their own personal beliefs. This affidavit was preprinted on the California School Immunization Record, making exemptions relatively easy. In fact, some parents signed it not because they were antivaccination, but sometimes because they just didn’t want to spend the time to dig up the records.

Previous research demonstrated an association between easier state exemption procedures and higher exemption rates. In light of that, AB2109 mandates that parents filing an exemption must submit an attestation signed by a licensed health care provider stating that the parents have received information about the risks and responsibilities of refusing immunizations.

The law also requires a letter signed by the parents indicating receipt of this information. Let’s be serious, this isn’t exactly a tough standard to exceed. But for the “lazy” antivaccination parent, who forgot about a vaccine appointment with the child’s doctor or can’t find the immunization record, this will probably spur them to provide the school with proper documents. But those children were or were going to be vaccinated anyways, so it has no meaningful effect other than to provide the California Department of Health with more accurate accounting of vaccine refusal in the state. (Most of this law will be superseded by SB 277 if it is finally passed and signed by the California governor, Jerry Brown.)

This law went into effect on 1 January 2014, Unfortunately, at the time of signing the bill into law, California’s governor, Jerry Brown, issued an opinion (pdf), the legality of which is subject to debate, that stated that parents whose religious beliefs are opposed to immunization should not have to submit an attestation from a health care provider. Dorit Reiss, a frequent contributor to this blog and Professor of Law at the University of California’s Hastings College of the Law, wrote on her school’s website that:

The governor is well within his rights to direct the Department of Public Health how to implement the law, and it is laudable that he did that in a transparent manner, via the signing statement, rather than behind the scenes. But instructing the Department of Public Health to create a religious exemption from this requirement is neither desirable nor legal.

Signing statements are useful and important. They can make transparent the views of the administration on how laws should be interpreted. They can provide courts with guidance on interpreting a statute (though scholars disagree about their role in interpretation), and can help direct public officials how to implement a statute. But a governor can only direct executive officials to act consistent with powers provided in the statute, and may not go beyond it. If the governor goes beyond the statutory mandate, his words in the signing statement do not carry the force of law. A governor is not authorized to add an exemption that is not in the statute without going through the legislative process.

It’s possible that Governor Brown’s signing statement may not have much material impact on the law. However, it’s not clear to me how useful this law is going to be over the long run. It’s going to make it an easier choice for some portion of those who have PBE’s to get their children’s vaccinations up-t0-date or to find the immunization record, because they’re already fully vaccinated. This is just a mere accounting change, though it is critical for providing infectious disease specialists accurate data for why a disease spreads through a small community.

Clustering of PBEs–herd immunity


Of course, those who are vehemently opposed to vaccines, will merely be inconvenienced, but will still go through the trouble of getting the attestation of receiving information about vaccinations. And there’s a middle group, who may be on the fence, who might listen to what is being told to them by the healthcare worker and change their attitude, which would be the best case scenario.


In a new article published in the American Journal of Public Health, the authors performed an extensive review of the effects of these PBEs in California by examining the rate of PBE by area. If you examine the maps (above), taken from the article, you will note that there appears to be large cluster areas of PBEs in specific areas. Northern California, parts of Los Angeles, Ventura and Santa Barbara Counties, and San Diego all appear to have fairly substantial clusters of PBE, presumably un vaccinated children. According to the authors,

Parental hesitancy regarding vaccines is on the rise. In addition to showing that PBEs in California have continued their upward trajectory, our data reveal strong geographic clustering. This pattern suggests information diffusion or contagion, perhaps within social networks. On the basis of the evidence presented  here, we expect that AB2109 may not be sufficient to combat continued increases in exemptions among the children at highest risk for vaccine-preventable childhood disease outbreaks: clustered subgroups of children whose parents have serious concerns about vaccine safety and efficacy. The reason is that these parents are unlikely to be swayed by increasing the opportunity cost of a PBE.

The data show us that some counties in California have up to 20% PBE rates, far above the level necessary to maintain a herd effect, and allowing for clusters of vaccine preventable disease outbreaks, as we’ve seen with whooping cough and measles in California.



I can’t believe I have to make these arguments in favor of vaccination in a modern world. What is this? The Middle Ages, where most children died before turning 5? Where wizards and dragons were believed to be real things? Where medicine was non-existent?

Editor’s note: This article was originally published in July 2014. It has been completely revised and updated to include more comprehensive information, to improve readability and to add current research.


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