“Syringe and Vaccine” by NIAID is licensed under CC BY 2.0

How Well Did California’s Interventions to Improve Vaccination Rates Work?

MedicalResearch.com Interview with:
Ms. Cassandra Pingali

Ms. Pingali worked on this paper while a a graduate student at Emory University, and completed it post-graduation.
She is currently an ORISE fellow at Centers for Disease Control and Prevention
Immunization Services Division

MedicalResearch.com: What is the background for this study?

Response: Despite high overall immunization coverage in the United States, we are currently experiencing the largest measles outbreak since measles was declared eliminated in 2000. In 2014, California grappled with a very large measles outbreak known as the “Disneyland” outbreak. Later investigation revealed that most of the affected children were unvaccinated against measles despite the availability of a safe and effective vaccine.

In order to prevent future outbreaks, California officials wanted to improve their declining childhood vaccination coverage. California passed two laws and implemented an educational program for school staff to increase vaccination rates in the state. We felt it was important to take a systematic look at these interventions and examine if public health initiatives such as these are working to improve vaccination rates.

MedicalResearch.com: What are the main findings?

Response: Our main finding was an impressive decrease in the number of children not-up-to-date on their vaccines after the three interventions were implemented. The state-level not-up-to-date rate fell from a high in 2013 of 9.84% to 4.87% in 2017. So, these interventions seem to be effective at increasing the number of up-to-date students.

We noted a big decrease in the likelihood that a not-up-to-date kindergartener would have contact with another not-up-to-date kindergartner within their school. We also found a decrease in the number of schools located inside geographic clusters with high rates of kindergartners who were not-up-to-date on their vaccines. In the 2012-2013 school year there were 3,206 schools located within a cluster, and this decreased to 1,613 schools in the 2016-2017 school year.

By decreasing the clustering and contact of not-up-to-date kindergartners, there is an improvement in community immunity. These results indicate that a vaccine preventable disease outbreak is less likely to occur, and the interventions were effective at increasing vaccination rates in California kindergartners.

MedicalResearch.com: What should readers take away from your report?

Response: I think our paper highlights the importance of preventing vaccine preventable disease outbreaks in our communities by maintaining high immunization coverage. California wanted to take action after the Disneyland measles outbreak in 2014 to improve vaccination rates and prevent future vaccine preventable disease outbreaks. They used multiple approaches to address this problem and started by tightening the requirements to obtain a personal belief exemption. Independent of this new legislation, they took a two-pronged approach by also starting the conditional admissions education program. They later decided to completely eliminate personal belief exemptions. These interventions did a good job at lowering the state-level not-up-to-date rate. We saw less contact between not-up-to-date kindergartners in school, and fewer schools were located within a geographic high not-up-date cluster after the interventions.

Despite the impressive decrease in the not-up-to-date rate in California, we do have concerns we are now seeing a replacement effect occurring. Our study published last month in Pediatrics found evidence of this replacement effect.1

We saw that nonmedical exemptions were largely replaced by other school entry mechanisms allowing kindergarteners not up-to-date on vaccinations to enter school. In the second-year post implementation of SB277 the not- up-to-date rate actually increased as improvements in the nonmedical exemption rate were eroded by other entry mechanisms such as medical exemptions. Unintended effects such as this need to be carefully evaluated and policymakers should consider such consequences when creating new legislation.

Another surprising finding was the very geographically stable clusters in Northern California. Almost all other areas of the state responded well to the interventions except this area. We aren’t sure why Northern California has such persistent vaccine hesitancy but the geographic clustering in that area was not greatly affected by the interventions.

1 Delamater, P. L., Pingali, S. C., Buttenheim, A. M., Salmon, D. A., Klein, N. P., & Omer, S. B. (2019). Elimination of Nonmedical Immunization Exemptions in California and School-Entry Vaccine Status. Pediatrics, 143(6), e20183301

MedicalResearch.com: What recommendations do you have for future research as a result of this work?

Response: I think there is some very exciting follow-up research that can be done on this topic this year. For example, California is currently voting on new legislation to allow the California Department of Health to review medical exemptions under certain circumstances. This may help address the replacement effect we’ve been seeing but we would need careful evaluation to understand effects of this legislation if passed. We’ve also just seen New York State pass legislation to eliminate religious exemptions to school mandated vaccination in response to the large measles outbreak in that state. In May, Maine passed legislation to eliminate religious and philosophical exemptions to the school mandated vaccine requirements. Interestingly in Washington they also just passed a new law that removes the personal and philosophical exemptions to the MMR vaccine but did not change access to religious and medical exemptions. California was the first state in over thirty years to ban personal belief exemptions, but we have a very dynamic vaccine policy environment currently. This is a great opportunity to compare the effects of similar vaccine legislation across states. 

No disclosures other than those in the paper. We thank NIH for their funding of this project.


S.C. Pingali et alAssociations of statewide legislative and administrative interventions with vaccination status among kindergartners in CaliforniaJAMA. Published online July 2, 2019. doi:10.1001/jama.2019.7924. 

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Last Updated on July 2, 2019 by Marie Benz MD FAAD