Examining the Spatiotemporal Evolution of Vaccine Refusal: Nonmedical Exemptions from Vaccination in California, 2000-2013

"There has been limited research on how vaccine refusal manifests throughout a region and whether there are observable spatial patterns that can shed light on the processes that drive hesitant or resistant parents to become vaccine refusers."

In the United States (US), some vaccine hesitant parents are choosing to exempt their children from school-entry vaccination requirements for nonmedical reasons, despite the demonstrated safety and effectiveness of vaccines. Vaccine hesitancy is not constrained to the US; numerous countries around the world are also contending with issues stemming from vaccine hesitancy, including reduced levels of coverage and corresponding vaccine-preventable disease (VPD) outbreaks. This paper shares the results of an analysis of how vaccine refusal, measured by the use of nonmedical exemptions (NMEs), based on personal or religious beliefs, from vaccination evolved across space and over time in California, US.

Vaccine hesitancy in the US, as expressed through the use of NMEs, remains a highly contested topic across the country. Each state and the District of Columbia develops and implements its own childhood vaccination laws, regulations, and procedures, which are generally enforced at school entry. For children with a contraindication for vaccination, all states have a provision for a medical exemption. Further, nearly all states allow parents to obtain a NME for their child based on personal, philosophical, or religious beliefs. The restrictiveness of state-level NME requirements has been shown to affect the number of parents exempting their children from vaccination, as states with more restrictive policies tend to have fewer NMEs. Although children with NMEs cannot be assumed to be fully unvaccinated (given limitations of some surveillance systems), numerous studies have shown a link between NME use and VPD outbreak risk.

As noted here, the ability of those opposed to vaccination to disseminate their message and engage with other vaccine-hesitant parents has been bolstered by the increasing availability and use of the internet. One particular concern is that parents will advance along the spectrum of hesitancy (e.g., from resistant to refusal) as the debate surrounding vaccines, vaccine safety, and parental rights continues to play out online.

Using school-entry data from the California Department of Public Health, the researchers examined NMEs for students entering kindergarten in California from 2000 to 2013. Over this 14-year time period, California was devoid of changes to large-scale vaccine-related policy; this allowed the researchers to focus on how the changes in vaccine-related behaviour manifested in space over time.

The article outlines the global and local spatial autocorrelation analysis that was conducted in order to determine whether NME use became more geographically clustered over the study period and whether the location of local clusters of high use were temporally stable. The researchers conducted a grouping analysis that identified the general temporal trends in NME use over the time period.

They found that the use of NMEs increased from 0.73% of all kindergarteners in 2000 to 3.09% in 2013 and became more geographically clustered over the study period. Local geographic clusters of high use were relatively isolated early in the study period, but expanded in size over time. The grouping analysis showed that 50% of the regions had consistently low NME rates (at or near 0%) over time. These findings show that the vaccine-related behaviour of parents in many California regions was not substantially affected by the changes in NME use occurring in other regions over this time period. Another 35% of all regions had moderate increases in NME use, mirroring the statewide behaviour. The remaining 15% of all regions in the state had large increases in NME use over this time period. Hence, the largest increases in vaccine hesitancy appear to have occurred in a relatively small proportion of regions throughout the state. In these regions, parents appeared to be influenced to use NMEs due to the relatively large proportion of other parents that already chose to use NMEs in prior years.

These findings suggest that vaccine refusal appeared to be 1) a self-reinforcing process, such that communities with the highest NME use also had the largest increases in NME use throughout the study period and 2) a spatially diffusive process, such that regions located nearby those with high initial NME use demonstrated larger increases in NME use over time than regions located further from the initial high use regions.

In the local geographic clusters, even if some of the students with NMEs had actually been vaccinated, there was an increased probability that unvaccinated (or undervaccinated) individuals could come into contact with one another, increasing the risk of disease transmission and the potential for outbreaks. This is especially salient when examining school age children, as schools offer an environment where large numbers of children interact on a regular basis.

While the findings are restricted to California, they do potentially provide information for other states and countries that are struggling with increases in vaccine refusal and declining childhood vaccination coverage. Specifically, the results demonstrate that efforts aimed at decreasing future NME use may be most effective if they are not only focused on regions where NME use is already high, but also on those located near to the high use regions, as they may be susceptible to future increases in vaccine refusal.