Geographic correlation between nonmedical exemption rates in Arizona kindergarten classes and rates of community pertussis infection

Abstract

Background: Every state in the US mandates specific vaccinations for all children prior to school entry. However, many states, such as Arizona, are permit nonmedical exemptions (NMEs), and thus, communities with high levels of NMEs are potentially more vulnerable to outbreaks of vaccine preventable diseases. Objective: The objective of this study was to detect spatial clusters of pertussis cases and kindergarten NMEs of DTaP vaccine in Arizona. Methods: Data detailing kindergarten NMEs for each AZ school in the 2012-13 and 2013-14 school years and pertussis cases with report dates during those time periods were obtained from the Arizona Department of Health Services databases. Addresses for each school and pertussis case were geocoded, and the coordinates were obtained for each corresponding census tract. Using a purely spatial analysis with Poisson probability model, areas with high rates of pertussis or NMEs were identified with SaTScan. Clusters for both the number of NMEs and pertussis cases were detected and presented in maps. Extremely large (>50km radius) clusters extending beyond state boundaries were considered irrelevant and removed. Results: Multiple clusters of NMEs and pertussis cases were identified in two school years. While the clusters were generally in highly populated regions, they were not located at the same region in both years. The NME clusters and case clusters did not always overlap. Conclusions: The current analysis does not support an association between kindergarten NME frequency and pertussis outbreaks within these time periods. Clusters of both NMEs and pertussis cases were located in different regions each year. Since kindergarten NMEs vary from year to year, it is possible that these NME rates are not reflective of those in the community. Further studies with larger datasets will be important in elucidating the spatial distribution of pertussis cases and NMEs.