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UCLA Electronic Theses and Dissertations UCLA UCLA Electronic Theses and Dissertations Title Exempted: Spatial Clustering of Intentionally Unvaccinated Children in California and its Potential Consequences for Measles Transmission Permalink https://escholarship.org/uc/item/3bt9k541 Author Gromis, Ashley Publication Date 2017 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA Los Angeles Exempted: Spatial Clustering of Intentionally Unvaccinated Children in California and its Potential Consequences for Measles Transmission A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Sociology by Ashley Renee Gromis 2017 © Copyright by Ashley Renee Gromis 2017 ABSTRACT OF THE DISSERTATION Exempted: Spatial Clustering of Intentionally Unvaccinated Children in California and its Potential Consequences for Measles Transmission by Ashley Renee Gromis Doctor of Philosophy in Sociology University of California, Los Angeles, 2017 Professor Ka Yuet Liu, Chair Rates of non-medical vaccine exemptions have been increasing in the U.S. since the early 1990s. These exemptions tend to be spatially clustered rather than randomly occurring in the population. In contrast to long-standing public health findings, under-vaccination due to non- medical exemptions tends to be found among children with educated, affluent, non-Hispanic white parents rather than those with relatively disadvantaged backgrounds. This dissertation uses data on Personal Beliefs Exemptions (PBEs) in schools in California from 1998-2014 and an empirically-calibrated large-scale simulation experiment to understand how spatial clusters of these exemptions form and their potential consequences for measles transmission. This dissertation emphasizes the importance of interaction in social networks in structuring both the local context surrounding parents‘ vaccine decisions and the patterns of ii physical contact through which disease spreads. Major findings show that residential sorting of parents into neighborhoods and schools based on socio-demographic characteristics associated with school-level PBE rates provides only a partial explanation for broader patterns of clustering. Additional analyses provide evidence of spillover effects between PBE rates and socio- demographic characteristics, particularly percent non-Hispanic white children, on PBE rates in nearby schools. A large-scale simulation of measles transmission using a synthetic population of youth in California in 2014 is then used to examine the potential consequences of PBEs for disease spread. Schools serve as hotspots for measles transmission regardless of the spatial locations of PBEs, although clustering of these exemptions within households and charter schools increase the opportunities for infections. Surprisingly, spatial pockets of PBEs may provide slight protection against measles transmission when outbreak sizes are small and the disease is introduced randomly into the population. Yet, clustering of exemptions can reduce protective effects of herd immunity to large measles epidemics, even when overall vaccination coverage in the population is high. This dissertation concludes by drawing connections between findings in the empirical chapters and linking the results to the larger theoretical context of social interaction in networks. Practical implications for vaccine policy and efforts to control vaccine-preventable disease are discussed. Overall, this project seeks to illustrate how decisions perceived as personal by parents contribute to collective health outcomes. iii The dissertation of Ashley Renee Gromis is approved. Gabriel Rossman William G. Roy Edward T. Walker Peter Bearman Ka Yuet Liu, Committee Chair University of California, Los Angeles 2017 iv TABLE OF CONTENTS Acknowledgments xii Vita xvi Chapter 1: Introduction 1 Research Questions 5 Personal Belief Exemptions in California 6 Outline of Chapters 12 Significance 14 Figures 15 Chapter 2: The Importance of Social Networks and Space in Vaccine Refusals Debates 17 Chapter Introduction 18 Evaluation of Risk 19 Trust in Healthcare Institutions 23 Homophily in Social Networks 26 (i) Selection 26 (ii) Confounding Effects 27 (iii) Social Influence 28 Social Ties 30 Biological Contagion 34 Disease Spread in Small-Worlds 36 v Robustness of Networks to Disease Spread 36 Social Behavior and Biological Contagion 38 Conclusion 40 Chapter 3: Clustering of Personal Belief Exemptions across Kindergartens in California, 1998-2014 43 Chapter Introduction 44 Personal Belief Exemptions 45 Costs and Benefits of Vaccination 47 Clustering of PBEs across Schools 53 Data and Methods 54 Data 55 Methods 57 Results 61 Discussion 69 Conclusion 71 Tables 72 Figures 76 Appendix 80 Chapter 4: The Structure of Spatial Dependence in Yearly Personal Beliefs Exemption Rates across Kindergartens in California, 1998-2014 88 Chapter Introduction 89 Spatial Dependence in PBE Rates 90 Spatial Heterogeneity 91 vi Unmeasured Covariates 92 Spatial Externalities 92 Spatial Diffusion 93 Data 96 Analytic Strategy 97 Spatial Neighbors 102 Results 104 Discussion 110 Conclusion 112 Figures 113 Appendix 120 Chapter 5: How Spatial Clustering of Non-Medical Vaccine Exemptions Affects Locations of Measles Transmission in Social Networks 125 Chapter Introduction 126 Social Contact and Disease Spread in Networks 127 Measles Transmission 129 Hypotheses 131 Synthetic Population 132 Simulation Model 135 Results 139 Schools and Childcare Centers 140 Households 143 Physicians 144 vii Neighborhoods and Malls 145 Discussion 148 Conclusion 151 Tables 152 Figures 153 Appendix 159 Chapter 6: Effects of Non-Medical Vaccine Exemptions on Overall Size of Measles Outbreak 161 Chapter Introduction 162 Spatial Clusters of PBEs as Disease Cores 164 Hypotheses 165 Data and Methods 167 Results 170 Discussion 176 Conclusion 178 Figures 179 Appendix 184 Chapter 7: Summary and Conclusion 186 Bibliography 195 viii LIST OF TABLES Table 3.1: Correlates of PBEs among public school kindergartens, 1998-2014 72 Table 3.2: Three-level model for correlates of PBEs among public school kindergartens, 2006-2014 73 Table 3.3: Effects of school type on PBEs among kindergartens, 1998-2014 74 Table 3.4: Effect of racial segregation on PBEs rates among public kindergartens 75 Table A3.1: Correlates of PBEs among public school kindergartens, 5 year lag, 1998-2014 80 Table A3.2: Correlates of PBEs among public school kindergartens, 1998-2010 81 Table A3.3: Correlates of logged PBE rates among public school kindergartens, 1998-2010 82 Table A3.4: Physician effects on PBEs, by physician type, 1998-2014 83 Table A3.5: Percent renters effects on PBE rates in public kindergartens, 1998-2014 84 Table A3.6: Effect of household income on PBE rates in public kindergartens, 1998-2014 85 Table A3.7: Correlates among public school kindergartens, additional local business variables, 1998-2014 86 Table A4.1: Yearly observations, by school type 120 Table A4.2: LaGrange Multiplier tests for spatial dependence, 1998-2014 121 Table 5.1: Descriptive Statistics of Synthetic Population 152 ix LIST OF FIGURES Figure 1.1: Statewide PBE rate in California kindergartens by school type, 1992-2014 15 Figure 1.2: County PBE rates in California kindergartens, 2014-2015 school year 16 Figure 3.1: PBE rates in California kindergartens, 1992-2014 76 Figure 3.2: Adjusted spatial clusters of PBEs across public schools, 2014 77 Figure 3.3: Effect of percent non-Hispanic white by school type 78 Figure 3.4: Effect of distance to nearest public school kindergarten in PBE cluster on PBE rates in childcare centers and private schools 79 Figure A3.1: Adjusted spatial clusters of PBEs by mother‘s education and percent non- Hispanic white, 1998-2013 87 Figure 4.1: Spatial concentration and dispersion of PBE rates in California kindergartens, 1998-2014 113 Figure 4.2: Diagrams of spatial relationships in autocorrelation among school PBE rates in spatial error, spatial lag, and spatial Durbin lag models 114 Figure 4.3: Local variation in covariate effects in GWR models, 1998-2014 115 Figure 4.4: Reduction in AIC (compared to OLS models) for spatial autocorrelation models 117 Figure 4.5: Direct and indirect effects of covariates 118 Figure 4.6: Effect of having neighboring schools with high PBE rates in the previous year 119 Figure A4.1: Reduction in AIC (compared to OLS) for spatial autocorrelation models, nearest 5 to 50 neighbors for all schools and schools of the same type 122 Figure A4.2: Effect of having neighboring schools with high PBE rates in the previous year, as measured by yearly standardized rates 123 Figure A4.3: Effect of neighboring schools with high PBE rates in the previous year, 1998-2014 124 Figure 5.1: Transmission settings for measles infections 153 x Figure 5.2: Ratio of percentage of infections to percentage of children enrolled in school and childcare centers 154 Figure 5.3: Distribution of outbreak sizes with and without shared vaccination status among siblings 155 Figure 5.4: Prominence of household transmission in reported measles outbreaks 156 Figure 5.5: Infections in children aged < 1 year 157 Figure 5.6: Average number of overall infections and percentage of infections acquired in malls and neighborhoods as proportion of contact increases 158 Figure A5.1: Transmission settings for measles infections, using 1 and 40 initial infections. 159 Figure A5.2: Transmission settings for measles infections, when PBEs are distributed as observed by siblings do not
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