Geographic Clusters in Underimmunization and Vaccine Refusal

Abstract

BACKGROUND AND OBJECTIVE: Parental refusal and delay of childhood vaccines has increased in recent years and is believed to cluster in some communities. Such clusters could pose public health risks and barriers to achieving immunization quality benchmarks. Our aims were to (1) describe geographic clusters of underimmunization and vaccine refusal, (2) compare clusters of underimmunization with different vaccines, and (3) evaluate whether vaccine refusal clusters may pose barriers to achieving high immunization rates. METHODS: We analyzed electronic health records among children born between 2000 and 2011 with membership in Kaiser Permanente Northern California. The study population included 154 424 children in 13 counties with continuous membership from birth to 36 months of age. We used spatial scan statistics to identify clusters of underimmunization (having missed 1 or more vaccines by 36 months of age) and vaccine refusal (based on International Classification of Diseases, Ninth Revision, Clinical Modification codes). RESULTS: We identified 5 statistically significant clusters of underimmunization among children who turned 36 months old during 2010–2012. The underimmunization rate within clusters ranged from 18% to 23%, and the rate outside them was 11%. Children in the most statistically significant cluster had 1.58 (P < .001) times the rate of underimmunization as others. Underimmunization with measles, mumps, rubella vaccine and varicella vaccines clustered in similar geographic areas. Vaccine refusal also clustered, with rates of 5.5% to 13.5% within clusters, compared with 2.6% outside them. CONCLUSIONS: Underimmunization and vaccine refusal cluster geographically. Spatial scan statistics may be a useful tool to identify locations with challenges to achieving high immunization rates, which deserve focused intervention.

Geospatial analysis of nonmedical vaccine exemptions and pertussis outbreaks in the United States

“…Our findings are consistent with previous studies that examined geographic clusters of nonmedical vaccine exemptions and geographic clusters of pertussis cases (27,28,36). The findings are consistent with previous studies indicating that waning immunity is associated with the acellular pertussis vaccine (9-18).…”

“…This method identifies localized, statistically significant (P < 0.05) geographic clusters of events adjusting for multiple testing (27,28,(36)(37)(38). We evaluated potential clusters between a 0-and 200-km radius by using the free software program SaTScan, version 9.4 (38).…”

“…However, if one takes a closer look at the community level, there is substantial local variation in vaccination rates throughout the United States (27)(28)(29)36). It is the local variation in vaccination rates that is of concern because more than 90% of the community needs to be vaccinated against pertussis for herd/community immunity to be effective (39).…”

“…The rates of nonmedical vaccine exemptions are not evenly distributed among communities, which indicates that obtaining nonmedical vaccine exemptions tends to be a socially and spatially clustered behavior (27,28,36). The clustering of vaccine refusal creates "hot spots" in communities where herd/community immunity against vaccine preventable disease is compromised.…”

“…It is clear that the rise in reported pertussis cases in the United States is a multifaceted problem not fully captured in national or state-level data. One approach to understanding the rise in reported pertussis cases in the United States is the identification of statistically significant geographic clusters of events, such as pertussis outbreaks and clusters of nonmedical vaccine exemptions (27,28,36). The aim of this investigation was to identify statistically significant geographic clusters of high rates of pertussis cases and geographic clusters of high rates of nonmedical vaccine exemptions at the county level to gain a better understanding of the roles that waning immunity and nonmedical vaccine exemptions may have had on the resurgence of pertussis in the United States in 2012.…”