January 2015, VOLUME 135 / ISSUE 1
Published online January 19, 2015
Geographic Clusters in Underimmunization and Vaccine Refusal
Tracy A. Lieu, MD, MPHa, G. Thomas Ray, MBAa, Nicola P. Klein, MD, PhDa,b, Cindy Chung, MDc, and Martin Kulldorff, PhDd
aDivision of Research, Kaiser Permanente Northern California, Oakland, California;
bVaccine Study Center, Kaiser Permanente, Oakland, California;
cDepartment of Pediatrics, San Rafael Medical Center, Kaiser Permanente Northern California, San Rafael, California; and
dDepartment of Population Medicine, Harvard Pilgrim Health Care and Harvard Medical School, Boston, Massachusetts
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.