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Study identifies geographic clusters of underimmunization in Northern California


Researchers used spatial analysis software and electronic medical records to identify clusters of underimmunization and vaccine refusal among Kaiser Permanente members in Northern California, according to a study published today in the journal Pediatrics.

Children are considered to be underimmunized when they miss one or more recommended vaccine doses before age three, while vaccine refusal means not allowing a child to receive any vaccines.

The study reviewed the immunization records of more than 154,000 Kaiser Permanente Northern California members between birth and 3 years old. All the children were born between 2000 and 2011 and were covered continuously by Kaiser Permanente, which provides health care for about 40 percent of the insured population in 13 Northern California counties. To identify the clusters of underimmunization and vaccine refusal, researchers used sophisticated spatial analysis software, which allowed them to examine health factors in relation to geography.

The Centers for Disease Control's immunization schedule recommends a minimum of 17 separate injections during a child's first two years of life, including for hepatitis A and B, diphtheria, tetanus, pertussis, influenza, polio, measles, mumps, rubella, and pneumococcal virus. Underimmunization is associated with elevated risk of vaccine-preventable disease, while nonmedical immunization exemption and refusal have been associated with increased community risk of measles and pertussis.

"This research confirms anecdotal reports of underimmunization clusters," said Tracy A. Lieu, MD, MPH, the study's lead author, a Kaiser Permanente pediatrician, and director of the Kaiser Permanente Division of Research. "In addition, we found clusters in places we hadn't anticipated."

Dr. Lieu noted that this study will be useful for targeting outreach efforts in particular areas, although it was not designed to figure out why children in some areas are less likely to be fully immunized.

"Everyone in the Kaiser Permanente health care system has access to vaccines, and since childhood vaccination is covered by insurance, financial barriers are not an issue," Dr. Lieu said. "Our findings raise awareness that there may be communities where parents have more vaccine hesitancy and may be interested in more information or more in-depth conversations with their children's doctors."

Underimmunization ranged from 18 percent to 23 percent within clusters, compared with 11 percent outside clusters. Between 2010 and 2012, geographic clusters of underimmunization were found in:

  • the East Bay (Richmond to San Leandro);
  • Sonoma and Napa counties;
  • a small area of east Sacramento;
  • northern San Francisco and southern Marin counties; and
  • a small area of Vallejo.

"Shot limiting," in which parents limit the number of injections or antigens that children receive during a pediatric visit to two or fewer, was found to cluster in similar areas.

Vaccine refusal ranged from 5.5 percent to 13.5 percent within clusters, compared with 2.6 percent outside clusters. Between 2010 and 2012, geographic clusters of vaccine refusal were found in:

  • the East Bay (El Cerrito to Alameda);
  • Marin and southwest Sonoma counties;
  • northeastern San Francisco;
  • northeastern Sacramento County and Roseville; and
  • a small area south of Sacramento

"Kaiser Permanente's electronic medical record system is among the richest in the world," Dr. Lieu said. "This is the first time that spatial analysis techniques have been applied to real-time vaccine data."


In addition to Lieu, co-authors of the study are G. Thomas Ray, MBA, Kaiser Permanente Division of Research; Nicola P. Klein, MD, PhD, Kaiser Permanente Vaccine Study Center; Cindy Chung, MD, Kaiser Permanente San Rafael Medical Center; and Martin Kulldorff, PhD, Harvard Pilgrim Health Care and Harvard Medical School.

About the Kaiser Permanente Division of Research

The Kaiser Permanente Division of Research (DOR) conducts, publishes and disseminates epidemiologic and health services research to improve the health and medical care of Kaiser Permanente members and the society at large. It seeks to understand the determinants of illness and well-being and to improve the quality and cost-effectiveness of health care. Currently, DOR's 500-plus staff is working on more than 250 epidemiological and health services research projects. For more information, visit or follow us at @kpdor

About Kaiser Permanente

Kaiser Permanente is committed to helping shape the future of health care. We are recognized as one of America's leading health care providers and not-for-profit health plans. Founded in 1945, our mission is to provide high-quality, affordable health care services and to improve the health of our members and the communities we serve. We currently serve approximately 9.5 million members in eight states and the District of Columbia. Care for members and patients is focused on their total health and guided by their personal physicians, specialists and team of caregivers. Our expert and caring medical teams are empowered and supported by industry-leading technology advances and tools for health promotion, disease prevention, state-of-the-art care delivery and world-class chronic disease management. Kaiser Permanente is dedicated to care innovations, clinical research, health education and the support of community health. For more information, go to:

Cyrus Hedayati | EurekAlert!

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