Study identifies geographic clusters of underimmunization in Northern California

1. Among children in 13 Northern California counties, 5 statistically significant geographic clusters were identified where rates of underimmunization ranged from 18% to 23%. Vaccine refusal clusters and vaccine limiting clusters were also identified and found to have significant overlap with the underimmunization clusters.

2. Children in neighborhoods with higher percentages of Asians and Hispanics were more likely to be fully immunized. In contrast, children in neighborhoods with more families below the poverty line, as well as in neighborhoods with higher percentages of graduate degrees, were more likely to be underimmunized. 

Evidence Rating Level: 2 (Good)  

Study Rundown: Parental refusal to vaccinate their children has increased over the past decade and led to an emergence of vaccine-preventable diseases. This study sought to identify geographic clusters of underimmunization and vaccine refusal in Northern California. “Underimmunization” was defined as missing ≥1 of the vaccines recommended by the Centers for Disease Control’s Advisory Committee on Immunization practices by age 36 months. “Vaccine limiting” was defined as children who did not receive ˃2 injections on any day by 36 months. “Vaccine refusal” was defined as those with ≥1 ICD-9 diagnoses for vaccine refusal by 36 months. There were 5 statistically significant geographic clusters of underimmunization identified; rates of underimmunization within these clusters ranged from 18% to 23%, compared to 11% outside the clusters. Vaccine refusal and vaccine limiting were also clustered and overlapped with the underimmunization clusters. Individual-level predictors of underimmunization included neighborhoods with more families below the poverty line and neighborhoods with more graduate degrees. This study is limited by the exclusion of uninsured children and the use of ICD-9 codes for vaccine refusal compared to chart review. However, the identification of these geographic clusters can help physicians focus efforts to increase vaccination rates and be cognizant of areas that may be at higher risk for vaccine-preventable diseases.

Click to read the study, published today in Pediatrics

Relevant Reading: Alternative vaccination schedule preferences among parents of young children

In-Depth [retrospective cohort study]: Participants included 154 424 children born between January 1, 2000 to December 31, 2011 in the Kaiser Permanente Northern California system. Enrolled children had continuous membership from birth until age 36 months and ≥2 primary care visits by their first birthday. The study population was 38% white, 25% Hispanic, 22% Asian, 6% black and 9% unknown. Underimmunization increased during the study period from a mean of 8.1% from 2002 to 2005 to 12.4% from 2010 to 2012. A larger percentage of Asians and Hispanics in neighborhood block groups was associated with being fully immunized. In contrast, whites (OR 2.21; 95% CI 2.03-2.42) and blacks (OR 1.92; 95% CI 1.68-2.20) were more likely to be underimmunized. There was a 2.19 increased odds of underimmunization for each percent increase in households with incomes below the poverty line (95% CI: 1.44-3.34). There was a 2.63 increased odds of underimmunization for each percent increase in persons with graduate degrees (95% CI: 1.72-4.02). Among the 5 statistically significant geographic clusters of underimmunized children identified, underimmunization rates ranged from 17.5% to 22.7%, compared to 10.9% outside. Rates of vaccine refusal within the clusters ranged from 5.5% to 13%, compared to 2.6% outside.

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Image: PD

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