Clusters of underimmunized, vaccine-refusal communities identified in California

Underimmunized communities and families that refuse vaccines were found to cluster geographically, which could significantly increase the risk for an outbreak and create barriers in achieving immunization quality benchmarks, according to study data.

Prior studies have found that vaccine refusal and delay are associated with increased risk for measles and pertussis outbreaks, as well as higher individual risks for measles, pertussis, varicella and pneumococcal infections.

“The ability to identify clusters of underimmunization and vaccine refusal would help providers focus their efforts on the parents in communities with the greatest concerns, as well as identify areas at elevated risk for outbreaks of vaccine-preventable diseases,” researcher Tracy A. Lieu, MD, MPH, director of the division of research at Kaiser Permanente, and colleagues wrote.

Tracy A. Lieu

To gauge the viability of identifying geographic clusters of events characterized by vaccine hesitancy, Lieu and colleagues examined electronic health records of 154,424 children born between 2000 and 2011 in Northern California.

The researchers defined children as “underimmunized” if, by 36 months of age, they missed at least one of the vaccines recommended by the CDC’s Advisory Committee on Immunization Practices.

To evaluate individual-level demographic and neighborhood-level predictors of underimmunization, the researchers performed logistic regression in which the dependent variable was whether the child was underimmunized and the individual-level independent variables were calendar year, gender and race/ethnicity. Additionally, researchers included neighborhood-level variables such as race/ethnicity, Spanish speakers, households with income below the poverty level, housing units occupied by owners and education.

Lieu and colleagues identified five statistically significant clusters of underimmunization among children aged 36 months during 2010-2012, with underimmunization rates of 17.5% to 22.7% compared with 10.9% in the surrounding areas.

Children in the most statistically significant groups exhibited 1.58 times the rate of underimmunization as surroundings areas (P<.001). In addition, families that refused vaccines also were found to be geographically grouped, with rates of 5.5% to 13.5% within groups vs. 2.6% among those living outside them.

“While we were able to identify geographic clusters, we did not have additional data to explain why they arose,” Lieu and colleagues wrote. “Our analyses adjusted by race/ethnicity and neighborhood income suggested that these were not dominant drivers of clustering, although individual-level underimmunization was higher in neighborhoods with more families in poverty, as well as those with more graduate degrees. Future analyses of Internet use or social media might provide further insight about what drives geographical clustering.”

Disclosure: One of the researchers reported research support from GlaxoSmithKline, MedImmune, Merck, Novartis, Nuron Biotech, Pfizer and Sanofi Pasteur.