It looks like a combination of the top and low ends of the socioeconomic distribution, Geographic clusters of underimmunization identified in Northern California:

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: