As an outbreak of measles linked to Disneyland sweeps across the state, researchers have identified five Northern California areas that have higher proportions of unvaccinated children than the general population.

The study, published online Monday in the journal Pediatrics, found five statistically significant clusters where 18 to 23 percent of kids were under-immunized. Outside of those areas, across the 13 Northern California counties included in the study, 11 percent of children hadn’t had all of the recommended vaccines.

Health officials worry that illnesses such as measles or whooping cough could spread more rapidly in areas that have lower vaccination rates.

In the five hotspots, the percentage of children who had not received any vaccines varied, from 5.5 to 13.5 percent. Outside of the clusters, the immunization refusal rate was 2.6 percent.

These are the five areas identified by the study as having a higher percentage of children who are unvaccinated:

-East San Francisco Bay, from Richmond to San Leandro
-Sonoma and Napa counties
-An area between Sacramento and Roseville
-Northern San Francisco and Southern Marin counties
-Vallejo

The researchers analyzed data from more than 154,000 children who were enrolled in Kaiser Permanente Northern California health insurance and born between 2000 and 2011.