James Watt, chief of the communicable disease control division with the California Department of Public Health, called high personal belief rates "a concern," especially "because in some schools you really do have situations where you fall below a herd immunity threshold, and disease transmission is even more possible."

"Herd immunity" means that there's such a high vaccination rate that those who cannot be vaccinated, such as newborn infants or people with compromised immune systems, are protected.

[Related: Five Things You Should Know About Vaccines]

Schools are ripe places for disease transmission if kids aren't vaccinated. Kids don't have good "cough etiquette," Watt noted, plus "kids in schools have a lot of contact with each other." He stressed that the best thing parents can do is have their own children vaccinated. "The vast majority of children who are vaccinated are protected."

A more worrisome issue, he said, is for children who cannot be vaccinated for a medical reason. In that case, parents may want to weigh a school's PBE rate and consider what that means for their own child.

"It's one piece of information that parents can use as they think about schools and their own children and what their risks may be," Watt said. "Different children have different needs."

In January, a new state law goes into effect requiring parents who want to opt out of vaccines to talk with a health care provider first about the pros and cons of both vaccines and communicable illnesses. Washington state passed a similar law in 2011, and their opt-out rate has since dropped by almost 40 percent.

On a countywide basis, the highest PBE rates are found in a collection of Northern California counties. Alpine, Calaveras, Del Norte, Humboldt, Mariposa, Siskiyou and Tuolomne counties all have PBE rates between 10.5 and 12.5 percent. Trinity County's rate is 15.93 and Nevada County's rate is 22.44, but both these counties have only a few hundred students in school, so it would take just a few children with PBEs to create the high percentage rate.

By contrast, San Francisco County's PBE rate is 1.33 percent, and Los Angeles County's rate is 1.88 percent.

Our methodology

The state conducts a kindergarten assessment of vaccination rates each fall. The data shown above are from the 2012-2013 school year and reflect a survey of more than 500,000 children at 8,220 public and private kindergartens in California. The assessment is done at the school level and reported to local health departments and to the state.

The state then compiles a report of the immunization status of all kindergarteners, a table with 26 columns of information. More than 90 percent of kindergarteners in the state start school fully immunized. But some children have "conditional" status, meaning they are lacking some vaccines but parents intend to get them. Officials told us they do not want to deny entrance to school because the child is not fully up-to-date, but is expected to be soon. Some parents are opposed to vaccines, so they have a "personal belief exemption" on file.

To make it easier for readers to look up the PBE rate at their own school, we simplified the state's table. We did not alter the data in any individual column. Instead, we selected six of the 26 columns that we thought readers would find most interesting. In addition, we made the data sortable by county, city, school district and PBE rate. The state publishes a PDF of its data, which is not sortable. We asked the state for its Excel spreadsheet so that we could post this sortable version.