Dangerously Low Vaccination Rates in California: Is the Golden State Heading for the End of Herd Immunity? Data Answers.

Alice Corona
Jul 16, 2015 · 6 min read

In late January 2015, a serious outbreak of measles began in Disneyland in Orange County, California. The outbreak did not results in deaths. It did frighten parents around California and spurred a public outcry against the “anti-vaxers” — the parents who are vehemently opposed to mandated vaccinations for various reasons (among which some very questionable ones, such as the unproven assertion that vaccinations are associated with increased risk of autism).

The Golden State has long been known for its high concentration of parents who have not vaccinated their children. In some parts of California, vaccination rates have dropped far below the required level to maintain herd immunity (ironically, some of the places with the lowest vaccination rates are among the highest income locations, including Marin County). In July 2015, the California State Legislature passed a law mandating vaccinations for all Kindergarten students entering schools. The law also eliminated the Personal Belief Exemption, a way for parents to exempt their children from vaccination requirements.

This article examines data on the California public school districts and schools with the lowest vaccination rates for kindergartens. All data is pulled from the California Department of Public Health and refers to the latest reporting period (2014–2015).

Click here for information on definition and terminology.

San Francisco, Los Angeles and Measles: Percentage of vaccinated Kindergarten students is below the “herd-immunity” threshold

According to public health standards for measles, “coverage rates must remain in the 92–94% range to ensure “herd-immunity” against measles, which involves maintaining a threshold level of vaccination across a community that protects even the unvaccinated”. The 2014–2015 data suggests that in roughly one-third of California’s cities, Kindergartners without the MMR vaccination (Measles-Mumps-Rubella).

Cities with the lowest MMR vaccination rates. (Percentage of Enrolled Students without Vaccination)

Most of the cities featured in the previous map are small. They have a couple of schools and a limited number of students enrolled. However, not all cities with vaccination rates for MMR lower than 90% are small.

For example, Los Angeles reports having 19% of its 30,000 kindergartners without the MMR vaccination. The rate for private schools (14%) is actually lower than that of public schools (Caveat: only two-thirds of private schools actually reported). San Francisco also reported a high overall MMR exemption rate (10.4%), well below herd immunity maintenance levels.

Percentage of K-grade students Without MMR in the 10 biggest cities

Statewide, the rate for California is just slightly above the 92.2% immunization threshold.But, as the previous graphs showed, the rate differs greatly between communities.

Nevada County: Almost One of Every Five Students Not Vaccinated Because of Parents’ “Personal Beliefs”

Personal Belief Exemption Mapped by Counties — Top 15 with Highest Rates

Personal Belief Exemptions are highly controversial. But in some parts of California, they are extremely common. For example, more than 21% of Nevada County’s 838 enrolled kindergartners are not up to date with mandatory vaccinations because of their parent’s personal belief. This is the highest rate in California. Nevada County is different in other ways. Usually, it’s private schools that have higher PBE rates. In Nevada County, however, the school with the highest PBE rate is public. Nevada City’s public school enrolls 264 kindergarteners, of which only 51% are up to date with all vaccinations. Almost 4% of them didn’t even have to provide a proof of documentation of counseling with a healthcare practitioner. In other words, this is a mid-sized elementary school with a shockingly low vaccination rate.

Kindergartner Vaccination Rates in Cities in Nevada County

Where Kindergartners Are Not Vaccinated for Parents’ Religious Belief

From January of last year, families applying for a Personal Belief Exemption (PBE) have to provide documentation of counseling with a healthcare practitioner (unless it is a religiously motivated PBE — such as for practitioners of Christian Science).

According to the Center for Disease Control and Prevention (CDC), of the 159 cases of measles reported in 2013:

  • 82% of cases were in unvaccinated persons. Plus 9% in people who weren’t sure.
  • 79% of those the unvaccinated cited philosophical differences as the cause of their exemption from the MMR vaccine.

Cities with Highest Religious PBE (% of all Enrolled) and Comparison with Health Care Practitioner Counseled PBE (% of all Enrolled)

Lake View Terrace, in LA County, has 63 kindergarteners enrolled in 2 schools, one public and one private. In the public school, only 82% of the students are up to date with vaccinations. This is well below the “herd-immunity” threshold, but still significantly better than its private counterpart.

Most of Lake View Terrace’s kindergarteners are in the private school. Of its 35 students, 25 are exempted from mandatory vaccinations because of their parents’ religious beliefs. Overall, only 9% of the K-graders attending Lake View’s private school are up to date with all vaccinations. This also brings down the overall vaccination rate in Lake View to 41%, when combining all of the city’s kindergarteners.

Cities and Overall PBE vs. Religious PBE

Toulumne County, Shasta County, and Siskiyou County have the highest incidence of religious PBE (3.2%). In the first two, religion accounts for about half of the Personal Belief Exemption cases. In Siskiyou County, together with Lassen County, the religious PBE actually surpasses the cases where the exemption is accompanied by a document proving a counseling with a health care practitioner. In Modoc County, religion accounts for all the PBE. These are all rural counties and are relative poor compared to urban counties.

Counties and Religious PBE (% of all Enrolled) vs. Health Care Practitioner Counseled PBE (% of all Enrolled)

The latest data on U.S. nationwide vaccination coverage suggests that herd immunity is not met in 10 U.S. States, in the District of Columbia, and in two U.S. territories. Physicians — and especially pediatricians — worry that the Disneyland outbreak is only the beginning of a resurgence of many diseases once well controlled by vaccines.

U.S. Vaccination Coverage — States with MMR Rates below 92%

NOTE:You can find all the data, together with more interactive charts and articles in Silk’s dedicated resource: Latest Measles and Vaccination Rates in California

Originally published at blog.silk.co.


Silk is a place to explore the world through data.


Silk is a place to explore the world through data. Each Silk contains data on a specific topic. Silk displays data as beautiful interactive charts, maps and web pages.

Alice Corona

Written by

Stories with data, from the data collection (or scrape) to the data visualization. Data storytelling instructor. Currently project leader at batjo.eu


Silk is a place to explore the world through data. Each Silk contains data on a specific topic. Silk displays data as beautiful interactive charts, maps and web pages.