Vaccination and Exemption Rates in California

Get Educated. Get Vaccinated. Stop the Outbreaks.

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Clusters of intentionally unvaccinated people are leading to outbreaks of vaccine-preventable diseases in California. Photo by Getty Images

Are vaccination rates in California rising?

That would certainly be good news, as California has more people than the 21 least populated states in the country combined. Ranking as the most populated state, California also ranks as a top tourist destination in America and internationally. Unlike Las Vegas, what happens in California doesn't always stay in California. The Disneyland measles outbreak spread throughout California and to six other states and to Mexico and Canada (159 cases).

How high are the vaccination rates in California?

Opponents of SB 277, a bill that would eliminate personal belief vaccine exemptions in California, like to claim that 97.46% of children in the state are vaccinated? And along with rising vaccine rates, they also claim that there are declining vaccine exemption rates.

California Vaccination and Exemption Rates

Can that be true? With vaccination rates that high, they would be leading the country and likely wouldn't have the highest numbers of measles and pertussis cases.

Assembly Bill 2109, passed in 2012, did require parents seeking a personal belief exemption to get documentation and education from a health care provider and that did seem to increase vaccination levels a little, but still, in 2014:

  • only 89.4% of preschool age children had all of their recommended vaccinations, including a 0.27% reduction in PBEs
  • only 90.2% of kids starting kindergarten had all of their recommended vaccinations, including a 0.61% reduction in PBEs

    So no, vaccination rates aren't that high and there is still a lot of work to do in getting their vaccination rates up and vaccine exemption rates down.

    Keep in mind that personal belief exemptions to vaccines in California rose 337% between 2000 and 2012.

    And California still ranks below the majority of states for vaccination rates.

    For example, in the latest CDC survey of kids entering kindergarten, at least 37 other states had higher rates of MMR vaccination. Conversely, at least 38 states had lower rates of vaccine exemptions.

    California's Clusters of Unvaccinated Kids

    The state average vaccination rates and exemption rates don't tell the whole story, though.

    An even bigger problem is that in some schools, even though the state average for PBEs was 2.67%, a much higher percentage of kids are intentionally unvaccinated and susceptible to vaccine-preventable diseases. In these schools, 50% or more of the kids may not be getting vaccinated.

    And it is not just that these intentionally unvaccinated kids are clustered together in one school - these schools are often clustered together in the same area of the state.

    Why is this a problem?

    Several studies have found that clusters of unvaccinated children have been responsible for outbreaks of vaccine-preventable diseases, including measles and pertussis.

    For example, in 2010, clusters of nonmedical exemptions (NMEs) were found to be a factor for the "California pertussis resurgence." One study, "Nonmedical vaccine exemptions and pertussis in California, 2010," found that "geographic areas with high rates of NMEs are associated with high rates of pertussis." This suggests that "communities with large numbers of intentionally unvaccinated or under-vaccinated persons can lead to pertussis outbreaks."

    And unfortunately, several studies have found the same thing with measles, Hib, and chicken pox.

    Herd Immunity in California

    Again, some people try to use the average immunization rates in California to suggest that enough kids are getting vaccinated to prevent outbreaks of vaccine-preventable diseases or that they are above the threshold for herd immunity.

    Of course, we have seen that this isn't true.

    To understand why and to understand the effects of clustering of unvaccinated kids and adults on herd immunity, it helps to understand that herd immunity is also referred to as community immunity. If enough of the people in your community are vaccinated, then even those who can't be vaccinated (or are intentionally not vaccinated are 'hiding in the herd') will likely be protected if a disease is introduced into the community.

    But who is in your herd? Your 'herd' is the people that you are around - the people that you or your child comes into contact with on a regular basis.

    Unfortunately, many people don't seem to understand that. The ACLU of California went so far as to say that the "ACLU of California states that in any event, herd immunity is a concept that applies to the entire population in a geographic area, not simply to public school students."

    This kind of idea that herd immunity only applies to large geographic areas is too simplistic. Herd immunity also applies to sub-populations.

    Consider a teacher with measles.

    He might not cause a large outbreak in a city with vaccination rates at or above 95%, but what happens when he goes to work at a school in that city where 20 to 50% of the kids have personal belief vaccine exemptions? Or when he goes home to his unvaccinated family?

    Exposing so many unvaccinated people will cause an outbreak. And if they are in a community of like-minded people who also don't vaccinate their kids or themselves, then it might be a very large outbreak that is hard to control. An outbreak that poses a risk even to those who aren't intentionally unvaccinated, including those too young to get vaccinated and protected yet and those with immune system problems.

    California Measles Outbreaks

    The California measles outbreaks should have been a good wake-up call that low vaccination rates and clusters of intentionally unvaccinated children are a problem and "can undermine measles elimination."

    No, not the 2015 measles outbreaks.

    I'm talking about the 2008 outbreak that:

    • started with an intentionally unvaccinated child in San Diego, said to be a patient of Dr. Bob Sears
    • quickly spread to eleven other children, including one who traveled to Hawaii while still contagious
    • led to the largest measles outbreak in San Diego since 1991
    • hospitalized one infant who was too young to be vaccinated and whose parents said that they spent "spent 3 days in the hospital fearing we might lose our baby boy."
    • put 121 intentionally unvaccinated children and infants too young to be vaccinated into 21-day quarantine
    • cost at least $10,376 per case to control, not including direct medical costs and direct and indirect costs for quarantined children, etc.

    And most importantly, the 2008 outbreak in California "was fueled by clusters of intentionally unvaccinated children."

    I could just as easily have been talking about the 2011 California measles outbreaks, though when California led the U.S. in measles cases.

    Or 2014, when California had at least 60 measles cases, including at least 22 in Orange County alone.

    Of course, those outbreaks have been dwarfed by the 2015 outbreak, the Disneyland outbreak, during which Orange County already has had at least 35 cases of measles and the state is up to 136 cases.

    Has California finally learned their lesson?

    Get Educated. Get Vaccinated. Stop the Outbreaks.


    Atwell, Jessica. Nonmedical vaccine exemptions and pertussis in California, 2010. Pediatrics Vol. 132 No. 4 October 1, 2013. pp. 624-630

    CDC. Vaccination Coverage Among Children in Kindergarten — United States, 2013–14 School Year. October 17, 2014 / 63(41);913-920

    Smith PJ, Chu SY, Barker LE. Children who have received no vaccines: who are they and where do they live? Pediatrics 2004;114:187–95.

    Sugerman DE. Measles outbreak in a highly vaccinated population, San Diego, 2008: the role of the intentionally undervaccinated. Pediatrics 2010;125(4):747–55.

    Thompson JW. Impact of addition of philosophical exemptions on childhood immunization rates. American Journal of Preventive Medicine; 2007;32(3):194–201.

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