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Whooping Cough Outbreaks Tied to Parents Shunning Vaccines

Study finds that areas with high rates of nonmedical vaccine exemptions also had high number of cases


But, the researchers also wanted to see if the clustering of people who had nonmedical exemptions for vaccines played a role in the outbreak. Omer said that previous research has shown that there do tend to be clusters of people with nonmedical vaccine exemptions.

In California, the rate of such exemptions has risen from 0.77 percent in 2000 to 2.33 percent in 2010, according to the study. Still, the state has relatively high vaccination rates. Almost 91 percent of children entering kindergarten in 2010 had received all the required immunizations, the study authors pointed out.

Omer and his team reviewed data on vaccine exemptions from school data, and they geographically coded that information to the census tract level. They then did the same thing with data on whooping cough cases.

The investigators found 39 clusters for high nonmedical exemption rates and two statistically significant clusters of whooping cough cases. Census tracts within a high nonmedical exemption cluster were 2.5 times more likely to be in a whooping cough cluster than areas without high exemption rates.

The risk of whooping cough was 20 percent higher inside a vaccine exemption cluster than outside of one, according to the study.

In California, whooping cough clusters and nonmedical exemption clusters were associated with a lower population density, lower average family size, fewer minorities, higher percentage of high school graduates, higher average household incomes and a lower percentage of families living in poverty, the study found.

"There was some nice analysis from the CDC years ago that people who refuse vaccines tend to be higher in socioeconomic status," added Omer.

An expert not involved with the study agreed with that analysis.

"The irony is that normally people with lower socioeconomic status have an increased risk of infectious diseases, but with vaccine-preventable infectious diseases, the risk is higher for those higher in socioeconomic status," noted Dr. Kenneth Bromberg, director of The Vaccine Research Center and chairman of pediatrics at The Brooklyn Hospital Center in New York City.

Some who seek these exemptions argue that it's a personal decision that only they can make for their family. But, Omer and Bromberg both expressed concern because the decision not to vaccinate is likely putting others at risk for infection.

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