Study: Parents protesting vaccines cause whooping cough outbreak

Researchers at Johns Hopkins think they now know what contributed to the biggest outbreak of whooping cough in California in 47 years. The new study ties it to a cluster of people who refused to have their children vaccinated for the disease.

"When you have conscientious objectors to immunizations, the rates of those diseases go up in the community," Dr. Ellen Rome, a pediatrician at Cleveland Clinic Childrens, said.

Researchers looked at the number of people who refused the whooping cough vaccine, otherwise known as non-medical exemptions. Their children entered kindergarten between 2005 and 2010 - the year the outbreak occurred.

Data show a link between geographic areas, with a large number of people who refused the vaccine, and cases of pertussis, also known as whooping cough.

Researchers say with diseases like measles and pertussis, about 95 percent of the population must be immunized to prevent outbreaks. They conclude that large numbers of unvaccinated or under-vaccinated people can lead to whooping cough outbreaks, putting vulnerable populations like infants at increased risk.

"Non-vaccinated babies who are exposed to whooping cough or pertussis are at risk of dying," Rome said. "If adults are non-vaccinated we may just get a cold or a bad cough, but for kids that can be fatal."

Complete findings for this study are in the journal "Pediatrics."

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