A 2010 outbreak of whooping cough infected 9,120 people and was linked to ten deaths. ( Getty Images/PhotoAlto)

The largest whooping cough outbreak since 1947 has been linked to clusters of unvaccinated children according to a new study.

The 2010 outbreak of pertussis, also known as whooping cough, in California infected 9,120 people and was linked to 10 deaths. Although health officials suspected the outbreak was the result of some children not being vaccinated, the new study confirms these suspicions.

Researchers examined the location of whooping cough cases and compared them with the areas where parents opted not to vaccinate their children for personal beliefs, not for medical reasons.

Published in the Pediatrics Journal last week, the study found that people living in areas where a large number of people had opted out of vaccines were 2.5 times as likely to live in an area with a large number of pertussis cases.

In addition, the outbreak did not come from areas with poorer socioeconomic households, where there may be less access to health care. Instead the clusters of both pertussis infections and those who opted out of vaccinations were in areas with higher socioeconomic characteristics.

In these regions people were more likely to be highly educated, have fewer children and make more money annually.

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"It's people who are consciously deciding not to do [vaccinations]," said Jessica Atwell, lead author on the study and a Ph.D. candidate at the Johns Hopkins University School of Public Health. "I think that they're basing these decisions on misinformation."

Atwell points out that whooping cough and measles are very virulent diseases. While a person with the flu can expect to infect one to two unvaccinated people, a person with pertussis can infect approximately 13 to 15 people if they are not vaccinated.

Atwell said in order to keep a virulent disease like pertussis from spreading, a community must have an immunity rate of between 93 percent and 95 percent.

"When you make that comparison, people understand this will go through a community like wildfire," Atwell said.

Pertussis is particularly dangerous for young infants. According to the Center for Disease Control, about half of the infants less than 1 year old who get pertussis will be hospitalized. Of those hospitalized, one or two out of 100 will die.

Only infants six months or older can be fully vaccinated against the disease, and adults or older children can be carriers for the disease without showing any symptoms.

"I think to be honest people didn't really fear pertussis anymore," Atwell said. "They don't realize how fragile our control of diseases like this is. They will come back."

The 10 deaths during the 2010 outbreak were all infants.

Atwell said following the outbreak, the state government took clear steps to help promote vaccinations.

The California health department has started to offer pertussis vaccinations for parents at birthing centers so their newborns will be more protected

In addition, a new law passed by the California state legislature and set to go into effect next year will make it more difficult for parents to get non-medical vaccine exemptions for their children.

"I think every parent has to do what they think is best for their child and [also think] how it will affect the rest of the community," said Atwell, who clarified that without a herd immunity, "it's much more dangerous."

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