WASHINGTON (WUSA9) -- California reported more than 9,000 cases of whooping cough in 2010 -- the state's highest rate in 66 years.
A new study shows a growing number of vaccine refusals contributed to the outbreak.
Whooping cough -- also called pertussis -- is a respiratory infection caused by the pertussis bacteria. It most commonly affects children.
In the past, researchers attributed pertussis outbreaks to the growing immunization of the vaccine. While immunity still plays a role, the new study shows vaccine refusals are also a major factor.
The study, published in the October 2013 issue of Pediatrics, says more than 95 percent of any population should be vaccinated to avoid an outbreak. But in California, more and more parents are refusing the whooping cough vaccine for non-medical reasons.
The data showed a significant connection between places where vaccine refusals were common, and where the outbreaks occurred. Areas with a high rate of vaccine refusals were 2.5 times more likely to see an outbreak.
The process to refuse a vaccine varies by state. In California, parents can exempt their children from vaccines with just a signed form that cites religious, philosophical or other non-medical reasons.