At the time, public health officials suspected the increase in numbers of parents refusing to vaccinate their children was a factor, and research published in Pediatrics now confirms that "clustering of unvaccinated individuals" played a significant role.
Whooping cough, or pertussis, is highly contagious and can spread quickly through a community.
Researchers found that people in areas with high rates of nonmedical vaccine exemptions, also called personal belief exemptions, were 2 1/2 times more likely to live in an area with high rates of pertussis.
Both clusters of vaccine refusals and clusters of pertussis cases were often found in high-income, educated neighborhoods.
Researchers note that many factors contributed to the outbreak, including the fact that pertussis is a cyclical disease. In addition, the protection offered by the current pertussis vaccine is shorter-lived than previously believed, which means some older children have lost their immunity. Adults require boosters.
Health officials warn that herd immunity is vital to protect infants and vulnerable individuals who cannot be vaccinated. When fewer than 95 percent of individuals are vaccinated, a community loses herd immunity.
In 2010, just 91 percent of kindergartners in California were up to date on their shots. Although researchers could see data for all nonmedical exemptions, they could not tell which specific vaccines parents had refused.