Jessica E. Atwell of the Johns Hopkins Bloomberg School of Public Health in Baltimore, Josh Van Otterloo of Emory University School of Public Health in Atlanta and Jennifer Zipprich of the California Department of Public Health said the 2010 outbreak of pertussis, or whooping cough, of 9,120 cases was the largest whooping cough outbreak since 1947.
Several causes of the outbreak were documented, including waning immunity of the acellular pertussis vaccine.
However, the study published in the journal Pediatrics analyzed non-medical exemptions for children entering kindergarten from 2005-10, and pertussis cases diagnosed in 2010 in California.
The researchers identified 39 statistically significant clusters of high rates of non-medical exemptions, and two statistically significant clusters of pertussis cases.
Census tracks within an exemption cluster were 2.5 times more likely to be in a pertussis cluster. With highly infectious diseases such as measles and pertussis, it is estimated that more than 95 percent of the population must be immunized to prevent outbreaks and to reduce the risk of the disease for those too young to be vaccinated or unable to receive vaccines.
The study findings suggest clustering of those who refused the whooping cough vaccine for non-medical reasons might have been one of several factors in the 2010 California pertussis resurgence.