In 2010, 9,120 cases of pertussis – or whooping cough – were reported in California, the most since 1947. Several causes of the outbreak have been documented, including waning immunity of the acellular pertussis vaccine. A new study in the October 2013 Pediatrics examines the role of clusters of individuals who refused the vaccine. The study,“Nonmedical Vaccine Exemptions and Pertussis in California, 2010,” published online Sept. 30, analyzes non-medical exemptions for children entering kindergarten from 2005 through 2010, and pertussis cases that were diagnosed in 2010 in California. Researchers identified 39 statistically significant clusters of high rates of non-medical exemptions, and 2 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 like 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. Study authors conclude that communities with large numbers of unvaccinated or under-vaccinated people can lead to pertussis outbreaks, putting vulnerable populations like young infants at increased risk.