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Vaccine refusals contributed to pertussis outbreak in California, study suggests

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In a 2010 video, Dr. Saad Omer, Emory University, on the "beauty of immunizations" and the workings of "herd immunity." Vaccines are one of the most effective tools available for preventing disease in humans, particularly children, says Omer.

High rates of non-medical exemptions (NMEs) from immunizations for kindergarten-aged children in some California geographic areas were among contributing factors to the state’s pertussis outbreak in 2010, according to researchers at Emory’s Rollins School of Public Health. 

In 2010, approximately 9,120 cases of pertussis – or whooping cough – and 10 deaths, were reported in California. These numbers represented one-third of all cases seen in the country that year and the most cases seen in the state since 1947. Several causes of the outbreak have been documented, but the role of vaccine refusal has not been explored. A recent study in the October 2013 edition of Pediatrics titled, "Non-medical Vaccine Exemptions and Pertussis in California, 2010," examines the role that non-medical vaccine exemptions played in the outbreak.

The researchers, led by Saad B. Omer, PhD, associate professor of global health at the Rollins School of Public Health, analyzed NMEs for children entering kindergarten from 2005 through 2010 along with pertussis cases with onset in 2010 in California, to determine the role of NMEs in the outbreak.

The team identified 39 statistically significant clusters of high rates of NMEs, and two statistically significant clusters of pertussis cases. Census tracts 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.

"Our findings suggest that communities with large numbers of intentionally unvaccinated or under-vaccinated individuals can lead to pertussis outbreaks," explains Omer. "In the presence of limited vaccine effectiveness and waning immunity, sustained community-level transmission can occur, putting those who are most susceptible to communicable diseases, such as young infants, at increased risk."

Complete findings of the study are available online at

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