HealthDay News — Vaccine refusal is likely one of the factors that contributed to the California whooping cough outbreak in 2010, researchers have found.

An analysis of the largest pertussis outbreak in California since 1947 indicates that incidence of the disease occurred in two large spatial and temporal clusters, Jessica E. Atwell, MPH, from the John Hopkins Bloomberg School of Public Health in Baltimore, and colleagues reported in Pediatrics.

They analyzed nonmedical vaccine exemptions for California children entering kindergarten from 2005 to 2010 and pertussis cases with onset in 2010 to examine whether clustering of unvaccinated individuals played a role in the state’s pertussis resurgence. Data was available for 8,521 of the 9,143 pertussis cases that occurred in 2010.

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Spatial clustering of nonmedical vaccine exemptions were observed, and these correlated with clusters of pertussis cases, the researchers found. Overall, there were 39 significant clusters of nonmedical vaccination exemptions among children entering kindergarten, Kulldorff’s scan statistics revealed.

The likelihood of being in one of the two pertussis clusters was significantly increased for census tracts within an exemption cluster (odds ratio 2.47, 95% CI: 2.22-2.75), and more pertussis cases occurred within versus outside exemption clusters (incident ORA 1.20, 95% CI: 1.10-1.30).The associations remained significant after adjustment for demographic factors.

A bivariate analysis indicated that both types of cluster were linked to factors “characteristic of high socioeconomic status,” the researchers reported, including such things as lower population density, lower average family size and higher median household income. However, data was not available to determine the reasons for nonmedical vaccine exemptions.

From 2000 to 2010 the overall rate of pertussis in California increased from 0.77% of children entering kindergarten to 2.33%. Although these rates are low, as many as 84% of new students claimed vaccine exemptions in 2010, according to the researchers.

Several other factors are also believed to have contributed to the pertussis outbreak, including the idea that the newer acellular pertussis vaccine formulation is less effective than the older whole cell version and that mutations in the Bordetella pertussis pathogen may play a role in the acellular vaccine’s efficacy.

“Our findings suggest that communities with large numbers of intentionally unvaccinated or undervaccinated persons can lead to pertussis outbreaks,” the researchers concluded. “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.”


  1. Atwell JE et al. Pediatrics. 2013; 132: 624–630; DOI: 10.1542/peds.2013-0878.