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In 2010, 9120 pertussis cases with 10 deaths were reported in California — the highest numbers since 1947. Although waning immunity has been proposed as a major cause, clustering of unvaccinated children might also have played a role.
To examine this possibility, researchers analyzed spatial clustering of nonmedical exemptions (NMEs) for children entering kindergarten from 2005 through 2010 and space-time clustering of pertussis cases with onset in 2010 in California. They hypothesized that if children obtaining NMEs and pertussis cases clustered spatially, and if there was statistically significant overlap between clusters of children with NMEs and cases, then clustering of NMEs might partially explain pertussis outbreaks despite a reported…