Nonmedical Vaccine Exemptions and Pertussis in California, 2010

Abstract

WHAT'S KNOWN ON THIS SUBJECT: Previous studies have shown that nonmedical exemptions (NMEs) to immunization cluster geographically and contribute to outbreaks of vaccinepreventable diseases such as pertussis. The 2010 pertussis resurgence in California has been widely attributed to waning immunity from acellular pertussis vaccines. WHAT THIS STUDY ADDS:This study provides evidence of spatial and temporal clustering of NMEs and clustering of pertussis cases and suggests that geographic areas with high NME rates were also associated with high rates of pertussis in California in 2010.abstract BACKGROUND: In 2010, 9120 cases of pertussis were reported in California, more than any year since 1947. Although this resurgence has been widely attributed to waning immunity of the acellular vaccine, the role of vaccine refusal has not been explored in the published literature. Many factors likely contributed to the outbreak, including the cyclical nature of pertussis, improved diagnosis, and waning immunity; however, it is important to understand if clustering of unvaccinated individuals also played a role. METHODS:We analyzed nonmedical exemptions (NMEs) for children entering kindergarten from 2005 through 2010 and pertussis cases with onset in 2010 in California to determine if NMEs increased in that period, if children obtaining NMEs clustered spatially, if pertussis cases clustered spatially and temporally, and if there was statistically significant overlap between clusters of NMEs and cases.RESULTS: Kulldorff' s scan statistics identified 39 statistically significant clusters of high NME rates and 2 statistically significant clusters of pertussis cases in this time period. Census tracts within an exemptions cluster were 2.5 times more likely to be in a pertussis cluster (odds ratio = 2.47, 95% confidence interval: 2.22-2.75). More cases occurred within as compared with outside exemptions clusters (incident rate ratios = 1.20, 95% confidence interval: 1.10-1.30). The association remained significant after adjustment for demographic factors. NMEs clustered spatially and were associated with clusters of pertussis cases. CONCLUSIONS:

Geospatial analysis of nonmedical vaccine exemptions and pertussis outbreaks in the United States

“…This method identifies localized, statistically significant (P < 0.05) geographic clusters of events adjusting for multiple testing (27,28,(36)(37)(38). We evaluated potential clusters between a 0-and 200-km radius by using the free software program SaTScan, version 9.4 (38).…”

“…However, if one takes a closer look at the community level, there is substantial local variation in vaccination rates throughout the United States (27)(28)(29)36). It is the local variation in vaccination rates that is of concern because more than 90% of the community needs to be vaccinated against pertussis for herd/community immunity to be effective (39).…”

“…Several studies have suggested reasons for the resurgence of pertussis in the United States, including waning immunity from the acellular pertussis vaccine that replaced whole cell vaccine (9)(10)(11)(12)(13)(14)(15)(16)(17)(18); medical advancements that have allowed more effective diagnosis and reporting of pertussis cases (19,20); asymptomatic transmission of B. pertussis from individuals vaccinated with the acellular pertussis vaccine (21,22); evolving variant strains of B. pertussis circulating that are less protected by the vaccine (23)(24)(25); and a decrease in vaccine coverage that has compromised herd/community immunity (1,(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). It is clear that the rise in reported pertussis cases in the United States is a multifaceted problem not fully captured in national or state-level data.…”

“…It is clear that the rise in reported pertussis cases in the United States is a multifaceted problem not fully captured in national or state-level data. One approach to understanding the rise in reported pertussis cases in the United States is the identification of statistically significant geographic clusters of events, such as pertussis outbreaks and clusters of nonmedical vaccine exemptions (27,28,36). The aim of this investigation was to identify statistically significant geographic clusters of high rates of pertussis cases and geographic clusters of high rates of nonmedical vaccine exemptions at the county level to gain a better understanding of the roles that waning immunity and nonmedical vaccine exemptions may have had on the resurgence of pertussis in the United States in 2012.…”

Public health: An injection of trust

“…But herd immunity breaks down when coverage drops even in small pockets of otherwise well-vaccinated regions. Omer and colleagues found that this effect may have contributed to a 2010 pertussis outbreak in California that infected more than 9,000 individuals 6 . "The clustering of refusals was associated with the clustering of pertussis, " says Omer.…”