While whole-cell and acellular pertussis vaccines have been responsible for reducing morbidity and mortality due to pertussis, since the start of the current decade there has been a dramatic increase in the number of pertussis cases identified in the United States (1;2). In 2004, a total of 25,827 pertussis cases were reported (3). Several hypotheses have been suggested to explain this resurgence including: low vaccination coverage in certain demographic groups, waning immunity, poor vaccine immunogenicity and efficacy, and possible emergence of Bordetella pertussis variant.
All states require receipt of certain vaccines at the time of school of entry; however, there are some differences between states in terms of the specific antigens covered by these requirements. State mandated school immunization requirements have played a major role in controlling the rates of vaccine-preventable diseases in the United States. State immunization laws allow certain exemptions to these requirements. There is considerable variability across states in terms of the type of exemptions allowed and the intensity of enforcement. Moreover, preliminary evidence points to pockets of high exemption rates within states. Such pockets can potentially provide the critical mass of susceptible individuals required to maintain the transmission of vaccine-preventable diseases in the community—even when the overall state-level vaccination coverage is high.
In this study, we intended to: (a) Evaluate associations between state-level incidence of pertussis and state policies for ensuring compliance with school immunization requirements; (b) Determine the geographical distribution and clustering of exemptions and associations with clustering of pertussis. For the first aim, we evaluated association between state-level pertussis incidence from 1986–2004 and ease of obtaining nonmedical exemptions; policy of granting personal belief exemptions; and acceptance of parental signature as sufficient proof of compliance with school immunization requirements. For the second aim, we used geographical distributions of exemptions and pertussis cases in Michigan to identify spatial clusters of exemptions and to evaluate their association with space-time clusters of pertussis.
In unadjusted analysis, the incidence rate in the states where the option of personal belief exemption was available was more than twice as high as the states where this option was not available (IRR = 2.06; 95%CI 1.77–2.40). Ease of obtaining nonmedical exemptions was associated with a 27% and 90% higher incidence of pertussis in states with medium and high ease of exemption, respectively—compared to states where it was difficult to obtain exemptions (IRRs: 1.27; 95%CI 1.06–1.51 & 1.90; 95%CI 1.60–2.28). The incidence of pertussis was 41% higher for states which considered parental signature as sufficient proof of immunization compared to the states that did not (IRR = 1.41; 95%CI 1.12–1.77). Availability of personal belief exemptions (IRR = 1.48; 95%CI 1.03–2.13) and high exemption ease (IRR = 1.53; 95%CI 1.10–2.14) remained associated with pertussis incidence in the multivariate analysis adjusting for demographics.
Twenty three statistically significant clusters of high exemption rates were identified. In bivariate analysis, likelihood of a census tract being in an exemptions cluster was associated with population density (in 100s of individuals per sq. mile) (OR = 1.02; CI, 1.01–1.02), percentage of minorities in a census tract (OR = 1.01; CI, 1.01–1.02), percentage of children younger than 5 (OR = 1.10; CI, 1.05–1.16), and mean family size (OR = 3.9; CI, 2.8–5.5). Population density (OR = 1.02; CI, 1.01–1.02) and mean family size (OR = 2.8; CI, 1.8–4.4) remained significantly associated in multivariate analysis.
Six statistically significant clusters of pertussis cases were identified with p values ranging from 0.001 to 0.049. The six clusters spanned the following timeframes: 8/9–9/93, 8/94–2/95, 5/98–6/98, 7/00–11/00, 4/02, 5/03–7/03, and 6/04–11/04. It was more likely for census tracts in exemptions clusters to be in pertussis clusters (OR = 3.0; CI, 2.5–3.6). The overlap of exemptions clusters and pertussis clusters remained significant after adjusting for population density, proportion of minorities, proportion of individuals aged 5 years or younger, and average family size (OR = 3.4; CI, 2.8–4.1).
Conclusion. State-level policies on nonmedical exemptions and documentation of immunization status should be reviewed as part of the efforts to control pertussis and maintain a low incidence of other vaccine-preventable diseases.
Moreover, geographic pockets of vaccine exemptors pose risk to the whole community. In addition to monitoring state-level exemption rates, state and local health authorities should be mindful of within-state heterogeneity in exemption rates and should actively follow trends in sub-state level exemption rates.