Geographical Clustering of Nonmedical Exemptions to School Immunization Requirements and Associations with Geographical Clustering of Pertussis

Learning Objectives for this Presentation:
By the end of the presentation participants will be able to appreciate the dynamics and impact of local clustering of vaccine exemptors.

Background:
School immunization requirements are important in controlling vaccine-preventable diseases in the United States. Forty-eight states offer non-medical exemptions to school immunization requirements. Children with exemptions are at increased risk of contracting and transmitting vaccine-preventable diseases. The clustering of non-medical exemptions can impact community risk to vaccine-preventable diseases.

Objectives:
To evaluate spatial clustering of nonmedical exemptions in Michigan and geographical overlap between exemptions clusters and clusters of reported pertussis cases.

Methods:
Spatial analysis of clustering of nonmedical exemptions and pertussis using Scan Statistics.

Results:
Six statistically significant clusters of pertussis cases were identified spanning the following timeframes: 8/93 – 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 minorities, proportion 5 years or younger, and average family size (OR = 3.4; CI, 2.8 – 4.1).

Conclusions:
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.