Community up to Date Rate As an Indicator for Pertussis

BACKGROUND:  Resurgence of vaccine preventable diseases (e.g., pertussis), due to less effective vaccine, suggests need for alternative public health strategies. Immunization information systems (IIS) may be a useful source of data for analysis. Herd immunity theory suggests that, independent of personal vaccination status, community immunity levels influence individual susceptibility to vaccine preventable disease. Low community up-to-date (UTD) rates may be a risk factor for vaccine-preventable disease incidence and spread. Assessing community UTD rates from IIS might allow health departments to identify and target place-based interventions.

METHODS: Colorado Immunization Information System (CIIS) records for Denver County residents in 2013 were received and cleaned. A software algorithm was written to generate a pertussis vaccine UTD status for individuals in CIIS who were less than 19 years of age.  Each person was assigned an “up to date” or “not up to date” status based on the Advisory Committee on Immunization Practices vaccine schedule. Last CIIS recorded patient home address was geocoded to census tract. Pertussis cases among Denver residents in 2013 were obtained from the Colorado Electronic Disease Reporting System (CEDRS). Case addresses were geocoded and point mapped. Population estimates for 2013 from the Colorado Department of Local Affairs were used as denominators in rate calculations. Rates of disease incidence and UTD by census tract-level were calculated.  Correlation (Pearson-Spearman Kendall) analyses between community “not up to date” status and pertussis case incidence were performed.  Disease incidence and UTD rates were mapped to census tracts using ArcGIS.  

RESULTS: Denver residents under 19 years of age totaled 149,830. Among 134,672 vaccinated individuals less than 19 years old, 69% were UTD for pertussis vaccination for their age.  UTD status varied widely across the 144 census tracts within the county (17-100%). CEDRS pertussis case reports for individuals <19 years were 95; pertussis incidence and UTD rates were mapped to show variance and visualize disparities across the county. By census tract, a correlation (p < 0.06) was found between community pertussis vaccine UTD status and pertussis incidence.  

CONCLUSIONS: In this population-based study, low community UTD rates were associated with increased incidence of vaccine preventable disease.  Maximizing immunization UTD status in communities or neighborhoods may be an effective strategy in reducing disease occurrence in an era of less effective vaccines. Using immunization registries to identify regions with low community UTD rates can help public health officials target community based interventions and communicate the importance of vaccination efforts.