Estimating Population-Level Effects of the Acellular Pertussis Vaccine Using Routinely Collected Immunization Data

Measuring and reporting the different population-level effects of the acellular pertussis vaccine on pertussis disease in addition to direct effects can increase the cost-effectiveness of a vaccine.

We conducted a retrospective cohort study of children born between 1 January 2008 and 31 December 2017, in King County, Washington, who were enrolled in the Washington State Immunization Information System. Diphtheria, tetanus toxoids, and acellular pertussis (DTaP) vaccination data from in the Washington State Immunization Information System was linked with pertussis case data from Public Health Seattle and King County. Census-level vaccination coverage was estimated as proportion of age-appropriately vaccinated children residing in it. Direct vaccine effectiveness was estimated by comparing pertussis risk in fully vaccinated and undervaccinated children. Population-level vaccine effectiveness (VE) measures were estimated by comparing pertussis risk in census tracts in the highest quartile for vaccination coverage with that in the lowest quartile.

For direct protection, estimated VE was 76% (95% confidence interval, 63%-84%) in low-vaccination-coverage clusters, and it decreased to 47% (13%-68%) in high-coverage clusters, after adjustment for potential confounders. The estimated indirect VE was 45.0% (95% confidence interval, 1%-70%), the total VE 93.9% (91%-96%), and the overall VE 42.2% (19%-60%).

Our findings suggest that DTaP vaccination provided direct as well as indirect protection in the highly immunized King County, Washington. Routine DTaP vaccination programs may have the potential to provide not only protection for vaccinated individuals but also for the undervaccinated individuals living in the same area.