For a study, in addition to direct benefits, measuring and reporting the different population-level effects of the acellular pertussis vaccination on pertussis disease improved the vaccine’s cost-effectiveness. The researchers conducted a retrospective cohort analysis of children born in King County, Washington, between January 1, 2008, and December 31, 2017, who were enrolled in the Washington State Immunization Information System. Data from the Washington State Immunization Information System on diphtheria, tetanus toxoids, and acellular pertussis (DTaP) vaccinations were matched to pertussis case data from Public Health Seattle and King County. The proportion of age-appropriately vaccinated children residing in a census district was used to estimate vaccination coverage. By comparing pertussis risk of incompletely vaccinated and under-vaccinated children, direct vaccination effectiveness was calculated. Pertussis risk in census tracts in the greatest quartile for vaccination coverage was compared to that in the lowest quartile for vaccination coverage to calculate population-level vaccine effectiveness (VE) metrics. After adjusting for relevant confounders, the estimated VE for direct protection was 76% (95% CI, 63%–84%) in low-vaccination-coverage clusters and 47% (13%–68%) in high-vaccination-coverage clusters. The indirect VE was calculated to be 45.0% (95% CI: 1%–70%), the total VE was 93.9% (91%–96%), and the overall VE was 42.2% (19%–60%). In the highly vaccinated King County, Washington, the researcher’s findings implied that DTaP immunization conferred both direct and indirect protection. Routine DTaP vaccination programs had the ability to protect not only those who had been vaccinated but also those who had not been vaccinated and lived in the same area.