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Charles LeBaron, MD
Trudy Murphy, MD
Centers for Disease Control and Prevention
Atlanta, GA 30333
Susan Lett, MD, MPH
Stephanie Schauer, PhD
Massachusetts Department of Public Health
Boston, MA 02108-4619
Our analysis favors adolescent vaccination, as do all published analyses that include adolescent vaccination strategies for the United States and Canada to date.1–5 However, Caro and colleagues feel that our analysis overstates the conclusions regarding adult vaccination based on a comparison to the work by Purdy et al.2 This comparison is difficult to make, because the adult strategy examined by Purdy et al was significantly different from the adult vaccination strategy considered in our analysis. Purdy et al modeled the impact of a 1-time booster of all adults 18 to 64 years of age without consideration of decennial boosters. Our analysis focused on vaccination of a single adult cohort with repeated boosters every 10 years. We believe both strategies are reasonable to consider in modeling exercises and that differences are to be expected given the different strategies considered and the different estimates for incidence used in each model.
We agree that herd immunity is a very important consideration. In our sensitivity analysis for herd immunity, we assumed that adult immunization would protect infants too young to be immunized, although we did not assume that unvaccinated adults would receive indirect protection because of other adults being vaccinated. Very scant data exist to support quantitative assumptions about the amount of indirect protection that unvaccinated infants or adults might receive. We chose the analysis without herd immunity as the base case analysis to focus on a pertussis vaccination program's direct benefits to those who would be vaccinated.
We agree with Caro and colleagues that the incidence of disease is key to the economic results regardless of the model that is considered. It is clear that if adult disease incidence is similar to adolescent disease incidence, strategies that include adult vaccination will look much more cost-effective. The key factor to consider is the incidence of disease in adults rather than the adolescent/adult ratio for either incidence or utilities. Using utilities to derive the cost per quality-adjusted life-year saved gives both adult and adolescent vaccination strategies appropriate credit for preventing morbidity.
We did not perform a mutually exclusive comparison as suggested by Caro and colleagues. The strategies in the model included an adolescent-only, adult-only, and adolescent + adult strategies. In our baseline analysis, the adolescent + adult vaccination strategy was not found to be cost-effective. However, in our sensitivity analyses, it is clear that if adult incidence is high enough and vaccine cost is low enough, adolescent + adult vaccination strategies are reasonably cost-effective. We agree with their statement that the results of our analysis should not preclude additional review of targeted or routine adult vaccination strategies. Models such as ours serve to clarify the issues in public health policy, but a single model rarely provides the final answer to any question. Future studies to refine our understanding of the benefits and costs of pertussis vaccination programs should be conducted as more information is put forward about pertussis vaccine efficacy in adults, disease incidence in adults, vaccine costs, and overall estimates of herd immunity.
REFERENCES
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