Epidemiologic methods in immunization programs

Epidemiologic studies and principles are central to the evaluation of newly developed candidate vaccines against infectious diseases. This review draws primarily on experiences with vaccine introduction in the US to illustrate the application of epidemiologic methods to this process. Before licensure, a vaccine must demonstrate its safety and efficacy in phased clinical trials. Continued close monitoring is needed after licensure, both of the vaccine and the immunization program itself. Surveillance for vaccine coverage, disease incidence, and adequacy of the cold chain are essential benchmarks for program evaluation. Vaccination coverage can be monitored through direct measurement or estimated indirectly through surveys and reports of doses of vaccine administered or distributed. Surveillance data on reported cases of disease are critical to assess whether the program is having an impact, to determine why disease is still occurring, to evaluate whether new strategies are necessary, and to detect problem areas and populations that require more intensive program input. Combined vaccines requiring fewer injections and visits are needed to maintain continued high population immunity with minimal discomfort and highest compliance. The increasing centralization of health care organization facilitates this task.