Evaluation of: Cortes JE, Curns AT, Tate JEet al.Rotavirus vaccine and health care utilization for diarrhea in U.S. children.N. Engl. J. Med.365(12), 1108–1117 (2011). Cortes et al. thoroughly analyzed the effect of the introduction of mass rotavirus vaccination on healthcare utilization in a population of approximately 2 million US children under 5 years of age. According to the results of the study, hospitalization has been reduced from prevaccination periods by 89% for rotavirus diarrhea and by 44% for diarrheal disease. The estimated reduction of hospitalizations during the two (single-year) periods amounted to 64,855 and the respective savings in treatment costs were US$278 million. The significance of this paper is its approach to the estimation of vaccine efficacy by the assessment of vaccination coverage and hospitalization, as well as emergency department and outpatient visits in a large population of children without any ‘selection’ (e.g., hospitalization) bias.