Oral Typhoid Vaccination for Travelers

To the Editor.——  We read with interest the recent article on typhoid vaccines by Schwartz et al.¹ We commend the authors for their intent to evaluate the efficacy of typhoid vaccination among travelers in developing countries; however, serious and important problems in methodology and analysis cast doubt on some of their conclusions.In this study, a case-exposure cohort study design² was used to estimate vaccine efficacy. The cases were ascertained via clinic-based surveillance, and the proportion of tourists vaccinated was estimated by a survey of clinic attendees. To use this study design to calculate vaccine efficacy, the following assumptions are needed: (1) vaccinated and unvaccinated ill tourists were equally likely to seek medical care at the study sites, and (2) the vaccination status of clinic patients was representative of the vaccination status of all tourists. No data were presented on the validity of these assumptions in this study,