Typhoid fever in group travelers: Opportunity for studying vaccine efficacy

Background: Typhoid fever (TF) is a rare disease among travelers to endemic areas, and little is known about its travel-related epidemiology. In addit ion, efficacy data on TF vaccines in travelers is scanty. During 3 months o f 1994/95, six cases of TF were reported in The Netherlands among participa nts of four package tours to Indonesia provided by the same operator. The p resent study was designed to describe the epidemiology of TF in these group s, and to assess whether travel groups can be used for studying the efficac y of TF vaccines in travelers. Method: Questionnaire-based historical cohort study of participants of 4 gr oups that stayed in the same hotels along their tours (n=156), TF was defin ed as blood culture-confirmed Salmonella typhi infection. Submitted isolate s were typed by antigen and phage typing. Immunization status was considere d documented if ascertained by written records. Results: Among 110 participants (71%), six cases of TF were identified (gro up specific attack rate AR 5.4%), three of which were from one travel group (AR 12.0%). There were no significant differences by age or sex. Three sub mitted S. typhi isolates showed three different types, two of which were in the same group. Eighty-three percent of respondents reported documented TF vaccination in the preceding 3 years. All cases occurred in recipients of the oral Ty21a vaccine (AR 10.2%, 95% CI 3.8-20.8%), but differences with n onvaccinees and recipients of the heat-inactivated whole cell or Vi-antigen polysaccharide vaccines were not significant. Conclusions: Although TF is rare in travelers, infections with different st rains of S. typhi can occur in one travel group. Travel groups offer an opp ortunity for retrospective assessment of vaccine efficacy, provided that eq ual chance of exposure is largely guaranteed: case ascertainment is maximal ly specific and similar in the vaccine groups; vaccine status is ascertaine d accurately; and prior immunity by previous exposures to and use of antibi otics effective against the infection are excluded from, or controlled for in, the analysis.