Preparatory studies for possible HIV vaccine trials in Northern Thailand

We studied several populations of human immunodeficiency virus (HIV)- seronegative individuals from northern Thailand who were believed to be at relatively high risk of HIV infection in order to assess their potential suitability for inclusion in a preventive HIV vaccine trial. Included were female commercial sex workers (CSWs), male sexually transmitted disease (STD) clinic patients, male conscripts into the Royal Thai Army (RTA), and men who were recently discharged from the army. We evaluated their HIV prevalence, their interest in study participation, their compliance with prospective follow-up, and their HIV incidence. Among 1068 female CSWs the baseline HIV prevalence was 38.3%; of 659 HIV seronegatives 395 (59.9%) agreed to enrollment in the study. Follow-up at 6-9 months was 73.2%; it was 58% in brothel-based CSWs and 85% in non-brothel-based CSWs. Overall HIV incidence in CSWs was 8.2 per 100 person-years; incidence was 29 per 100 person-years in brothel-based CSWs and 4 per 100 person-years in non-brothel-based CSWs. Among 1031 male STD clinic patients, baseline HIV prevalence was 15.9%, follow-up was successful in 98.1%, and HIV incidence was 4.0 per 100 person- years. Among four cohorts of RTA conscripts who entered the military in 1991 and 1993, baseline HIV prevalence has been about 12%, follow-up about 90%, and HIV incidence has varied from 1.2-3.2 per 100 person-years. Discharged RTA conscripts have had baseline HIV prevalence of about 13%, successful follow-up of 94%, and an HIV incidence of about 5 per 100 person-years. Our data suggest that male STD clinic patients may be the most suitable population, of those studied, to consider for an HIV vaccine trial in northern Thailand. Additional follow-up of STD clinic patients and other populations will be required to determine whether the high HIV incidence and good compliance found in these preliminary studies is maintained.