Determinants of HIV infection among female commercial sex workers in northeastern Thailand: results from a longitudinal study.

The risk factors of human immunodeficiency virus (HIV) seroconversion among female commercial sex workers in northeastern Thailand were investigated through a prospective study of two cohorts. The first group consisted of 240 sex workers from Khon Kaen who were seronegative at the time of a November 1990 cross-sectional study conducted by the Ministry of Public Health; the second was comprised of 305 seronegative prostitutes enrolled in March June and September of 1991. The average follow-up time was 8.1 months. Of the 489 participants for whom data were available 61 (12.5%) were HIV-positive at follow-up. With 1947 person-months of observation from the 240 sex workers who were uninfected at baseline and tested at least twice since then the cumulative incidence of HIV seroconversion between November 1990 and December 1991 was 9.4% and the average incidence rate of seroconversion was 9.2 per 100 person-years. In the first group of sex workers the significant risk factors for seroconversion were later date of study entry less than three months experience in commercial sex work and use of injectable contraception. In the second group seroconversion was significantly associated with current syphilis infection. These findings suggest that tighter enforcement of laws prohibiting prostitution particularly involving young girls could reduce HIV infection rates in Thailand. Clarification of the role of injectable contraception in increasing the risk of HIV infection (possibly through thinning of the vaginal epithelium) demands attention due to its implications for family planning programs.