PROBABILITY OF FEMALE-TO-MALE TRANSMISSION OF HIV-1 IN THAILAND

The epidemic of human immunodeficiency virus type 1 (HIV-1) infection in Thailand has allowed an estimate to be made of the probability of f emale-to-male HIV-1 transmission per sexual contact. In a study of 111 5 21 year-old male military conscripts, of whom 77 (6.9%) were HIV-1 s eropositive, sex with female prostitutes was identified as the princip al mode of HIV-1. transmission. With a mathematical model including da ta on conscript's age at first sexual contact, frequency of sex with f emale prostitutes, and province of origin; as well as province-specifi c HIV-1 seroprevalence of prostitutes, we estimated the probability of HIV-1 transmission per sexual contact to be 0.031 (95% confidence lim its [CL] 0.025-0.040). Allowing for random error in the self-reported frequency of contacts, the estimate was 0.056 (95% CL 0.041-0.075). Th e transmission probability was significantly greater among men with a history of sexually-transmitted diseases. These estimates are substant ially higher than analogous estimates made in North America. This high per-act probability of heterosexual transmission helps to explain the rapid spread of HIV-1 in the emerging epidermic in Thailand and perha ps in other countries where HIV-1 transmission is predominately hetero sexual.