The epidemic of human immunodeficiency virus type 1 (HIV-1) infection in Thailand has allowed an estimate to be made of the probability of female-to-male HIV-1 transmission per sexual contact. In a study of 1115 21-year-old male military conscripts, of whom 77 (6.9%) were HIV-1 seropositive, sex with female prostitutes was identified as the principal mode of HIV-1 transmission. With a mathematical model including data on conscript's age at first sexual contact, frequency of sex with female prostitutes, and province of origin; as well as province-specific HIV-1 seroprevalence of prostitutes, we estimated the probability of HIV-1 transmission per sexual contact to be 0.031 (95% confidence limits [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). The transmission probability was significantly greater among men with a history of sexually-transmitted diseases. These estimates are substantially 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 epidemic in Thailand and perhaps in other countries where HIV-1 transmission is predominantly heterosexual.
PIP: Approximately 75% of the estimated 13 million HIV-1 infections in adults worldwide are the result of heterosexual intercourse. Epidemiological investigations in Thailand suggest that sex with female prostitutes is the principal mode by which young men contract HIV. Sex with nonprostitute women, sex with men, IV drug use, blood transfusion, and tattooing have not appeared in augment the risk of infection. This study reports findings from a study conducted to determine the probability in Thailand of female-to-male transmission of HIV-1 per sexual contact. 1115 male military conscripts, aged 21 years of whom 6.9% were HIV-1 seropositive, were subjects of the study. Sex with female prostitutes was identified as the principal mode of HIV transmission among these young men. None of the 46 men who reported engaging in receptive anal intercourse was HIV-seropositive. Data on conscript's age at first sexual contact, frequency of sex with female prostitutes, province of origin, and province-specific HIV-1 seroprevalence of prostitutes was put to a mathematical model designed to produce an estimate of transmission probability. The probability of female-to-male transmission of HIV-1 per sexual contact was estimated to be 0.031 under 95% confidence limits. Allowing for random error in the self-reported frequency of contacts yielded a higher probability of 0.056. Transmission probability was significantly higher among the 43% of men who reported histories of STDs. Circumcision was very uncommon among these men who were 98% Buddhist. These estimates are substantially higher than comparable ones produced in studies in North America.