Background: In Thailand the epidemic of human immunodeficiency virus (HIV) infection is of recent origin. Because of the high seroprevalence of HIV among sex workers, the Ministry of Public Health began a program in 1990 and 1991 to promote the use of condoms during commercial sex. We evaluated the effect of this and other programs to prevent HIV infection in Thailand. Methods: Using direct interviews, we studied five cohorts of 21-year-old men from northern Thailand who were conscripted into the army by a lottery in 1991, 1993, and 1995. In all, 4311 men were tested for HIV antibodies by enzyme-linked immunosorbent assay, with confirmation by Western blot assay. Results: In the 1991 and 1993 cohorts, the prevalence of HIV infection was 10.4 to 12.5 percent. In 1995, it fell to 6.7 percent (P<0.001). The seroprevalence was only 0.7 percent among men who did not have sexual relations with a sex worker before 1992. Over the study period, the proportion of men who reported having sexual relations with a sex worker fell from 81.4 percent to 63.8 percent (P<0.001). From 1991 to 1995, the men's reported use of condoms during the most recent sexual contacts with sex workers increased from 61.0 percent to 92.5 percent (P<0.001); and in 1995, 15.2 percent of men had a history of a sexually transmitted disease, as compared with 42.2 percent in 1991 (P<0.001). Conclusions: Public health programs in Thailand have led to substantial changes in sexual behavior among young men, especially an increased use of condoms, and the rate of new HIV infections has declined.
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