RESUMO
<p><b>OBJECTIVE</b>To estimate the probability of unprotected heterosexual vaginal transmission of HIV-1 from clients to Female Sex Workers (FSWs) in Kaiyuan County, Yunnan province, China, and analyze factors which impact the transmission probability.</p><p><b>METHODS</b>An open cohort research of FSWs in Kaiyuan was created from surveys conducted in April 2008, October 2008, and April 2009. Two cross-sectional surveys of local clients were also carried out in May 2008 and November 2008. A model was developed to estimate the probability of unprotected heterosexual vaginal transmission of HIV-1 from clients to FSWs.</p><p><b>RESULTS</b>The transmission probability from clients to FSWs was estimated as 0.0023 [95%CI 0.0014-0.0032] per unprotected heterosexual act.</p><p><b>CONCLUSIONS</b>The transmission probability among this group engaging in commercial sex in Yunnan province differs from that found in studies from other countries, and future studies should estimate the influence factors of HIV-1 transmission probability, such as stage of HIV infection, co-morbid sexually transmitted infections (STIs).</p>