Feasibility of a preventive HIV-1 vaccine cohort among persons attending sexually transmitted disease clinics in Thailand

Persons attending sexually transmitted disease clinics at three sites in Th ailand were recruited to participate in a I-year study of HIV-1 incidence. Between September 1995 and February 1996, 31% (371 of 1205) of eligible men and 24% (161 of 659) of eligible women agreed to participate. At enrollmen t, HIV-1 seropositivity was 3.8% among men and 2.5% among women. Follow-up of the 514 participants who were seronegative at baseline was 78% at the 12 -month visit. During the study period, 53% of men reported 2 or more sexual partners, 31% reported sex with a commercial sex worker (CSW), and 33% wit h a casual partner. Of those visiting CSWs, 72% reported consistent condom use. Among women, 11% reported 2 or more sexual partners. Decreased HIV ris k behaviors among men were observed during the study. Four incident infecti ons occurred in men (1.4/100 person-years, 95% confidence interval [CI] = 0 .4-3.6) and none among women. Based on the observed HIV-I incidence, HIV va ccine efficacy trials in such populations would have to be larger than prev iously planned or more selective of high risk subgroups for recruitment.