Utility of behavioral changes as markers of sexually transmitted disease risk reduction in sexually transmitted disease/HIV prevention trials.

Most sexually transmitted disease (STD)/HIV sexual risk reduction intervention trials are evaluated using behavioral outcomes as their main indicators of intervention effectiveness. How good are behavioral measures as surrogate markers for STD infection? Do the behavioral changes that are commonly assessed in risk reduction interventions accurately reflect changes in STD risk? We applied a mathematical model of STD/HIV transmission to empiric data from a large HIV prevention intervention to estimate pre- to postintervention changes in intervention participants' STD risk. We then used the coefficient of determination (R(2)) to assess the strength of association between changes in STD risk and changes in three behavioral measures: proportion of acts of intercourse for which condoms were used, number of sex partners, and number of acts of unprotected intercourse. The results indicate that change in the number of acts of unprotected intercourse is a superior marker of STD risk changes for less infectious STDs such as HIV, whereas change in the number of partners may be preferable for highly infectious STDs such as gonorrhea. Changes in the proportion of acts of intercourse for which condoms were used were not strongly correlated with changes in STD risk under most of the conditions examined in this analysis. The utility of different measures of sexual behavior change as markers for changes in STD risk and, hence, expected incidence, depends on the infectivity and prevalence of the target STD.