Aggregate versus individual-level sexual behavior assessment: how much detail is needed to accurately estimate HIV/STI risk?

Abstract

The sexual behaviors of HIV/sexually transmitted infection (STI) prevention intervention participants can be assessed on a partner-by-partner basis: in aggregate (i.e., total numbers of sex acts, collapsed across partners) or using a combination of these two methods (e.g., assessing five partners in detail and any remaining partners in aggregate). There is a natural trade-off between the level of sexual behavior detail and the precision of HIV/STI acquisition risk estimates. The results of this study indicate that relatively simple aggregate data collection techniques suffice to adequately estimate HIV risk. For highly infectious STIs, in contrast, accurate STI risk assessment requires more intensive partner-by-partner methods.

Figure 1

Risk (probability) of acquiring HIV or a highly infectious sexually transmitted infection (STI) from a potentially infected partner, as a percentage of the risk associated with a maximum of 25 unprotected sex acts. For highly infectious STIs, risk grows rapidly in the first several sex acts and then more slowly as saturation is approached. For HIV transmission during receptive vaginal or anal intercourse, the per-partnership risk is essentially a linear function of the number of unprotected sex acts with that partner.

Figure 2

Risk of acquiring a highly infectious sexually transmitted infection (STI) as a function of the number of unprotected sex acts and the number of partners. Each new partner essentially “resets” the risk accumulation process but at a higher baseline level. Consequently, the number of sex partners is a critical determinant of the risk of acquiring a highly infectious STI.

Figure 3

Risk of acquiring HIV through receptive anal intercourse as a function of the number of unprotected sex acts and the number of partners. The risk associated with each sex act is approximately the same, regardless of the number of sex partners.