All STDs are not created equal: an analysis of the differential effects of sexual behaviour changes on different STDs

The same sexual behaviours that transmit HIV are implicated in the transmission of certain other STDs, including chlamydia, gonorrhoea, and syphilis. Consequently, it is often assumed that preventive methods that are effective against HIV should be equally effective against other STDs. The purpose of this study was to examine this assumption. We applied a mathematical model of HIV/STD transmission to empirical data from a large HIV prevention intervention that stressed sexual behaviour change. We modelled the effects of two behavioural strategies - reducing the number of sex partners and increasing condom use-on the proportionate change in intervention participants' cumulative risk of acquiring HIV or a highly-infectious STD, such as gonorrhoea. The results of this modelling exercise indicate that decreasing the number of partners is a more effective strategy for reducing STD risk than it is for HIV risk. In contrast, condoms are somewhat more effective at reducing the cumulative transmission risk for HIV than for highly infectious STDs. The protection provided by condoms for multiple acts of intercourse critically depends on the infectiousness of the STD. The results of this study suggest caution in extrapolating from one STD to another, or from one behavioural risk reduction strategy to another.