Fewer partners or more condoms? Modelling the effectiveness of STI prevention interventions

Some advocates of HIV prevention have counterposed programmes aimed at reducing numbers of sex partners to programmes aimed at increasing condom use. In this polarised debate, arguments about their relative effectiveness have been based on limited evidence.

To explore the theoretical impact of changing partner acquisition rates and condom use on the population level incidence of sexually transmitted infections different in different populations.

A standard deterministic compartmental model of the transmission dynamics of a sexually transmitted disease is solved numerically. The change in behaviour required to prevent a single case of infection is calculated for a range of scenarios.

When the transmission probability per sex act is low across partnerships, the incidence of infection responds similarly to changes in partner numbers and numbers of unprotected sex acts. High transmission probabilities alter the relationships, with the effectiveness of increasing condom use improving as the background number of unprotected sex acts decreases. The divergence in patterns of impact is marked when there is heterogeneity in the transmission probability per act across partnerships.

Both reducing numbers of sex partners and increasing condom use can lower the incidence of sexually transmitted infections. Unfortunately, there is no simple and general rule that will allow the efficiency of interventions to be calculated. Heterogeneity in transmission probability across infections, which may occur for both bacterial and viral infections, can reduce the effectiveness of condom use as an intervention if that use is inconsistent.