Abstract

Background: Sexual behavioural change must lie at the centre of efforts to alleviate the global scourge of HIV/AIDS by preventing new infections. Statistical analysis of risk factors at the individual-level may be used to design behaviour interventions. The importance of reducing cross-generational sex (young women having sex with older men) and delaying age at first sex on the spread of HIV at the population-level has been presumed but not scientifically investigated and quantified.

Methods: A mathematical model of heterosexual spread of HIV was developed and used to predict the population-level impact of reducing cross-generational sex, delaying sexual debut and increasing condom use.

Results: The impact of behaviour change on the spread of HIV is sensitive to the structure and reaction of the sexual network. Reducing cross-generational sex could have little impact on the risk of infection unless it is accompanied by a reduction in the number of risky sexual contacts; even exclusive peer-to-peer sexual mixing patterns can support high endemic levels of HIV. The benefit of delaying age at sexual debut is comparatively small and is reduced if males continue to prefer young partners or if young women spend more time unmarried. In Manicaland, Zimbabwe condom use by older men is especially low but if they were to use condoms as frequently as young men, the reduction in average risk of infection could exceed that generated by a two-year delay in first sex.

Conclusions: At the individual-level avoiding sex with older partners and delaying sexual debut can decrease the risk of infection but at the population- level these interventions may do little to limit the spread of HIV without wider-ranging behavioural changes throughout the sexual network. Mathematical models are essential to explore the population-level impact of individual-level behavioural changes.