in a population (summarised in the basic reproductive number, R0: the number of new infections generated by one case in a susceptible population ) and the equilibrium level of HIV prevalence exhibits threshold and non‐linear behaviour (Figure 3) [34]. If R0 is high, then substantial behavioural changes among some individuals that makes them fully protected from infection (e.g. perfectly consistent condom use), may not lead to a great reduction in prevalence. Alternatively, if behaviour change is restricted to a part of the population with less risky behaviour where transmission is not sustainable (R0 less than one), then changes which further reduce the riskiness of their behaviour would have a limited effect on the overall rate of infection.
HIV Prevalence (%)
30 25 20 15 10 5 0 0
2
4
6
Basic Reproductive Number, R0
8
10
Figure 3: The basic reproductive number (R0) determines the endemic level of HIV. (The model population is composed of two sub-populations: one with 70% of the population at low risk of infection and another with 30% of the population at high risk). Source: Authors’ construction, using a simplified version of the model presented in the appendix.
For these reasons, it is important to base assessments of the trend in a national epidemic or evaluations of the impact of specific preventive interventions on ‘hard’ outcomes – reported sexual behaviour indicators can change through bias or the natural course of the epidemic, and may have little bearing on the rate of HIV transmission. The prevalence of certain sexually transmitted infections (STI) has often been suggested a useful “proxy” indicator for the risk of HIV transmission. The relative abundance of STIs and the better correlation between incidence and prevalence for short‐duration bacterial infections makes this idea superficially attractive. However, using STI proxies is not reliable since the way in which different sexually‐transmitted infections respond to patterns of risk and change in behaviour varies substantially and is determined by their various natural histories. For instance, the prevalence of some STIs is tightly linked only to the behaviours of those at the 6