Combination HIV Prevention
A Simulation-based Method for Efficient Resource Allocation of Combination HIV Prevention
Over the past three decades there has been a wealth of operational research into e ffectively and effciently combating human immunode ficiency virus (HIV). These interventions have had varying results. Condoms, for example, have been shown to decrease the probability of transmission per sexual act (PTSA) by 95%, but they tend to be used inconsistently. Male circumcision has been shown to reduce the PTSA by 50%, but provides consistent partial protection by design. Antiretroviral therapy (ART) is a medical treatment that slows the reproduction of HIV. ART has been associated with 96% reduction in PTSA, and has been shown to prolong the life of an infected individual. However, it is diffcult to determine how to optimally distribute limited HIV prevention
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Young women in sub-Saharan Africa are at a disproportionately high risk of HIV infection. Biological and behavioural risk factors, in combination with a complex sexual age-mixing pattern, have been proposed to explain this gender discrepancy. Age-mixing patterns characterised by the frequent occurrence of large age differences between sexual partners are thought to be the result of socio-economic inequalities in society.Informed by behavioural, clinical and socio-demographic data from South Africa and Malawi, we plan to use Simpact to investigate the role of age-mixing patterns in the sustained spread of HIV. Directly linked to this question is that of HIV prevention: We will use Simpact to explore how changes in the age-mixing pattern affect the risk of acquiring HIV at the individual level, and alter the course of the epidemic, taking into account the biology, sociology and behavioural science behind the epidemiology of HIV in young women in sub-Saharan Africa. In 2012, the results of a cluster randomised trial in Malawi suggested that structural interventions can successfully modify the age-mixing pattern in young girls of school-going age, and that this may result in fewer new HIV infections among adolescent and young adult women. What remains unclear, however, is what the short- and long-term impact of such interventions might be on HIV incidence and prevalence in women and men of all ages.
HIV Treatment as Prevention (TasP)
The HPTN 052 study showed that early antiretroviral treatment (ART) could largely reduce the heterosexual HIV transmission while a recent simulation study indicated that universal immediate ART might be able to eliminate the epidemic in the near future. However, universal and immediate ART remains unfeasible in most settings due to limited resources. Therefore alternative options of providing ART have been proposed and discussed.We used Simpact to simulate HIV transmission and the effects of HIV Treatment as Prevention (TasP) interventions in sexual networks. We considered prioritising five HIV-infected population subgroups (CD4<500, long-term serodiscordant couples, pregnant women, female sex workers, the elders, all positives) under three schemes. The expected HIV prevalence over 15 years was projected for South Africa.Results show that HIV prevalence in South Africa can be suppressed to 10% by 2023 if the current treatment access level is maintained. Expanding ART to the mentioned subgroups might not lead to a dramatic drop in HIV prevalence in the short run. Differences between the impact estimates from alternative interventions emerge after 10 years. In the scenario where 80% of HIV positives were treated immediately after diagnosis, the prevalence decreased to 5% within twenty years. Prioritisation for those with CD4 counts below 500 and serodiscordant couples might be the most effective options, spending less than 3/4 than non-prioritising strategies for per averted infection. Furthermore, the demographic structure of the HIV-positive population would be changed by shifting resources to a particular subgroup.

Our simulations suggests that the impact of TasP interventions is complex and that long-term outcomes need to be considered according to specific objectives. Expanding ART to a larger proportion of the subgroups with higher CD4 levels or in serodiscordant relationships may have the most favourable incremental cost-effectiveness ratios.