We have explored the changing role of STI cofactors in HIV transmission during the HIV epidemics in four cities in West and East Africa. Throughout the HIV epidemics the contribution of STIs to HIV transmission remained high, with 50% or more of HIV transmission attributed to STIs in all four cities. This relative stability in the overall PAFs concealed opposing trends in the contribution of curable and incurable STIs. The attributable-fraction for curable STIs was predicted to fall during the HIV epidemic, while the attributable-fraction for HSV-2 was predicted to rise.
This has important implications for the(cost-)effectiveness of interventions seeking to target curable STIs. Over time in all four cities, the relative impact of STI treatment on HIV incidence fell, tending to increase the cost per HIV-infection-averted. However, in populations with a rapidly expanding HIV epidemic this increase in cost was offset by the rapidly rising HIV incidence that increased the absolute impact of the intervention. Our findings suggest that in African populations with mature HIV epidemics, STI treatment interventions are likely to remain highly cost-effective and may even be cost-saving, particularly in populations in which safer sexual behaviours have not adequately controlled STIs and HIV incidence remains high.
These results should be interpreted with some caution. The simulated HIV epidemics in the four cities were largely parameterised using data from a cross sectional study carried out in 199711
with limited data on STI trends over time. Although the observed sexual behaviours and STI rates in 1997 and HIV prevalence trends over time were fitted well, actual trends in STI rates may have differed from those simulated. This may affect the quantitative findings of this study, but is less likely to affect the qualitative conclusions.
The magnitudes of STI cofactor effects on HIV transmission remain poorly quantified20, 21
. However our sensitivity analysis showed that the cost-effectiveness of the intervention was relatively insensitive to their variation.
Although the simulated STI treatment intervention was based on extensive review of data from the RCT in Mwanza, and adequately explained the observed impacts of this empirical trial and the observed impacts of the STI treatment trials in Masaka and Rakai9
, the coverage achieved by this intervention may vary between populations. However, as our sensitivity analysis showed, in the short term this may not affect the cost-effectiveness of the intervention. Over the longer term, increased cure rates would be expected to increase the cost-effectiveness of the intervention.
The adjustments to the cost data were based on the results of an analysis of data from developing countries. Although these adjustments enabled us to estimate the true costs of the intervention more accurately, they are approximations and cannot fully control for differences in implementation and changes over time.
Our findings may help to explain why the STI treatment strategies tested since the trial in Mwanza have failed to show a significant impact on HIV incidence. Our results suggest that even if the STI treatment interventions in Rakai, Masaka and Zimbabwe5, 6, 8
had led to large reductions in STI rates, in these generalised HIV epidemics the relative impact on HIV incidence would have been relatively small and therefore undetectable.
This study suggests that despite the small relative impact on HIV incidence, STI treatment in the general population is likely to remain highly cost-effective and even cost-saving in contemporary African populations with HIV prevalences over ~5.5%, because of the substantial absolute impact on HIV incidence. In African populations with lower HIV prevalences, syndromic STI treatment may not be as cost-effective for HIV prevention, but is recommended for STI control and to reduce HIV transmission among STI patients and their sexual partners.
Rational health policy requires that scarce resources be allocated to interventions with the best cost-effectiveness even if relative impact at population level is modest. Effective STI management may prevent a decreasing fraction of new HIV infections as the epidemic expands, but is an inexpensive intervention with important collateral public health benefits and which effectively protects STI patients from the enhanced risk of HIV acquisition and transmission.
In conclusion, our study has shown that throughout HIV epidemics in sub-Saharan Africa, a large proportion of HIV transmission may remain attributable to other STIs, but that the proportion attributable to curable STIs is likely to fall. Despite this, we have shown that in populations with generalised HIV epidemics, interventions that target curable STIs may remain highly cost-effective and even cost-saving if changes in risk behaviours have not adequately controlled STIs.