Background: Antiretroviral therapy (ART) markedly reduces HIV transmission, and testing and treatment programs\nhave been advocated as a method for decreasing transmission at the population level. Little is known, however,\nabout the extent to which sexually transmitted infections (STIs), which increase the HIV infectiousness of untreated\nindividuals, may decrease the effectiveness of treatment as prevention.\nMethods: We searched major bibliographic databases to August 12th, 2014 and identified studies reporting differences\nin HIV transmission rate or in viral load between individuals on ART who either were or were not co-infected with\nanother STI. We used hierarchical Bayesian models to estimate viral load differences between individuals with and\nwithout STI co-infections.\nResults: The search strategy retrieved 1630 unique citations of which 14 studies (reporting on 4607 HIV viral load\nmeasurements from 2835 unique individuals) met the inclusion criteria. We did not find any suitable studies that\nestimated transmission rates directly in both groups. Our meta-analysis of HIV viral load measurements among\ntreated individuals did not find a statistically significant effect of STI co-infection; viral loads were, on average,\n0.11 log10 (95 % CI ?0.62 to 0.83) higher among co-infected versus non-co-infected individuals.\nConclusions: Direct evidence about the effects of STI co-infection on transmission from individuals on ART is very\nlimited. Available data suggests that the average effect of STI co-infection on HIV viral load in individuals on ART\nis less than 1 log10 difference, and thus unlikely to decrease the effectiveness of treatment as prevention. However,\nthere is not enough data to rule out the possibility that particular STIs pose a larger threat.
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