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An analysis of the ecological association between the human immunodeficiency virus (HIV) and syphilis was undertaken using joint mapping modelling based on data from South African national HIV and syphilis sentinel surveillance surveys carried out between 2007 and 2009. The syphilis prevalence, taken as proxy for sexual behaviour and increased HIV transmission, was first used with health district-level deprivation and population density as a covariate in a HIV prevalence spatial regression model and, secondly, together with HIV as a bivariate outcome. HIV was more highly prevalent in deprived and populated urban areas than elsewhere, while syphilis had a high prevalence in less deprived and less populated rural areas. Spatially, the HIV prevalence was lowest in the southwestern and highest in the northeastern parts of the country. This was in discordance to the syphilis prevalence, which revealed negative correlations with the HIV prevalence. Considerable variations across the districts remained after adjusting for the contextual covariate factors. Divergent spatial patterns between HIV and syphilis were identified, regarding both observed and unobserved covariate effects. The differing disease-specific spatial prevalence patterns may point to inconsistent successes in interventions between the two diseases. Overall, the results emphasize the need to develop and test plausible aetiological hypotheses relating to ecological correlations and causes of the disease-specific interjectory between the districts.
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