Estimating the annual risk of HIV transmission within HIV sero-discordant couples in sub-Saharan Africa

OBJECTIVE: To estimate the annual risk of HIV transmission (ϕ) within HIV sero-discordant couples in 23 countries in sub-Saharan Africa (SSA), by utilizing newly available national population-based data and accounting for factors known to potentially affect this estimation. METHODS: We used a recently developed pair-based mathematical model that accommodates for HIV-dynamics temporal variation, sexual risk-behavior heterogeneity, and antiretroviral therapy (ART) scale-up. RESULTS: Estimated country-specific ϕ (in absence of ART) ranged between 4.2% (95% uncertainty interval (UI): 1.9%-6.3%) and 47.4% (95% UI: 37.2%-69.0%) per person-year (ppy), with a median of 12.4%. ϕ was strongly associated with HIV prevalence, with a Pearson correlation coefficient of 0.92, and was larger in high- versus low-HIV-prevalence countries. ϕ increased by 1.31% (95% confidence interval: 1.00%-1.55%) ppy for every 1% increase in HIV prevalence. CONCLUSIONS: ϕ estimates were similar to earlier estimates, and suggested considerable heterogeneity in HIV infectiousness across SSA. This heterogeneity may explain, partly, the differences in epidemic scales.