Sexual networks, partnership mixing patterns and the sex ratio of HIV infections in generalized epidemics

Empirical estimates of the female-to-male sex ratio of infections in generalized HIV epidemics in sub-Saharan Africa range from 1.31 in Zambia to 2.21 in Ivory Coast. Gender inequalities in the sex ratio of infections can arise because of differences in exposure (to HIV positive partners), susceptibility (given exposure), and survival (once infected). Differences in susceptibility have to date received most attention, but neither the relatively high sex ratio of infections nor the heterogeneity in the empirical estimates in generalized epidemics is fully understood. In this contribution we focus on partnership network attributes and sexual mixing patterns that could lead to gender differences in the exposure to HIV positive partners. Using agent-based simulations, we show that gender asymmetric partnership concurrency, rapid partnership turnover, elevated partnership dissolution in female positive serodiscordant couples and lower partnership re-entry rates among HIV positive women can produce (substantial) differences in the sex ratio of infections. Coital dilution and serosorting have modest moderating effects.