Sexual bridging socially and over time: A simulation model exploring the relative effects of mixing and concurrency on viral sexuall transmitted infection transmission

Background: Sexual partnerships between people at higher and lower risk for sexually transmitted infections (STIs) (i.e., bridging) occur through dissortative mixing and concurrent partnerships, yet the relative effects of these network patterns on population STI spread are poorly understood. Goal: Using a stochastic model, the authors investigated the impact of mixing and concurrency on the spread of a persistent viral STI. Study Design: A total of 1050 populations were simulated of 1000 subjects over 400 weeks with varied concurrency levels and mixing patterns. STI prevalence and the average number of secondary transmissions per subject were analyzed with regression. Results: Mixing had a greater impact on prevalence for all groups, whereas concurrency was significant for only the lowest activity group. Mixing patterns moderated the magnitude of concurrency's impact on secondary transmissions. Conclusions: Through connecting subgroups of differential risk, sexual mixing facilitates dissemination of STIs throughout a population. Concurrency expedites transmission by shortening the time between sexual contacts among infected and susceptible persons, particularly during the highly infectious period.