Modelling study of generalized HIV epidemics demonstrates the importance of HIV transmission in both casual and stable partnerships

HIV sexual transmission is predominantly driven by single individuals rather than discordant couples: a model-based approach.

Champredon D, Bellan S, Dushoff J. PLoS One. 2013 Dec 20;8(12):e82906. doi: 10.1371/journal.pone.0082906.

Understanding the relative contribution to HIV transmission from different social groups is important for public-health policy. Information about the importance of stable serodiscordant couples (when one partner is infected but not the other) relative to contacts outside of stable partnerships in spreading disease can aid in designing and targeting interventions. However, the overall importance of within-couple transmission, and the determinants and correlates of this importance, are not well understood. Here, we explore how mechanistic factors - like partnership dynamics and rates of extra-couple transmission - affect various routes of transmission, using a compartmental model with parameters based on estimates from sub-Saharan Africa. Under our assumptions, when sampling model parameters within a realistic range, we find that infection of uncoupled individuals is usually the predominant route (median 0.62, 2.5%-97.5% quantiles: 0.26-0.88), while transmission within discordant couples is usually important, but rarely represents the majority of transmissions (median 0.33, 2.5%-97.5% quantiles: 0.10-0.67). We find a strong correlation between long-term HIV prevalence and the contact rate of uncoupled individuals, implying that this rate may be a key driver of HIV prevalence. For a given level of prevalence, we find a negative correlation between the proportion of discordant couples and the within-couple transmission rate, indicating that low discordance in a population may reflect a relatively high rate of within-couple transmission. Transmission within or outside couples and among uncoupled individuals are all likely to be important in sustaining heterosexual HIV transmission in sub-Saharan Africa. Hence, intervention policies should be broadly targeted when practical.

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Editor’s notes: In 2008, Dunkle and colleagues argued that serodiscordant couples now contribute the majority of new HIV infections in generalized epidemics in sub-Saharan Africa (SSA). Ever since, the relative importance of discordant couples in the spread of HIV in generalized epidemics has been the subject of intense debate. Ultimately, this is a debate about resource allocation and revolves around the proposition that serodiscordant couples ought to be treated as a priority population for targeted HIV prevention.

In this study, a partnership-based compartmental model is used to compare the relative magnitude of transmissions within stable couples, transmission to and from uncoupled individuals, and extra-couple transmission to and from coupled individuals. The results lend support to other recent modelling work that suggests that all of these transmission routes are important to sustain heterosexual HIV epidemics in SSA (e.g., Bellan et al. 2013). Further, they argue that high levels of serodiscordance in the population are the result of past “non-couple routes of HIV transmission and their interactions with the partner switching dynamics”. High levels of serodiscordance, in turn, generate a potential for future transmissions within couples. 

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