**2. Definition and prevalence of sero-discordance globally, SADC and SA**

In sub-Saharan Africa, sero-discordance is a critical factor for the transmission of HIV [7]. Sero-discordance, refers to couples with a mixed HIV status. In such relationships one partner has a known HIV positive status while his or her partner is HIV negative [8, 9]. Hence, for the purpose of this chapter, we define a sero-discordant couple as two individuals who are in a current sexual relationship in which both partners are aware of the other's HIV status. Some authors have argued that sero-discordant sexual relationships are high risk as HIV transmission is more likely to happen in longer-term relationships [10–12].

Despite the misconception that a greater proportion of men are likely to be the index partner, through a systematic review Eyawo et al. [13] established that nearly half (47%) of the index partners were women. This indicates that men and women are equally likely to be the index partner in sero-discordant couples in the sub-Saharan African region. These findings also speak to the prevention and marketing strategies that are meant to be gender balanced in heterosexual sero-discordant couples.

In regions with high HIV prevalence, proportions of sero-discordant intra-couple transmission range from 13.0 to 55% of new HIV infections [14]. For South Africa, the estimated proportion of sero-discordant couples is unclear, however, transmission among longer-term couples were estimated above 10.0% per year [15]. Thus the prevention of intra-couple HIV transmission may delay the progression of the epidemic. As such, sero-discordant couples are a key target population in the context of HIV prevention.
