There are relatively few long-term observational studies of HIV discordant couples in the pre-ART era. Strengths of this study include the availability of data since 1990, before many interventions became widely available to the population, and the observational study design rather than a trial population, which is likely to be generalisable. This study found a high HIV incidence rate in HIV serodiscordant couples (7.11/100 PYAR), and incidence was twice as high in females as in males.
HIV negative partners in steady HIV serodiscordant partnerships are at high risk for HIV acquisition if the HIV positive partner is not on ART. The HIV rate in this study is comparable to those reported from HIV serodiscordant couples elsewhere in sub-Saharan Africa (range 4–10/100 PYAR) 
. In contrast, the highest recorded annual HIV incidence in the general study population from which these couples were drawn during the period between 1990 and 2004 was 0.8/100 PYAR 
. A previous study in this population found that the rate ratios for serodiscordant versus concordant negative couples were 11.6 in HIV negative men and 105.8 in HIV negative women 
. These results highlight that urgent efforts are needed to identify discordant couples through increased uptake of counselling and testing, by ensuring that services are widely available and accessible for couples 
The main factors associated with HIV transmission within a couple were a male index partner, non-Muslim couple, high viral load in the index partner, and a greater age difference between spouses. The median age at HIV seroconversion was substantially higher in men (40 years) than in women (28 years), and this is likely to partly reflect the fact that in this population men tend to be older than their female partners. However, this difference was also observed in the general population from which the cohort came, in which median age at seroconversion was higher in men 
, likely due to the increased risk of HIV acquisition in females than males due to greater biological susceptibility discussed further below. Additionally, young women tend to have sex with older men who are more likely to be at higher risk through multiple partnerships 
. Among serodiscordant couples, the overall median age at seroconversion was older than in the general population, which may reflect selection bias, as we have excluded couples who are seroconcordant positive. However, these findings highlight that the older age at seroconversion provides an opportunity for prevention in younger discordant couples. Further, following the first known HIV status for the couple, the median duration of follow-up before seroconversion was 2 years, however more frequent testing and counselling of couples is likely to identify couples in whom HIV discordance is recent. This would then provide an opportunity for risk reduction and prevention of transmission.
HSV-2 infection in the HIV negative partner was associated with a doubling of the rate of seroconversion (although the confidence intervals were wide). This magnitude of association is consistent with a meta-analysis of 25 cohort studies in which prevalent HSV-2 increased the risk of HIV acquisition three-fold (adjusted RR 2.8 (95% CI 2.1–3.7) in men and 3.4 (95% CI 2.4–4.8) in women) 
. There is good biological plausibility for an association between HSV-2 infection and HIV. HSV-2 is known to cause breakages of the genital mucosa and thereby increase the risk of entry of HIV. In addition HSV-2 recruits HIV target cells in the genital mucosa thereby increasing the risk of HIV infection 
. However the increased risk of HIV acquisition associated with HSV-2 infection seen in epidemiologic studies may be partly due to unmeasured confounding from high risk behaviour given that both infections are acquired sexually 
. Despite this evidence, two RCTs of HSV-2 suppressive treatment found no evidence of a reduction in HIV incidence rates 
. It is possible that the dosage given (twice daily 400 mg of acyclovir) may have been inadequate to achieve sufficient suppression of HSV-2 to prevent HIV acquisition. Also, adherence to acyclovir in these trials may have been sub-optimal.
HSV-2 infection in the positive partner was associated with a slightly lower rate of HIV transmission to the HIV negative partner (adjusted HR 0.61 [95% CI: 0.24, 1.57], p
0.4). The result was unexpected as HSV-2 is thought to increase the infectiousness of HIV in co-infected persons. Previous observational studies of the association of HSV-2 with HIV incidence in discordant couples have also been inconclusive, with one study 
reporting no increase in risk of transmission in association with HSV-2 infection and the other 
reporting more frequent diagnosis of HSV-2 among seroconverting couples compared to couples remaining serodiscordant (46.2% vs 3.6%) after 6 to 12 months of follow up.
Over half the index partners in this study were male (54%). This is in contrast to the Partners in Prevention study from 7 eastern and southern African countries, in which 33% of index partners were male 
. Men were older than women in this study population and therefore more likely to have prevalent HIV. In addition, it is possible that HIV positive men are more likely to remarry e.g. after separation of death of a spouse, than HIV positive women. However, HIV incidence in the cohort was higher among women. Women might be at higher risk of HIV incidence because of the larger mucosa area in the vagina than the male foreskin, and because the low vaginal pH is hostile to HIV therefore vaginal secretions may carry less virus than semen thereby potentially rendering women to be less infectious than men, and further, semen increases vaginal pH thereby rendering it less hostile to the HIV. In addition, semen stays in the vaginal column for longer than vaginal secretions stay on the penis, and so women may have longer exposure to infection and therefore be at higher risk 
The rate of HIV transmission was higher when the man was older by more than 15 years, especially if the HIV negative partner was female. Similar results were seen in a longitudinal study in Zimbabwe that reported increased vulnerability to HIV in young women who have sexual relationships with older, and usually high risk men 
. One explanation for this is that younger women (and those in relationships with older men) are more likely to engage in extramarital sex, and hence the HIV infection is externally acquired. Younger women may also be vulnerable because of larger area of cervical ectopy as compared to older women, and lack of power to negotiate safer sex with their partners 
Muslims were at a significantly lower risk of HIV acquisition in our study (aHR
0.27, 95%CI 0.11–0.68) presumably because of the almost universal practice of male circumcision among Muslims in this population. There is little evidence that male circumcision directly reduces risk of male to female HIV transmission 
but because marriages tended to be between partners of the same religion, Muslim women may have had lower risk owing to lower incidence rates in the male partners for extramarital infection.
Rates of HIV seroconversion reduced over time. During the early period (1990–1994), there was low awareness of one’s own, or partners’, HIV status. Counselling advice e.g. for the use of condoms in the context of serodiscordant partnerships was also not widely available 
. As a consequence, little was done to prevent HIV transmission in marriage or longstanding sexual partnerships. Counselling and testing for HIV, condoms, treatment of opportunistic infections and antiretroviral treatment have become increasingly available in recent years and are likely to explain the reduction in seroconversion rates over time. No seroconversions occurred among couples in which the HIV positive partner was on HAART. Reduced risk of HIV transmission in the present of HAART has been reported in other observational studies 
and recently confirmed in a randomised clinical trial 
This study had a number of limitations. Firstly it was not ascertained whether HIV seroconversions occurred as a result of transmission from within the partnership or from an external partner. Genetic sequencing of couples’ virus has found up to 30% non-matching virus indicating infection acquired outside of the partnership 
. Therefore the rates of seroconversion reported in this study may be higher than within-couple transmission rates. Secondly, average coverage for the annual survey was about 60%, and we further excluded couples with incomplete HIV status and couples seen once only. The couples included in the incidence analysis were more likely to have a female negative partner than those seen once only, and this may have resulted in a higher estimate of overall HIV seroconversion than would be expected in this population. However, our estimated incidence is comparable to estimates from serodiscordant couple studies in neighbouring populations 
. Finally, we did not have data on other STIs, knowledge of own or partners HIV status, and had relatively few data on viral load and CD4 count. As a consequence, we may have failed to measure the potentially confounding effect from these factors. For example knowledge of one’s own HIV status or that of the partner may influence behaviour and cause one to adopt preventive measures including abstinence, condom use, seeking counselling, or treatment thereby reducing risk of HIV transmission despite their HSV-2 status. We did not have data on condom use. However condom use in the context of stable partnership is rare.
HIV negative partners in serodiscordant couples have a high incidence of HIV infection if the index partner is not on anti-retroviral therapy. Before these become available, there should be continued emphasis on couples counselling and testing (for example within the programmes of increased voluntary medical male circumcision scale-up), and HIV serodiscordant couples should be strongly advised to use the existing interventions to minimise risk of HIV transmission.