Our work constitutes a formal test of the effect of concurrent partnerships on HIV incidence in this setting (panel
). By shifting the focus away from an individual's own sexual behaviour patterns and onto the sexual behaviour profiles of the surrounding local community, we have tested the transmission dimension of the concurrency theory. The study took place in an area with one of the highest population-based prevalences of HIV documented worldwide19
and where there are large variations in the level of male partnership concurrency across the population. However, these differences in the prevalence of male concurrency did not translate into detectable differences in prospective incidence of HIV in women. The results were robust to differing definitions of community and held after we controlled for demographic, socioeconomic, behavioural, and environmental factors. Furthermore, the results also held at an ecological level and for the secondary analysis that quantified the effect of combined male and female sexual behaviour at a community level on the incidence of all participants (male and female) in the cohort. We were therefore unable to find any evidence to support the belief that concurrent partnerships are an important driver of the rate of spread of HIV infection in this hyperendemic setting. At the same time, the relation between numbers of lifetime partners in the community and risk of new infection provides strong and robust evidence of the effect of multiple partnering on HIV transmission and emphasises the importance of the characteristics of local community on spread of the virus (over and above an individual's characteristics and behaviours). This strong independent association is indicative of the fact that mean number of lifetime partners proxies for rate of partner turnover during the study period and thus men living in communities with high numbers of lifetime partners (relative to the age-profile of the community) continue to have (on average) higher numbers of sexual partnerships.
Panel. Research in context
Although theoretical mathematical models suggest that concurrent sexual partnerships could account for the rapid spread of HIV in sub-Saharan Africa, a recent systematic review13
concluded that there is no empirical evidence to show that the kinds of concurrent partnerships found in Africa produce more rapid spread of HIV than do other forms of sexual behaviour. Provision of such evidence is not straightforward, however, because partnership concurrency is a risk factor for disease transmission and spread through a population and not of individual risk of disease acquisition (provided that having concurrent partners does not increase an individual's cumulative number of sex partners or unprotected sex acts).
Our study is the first to examine the effect of prevalence of concurrency in the surrounding local community on an individual's risk of HIV acquisition. By shifting the focus away from an individual's own sexual behaviour patterns and onto the sexual behaviour profiles of the surrounding local community, our study has tested the transmission dimension of the concurrency theory in a typical rural South African population with high HIV prevalence. Although the mean number of lifetime partners of men in the immediate local community was independently predictive of hazard of HIV infection in women, a high prevalence of partnership concurrency in the same local community was not associated with any increase in risk of HIV acquisition. Our data therefore provide no evidence to suggest that the high rate of new HIV infections is being driven by the segment of the sexually active population reporting concurrent sexual partners (29% of men and 2% of women). Our findings suggest that in similar hyperendemic sub-Saharan African settings, there is a need for clear messages aimed at the reduction of multiple partnerships, irrespective of whether those partnerships overlap in time. However, the absence of an effect of concurrency on HIV incidence in this setting should not be taken to necessarily mean that high levels of concurrent partnerships could not have played an important part in the initial stages of the HIV epidemic in this population or continue to play a part in other specific epidemic settings.
As in any observational study involving collection of data for sexual behaviour and HIV acquisition, the possibility of bias affecting the results must be considered and discussed. Importantly, selection on the independent variables in the multivariable survival analysis would not have biased the hazard coefficient estimates. Thus, the hazard coefficients will not be affected by selection on age, education, wealth, urban versus rural residence, marital status, HIV prevalence, community-level concurrency, and partners reported in the last 12 months. Of course, a randomised controlled trial could further improve the strength of the evidence for concurrency effects, because it would allow us to control for selection on both known or unknown factors, but such a study is not feasible. By using a community-level estimate of concurrency as a surrogate measure for the concurrency practices of a participant's partner or partners in our analyses, we would naturally expect some attenuation of any concurrency effect. However, this attenuation would also apply to the community lifetime partners covariate, which is a significant predictor of HIV acquisition in our analysis, and thus attenuation would be unlikely to account for the null finding on the effect of concurrency on HIV acquisition.
A limitation of the study is that data for concurrency and lifetime partners were obtained only at the beginning of the study in 2004. We therefore assumed that no large systematic shifts in patterns of male sexual behaviour at a community level have taken place during the study. Such systematic shifts are unlikely, but nevertheless the theoretical possibility remains. Another limitation is that we were unable to control for all exposures outside an individual's geographically defined community—for example, women with migrant partners who return home periodically. However, since partner choice has a strong local geographical dimension, we would still expect to be able to show an effect (at an ecological or individual level) if concurrent partnerships were playing an important part in transmission.
The evidence for the concurrency hypothesis has been reviewed extensively elsewhere.13
However, our findings using individual-level longitudinal data for HIV seroconversion are in agreement with those of other studies that have not shown any association between partnership concurrency and HIV prevalence at a city or a country level.30,31
Similarly, a study in South Africa32
on intimate partner violence and HIV incidence in women found no relation between respondents' reports of the likelihood that their partner had other partners and risk of HIV acquisition. A small study in Malawi33
that attempted to investigate the association between partnership measured the correlation between HIV serodiscordance and concurrency in couples in Likoma Island. The results suggested that concurrent partnerships increased exposure to HIV infection. However, the use of prevalent (as opposed to incident) infection, the small sample size (142 couples), and the possibility of selection bias (in only 23% of concurrent partnerships were both partners tested for HIV) make it difficult to draw any robust inferences.
To spread widely, HIV infection has to have a high basic reproductive number, which will depend on the likelihood of transmission and the contact pattern throughout the population.34
Theoretically, concurrency along with high rates of partner turnover in key individuals can increase the spread of infection, as can factors such as high viral loads35
and the presence of other sexually transmitted infections.36
The present high incidence of infection in this population is possible because of the high prevalence of infectious individuals and the number of partners they have over time. What initially drove the rapid spread of infection is uncertain, but it has to have been a function of contacts and transmission patterns to which concurrency might have contributed. Hence, the absence of a positive concurrency finding in this hyperendemic setting should not be taken to necessarily mean that high levels of concurrent partnerships could not have played an important part in the initial stages of the HIV epidemic in this area or indeed continue to play a part in other specific epidemic settings. Additionally, as noted previously, formal polygamous marriages (a form of partnership concurrency that could be protective for acquisition of HIV37
) occur in the study area although fairly rare (3% of women in the cohort). Therefore, application of our methods to other study sites where the local epidemiology of HIV differs will be important. In the early stages of the HIV epidemic, when the disease is concentrated mainly in high-risk populations, it seems plausible that partnership concurrency could increase the rate at which HIV spreads outside these groups to the general population.
There is a growing debate about the relative merits of, and empirical evidence for, the effect of concurrent partnerships on the HIV epidemic in Africa.2,10,12,13,15
A systematic review of the empirical evidence for the concurrency theory concludes, “promoters of the concurrency hypothesis have failed to establish that concurrency is unusually prevalent in Africa or that the kinds of concurrent partnerships found in Africa produce more rapid spread of HIV than other forms of sexual behaviour.”13
Currently, several countries are planning or implementing HIV prevention strategies that specifically target the reduction of concurrent sexual partnerships.25
The dwindling funds available for HIV programmes worldwide38
and the difficulties of design and implementation of culturally sensitive messages around partnership concurrency, combined with the absence of empirical evidence for an effect, make these interventions a difficult investment to justify. Furthermore, there is a danger of such messages inadvertently giving the impression that having many serially monogamous partnerships does not place an individual (and his or her partners) at significant risk of infection. Thus, rather than “developing a hierarchy of messages beneath the core theme of concurrent partners reduction”,11
messages should be limited to specific behaviours and biological factors with proven effect on HIV acquisition or transmission as part of a combination prevention approach. Even where campaigns refer to both multiple and concurrent partnerships, the unnecessary appeal to reduction of concurrent partnerships is likely to dilute the message. Conversely, simplifying the public health message to reduction in multiple partnerships alone is likely to improve message clarity and effectiveness.39
Our results provide clear evidence of the effect of multiple partnering on HIV transmission in this typical rural, high-prevalence African population. However, we find no evidence to suggest that the high rate of new infections is being driven by the segment of the sexually active population reporting concurrent sexual partners (29% of men and 2% of women). Our findings suggest that in similar hyperendemic sub-Saharan African settings, there is a need for straightforward, unambiguous messages aimed at the reduction of multiple partnerships, irrespective of whether those partnerships overlap in time.