Role of concurrency in generalised HIV epidemics – Authors' reply
Authors: Frank Tanser, Till Bärnighausen, Lauren Hund, Geoffrey P Garnett, Nuala McGrath, Marie-Louise Newell
Journal: The Lancet (2011)

In response to our population-based study in which we followed up more than 7000 HIV-negative women over 5 years and failed to find evidence that concurrent sexual partnerships are an important driver of HIV incidence, James Shelton, Martina Morris, and Helen Epstein cite studies on HIV incidence in stable concordant HIV-negative partnerships as "direct evidence" for the concurrency hypothesis. However, these studies do not test the concurrency hypothesis (that concurrent sexual partnerships increase the rate of spread of HIV in a population) because they lack a meaningful counterfactual—ie, HIV incidence in people with the same total number of partners over the observation period but in serially monogamous partnerships. Furthermore, Morris and Epstein's claim that the concurrency hypothesis has already been tested and shown to hold runs counter to ongoing public debates on the topic,1Lurie MN Rosenthal S Concurrent partnerships as a driver of the HIV epidemic in sub-Saharan Africa? The evidence is limited.AIDS Behav. 2010; 14: 17-24Crossref PubMed Scopus (140) Google Scholar, 2Mah TL Shelton JD Concurrency revisited: increasing and compelling epidemiological evidence.J Int AIDS Soc. 2011; 14: 33Crossref PubMed Scopus (41) Google Scholar other empirical data,3Lagarde E Auvert B Carael M et al.Concurrent sexual partnerships and HIV prevalence in five urban communities of sub-Saharan Africa.AIDS. 2001; 15: 877-884Crossref PubMed Scopus (123) Google Scholar, 4Mishra V Bignami-Van Assche S Concurrent sexual partnerships and HIV infection: evidence from national population-based surveys. Macro International, Calverton2009Google Scholar, 5Maher D Waswa L Karabarinde A Baisley K Concurrent sexual partnerships and associated factors: a cross-sectional population-based survey in a rural community in Africa with a generalised HIV epidemic.BMC Public Health. 2011; 11: 651Crossref PubMed Scopus (23) Google Scholar and the results of a recent systematic review.6Sawers L Stillwaggon E Concurrent sexual partnerships do not explain the HIV epidemics in Africa: a systematic review of the evidence.J Int AIDS Soc. 2010; 13: 34Crossref PubMed Scopus (125) Google Scholar The claim is also inconsistent with Morris and Epstein's call for a randomised controlled trial to test the hypothesis, which would not be ethically permissible if it had indeed already been shown that the hypothesis held true. Morris and Epstein write that the coefficient on men's partners does not change after controlling for men's HIV prevalence and that therefore either "there is something wrong" with our study or that "men's previous lifetime partners affect a woman's future incidence, even in a neighbourhood with no HIV infected men". This argument is erroneous for three reasons. First, the premise is not based on our study. Nowhere in the article do we examine how the magnitude and significance of the coefficient on men's partners change when we additionally control for HIV prevalence. For this letter, we have added HIV prevalence to the regression with the men's partners variable and, as expected, the magnitude of the coefficient on the partners variable is reduced (by 11%) and the standard error increased (by 5%). Second, our study does not include any neighbourhoods with zero HIV-infected men, as Morris and Epstein imply. Third, as discussed in our article, men's previous lifetime partners proxy for the rate of partner turnover over the duration of the study, which affects women's future HIV incidence over and above HIV prevalence. Shelton, Morris, and Epstein, and Chris Kenyon and Bob Colebunders, argue that community-level concurrency variables would be insensitive to detecting a relation to HIV incidence because of their ecological nature. In our analysis, the community-level variables men's lifetime partners and HIV prevalence have the expected large and significant effects on HIV acquisition, whereas the adjusted hazard ratio of the concurrency variable is very close to 1 (0·99, p=0·730). This finding makes it unlikely that we failed to detect a concurrency effect on HIV acquisition owing to insensitivity in ecological measurement, because such an insensitivity would affect all community variables. Further, Morris and Epstein have themselves used community-level concurrency variables in their studies, arguing that "ecological analysis is an appropriate method" to examine the hypothesis.7Morris M Epstein H Wawer M Timing is everything: international variations in historical sexual partnership concurrency and HIV prevalence.PLoS One. 2010; 5: e14092Crossref PubMed Scopus (104) Google Scholar Stéphane Helleringer and Hans-Peter Kohler suggest that the concurrency variable could be more affected by measurement error than the lifetime partners variable. However, this suggestion remains unsupported by evidence, since neither of the two studies Helleringer and Kohler cite directly compared measurement error in the two variables. Morris and Epstein write that empirical evidence is lacking that respondents in our study preferentially chose partners from their immediate neighbourhoods. This is incorrect. As stated in the paper, our data show that 61% of women reported at least one partnership with a man who lived in the same isigodi (a Zulu term for a small neighbourhood community headed by a local chief). Morris and Epstein further claim that the data in our study have "serious problems" and extract other data from our past publications to back up this claim.8Tanser F Hosegood V Bärnighausen T et al.Cohort profile: Africa Centre Demographic Information System (ACDIS) and population-based HIV survey.Int J Epidemiol. 2008; 37: 956-962Crossref PubMed Scopus (271) Google Scholar However, they extract the wrong values and fail to consider the relevance of these cross-sectional data for the study at hand. From the extracted data, Morris and Epstein deduce that our contact rate was "low". In fact, the contact rate for this longitudinal study (ie, the proportion contacted of those eligible to be contacted by the HIV surveillance at least twice over the observation period) was 99·7%. The overall participation rate in the HIV cohort is thus correctly reported in our article and does not relate to any of the values Morris and Epstein calculate. As we state in our article, "in any observational study involving the collection of data for sexual behaviour and HIV acquisition, the possibility of bias affecting the results must be considered" and can never be completely ruled out. However, our work provides the most rigorous test of the concurrency hypothesis in a general population in sub-Saharan Africa to date. We fully agree with Shelton that it is important to raise awareness that "having multiple partners puts one's family at risk". Indeed, our results suggest that this is precisely what a clear, well constructed message around multiple partnering would help achieve. We declare that we have no conflicts of interest. Role of concurrency in generalised HIV epidemicsIt is heartening that Frank Tanser and colleagues (July 16, p 247)1 support direct messaging about sexual partner reduction for HIV prevention. However, it is not surprising that they did not see a role for concurrent or overlapping sexual partnerships in their study from South Africa. Their approach is very insensitive: it lacks individual exposure data for women, uses only a single initial point-in-time measurement of men's behaviour to construct a remote secondary geography-bound aggregate surrogate for women's risk, and has low response rates. Full-Text PDF Role of concurrency in generalised HIV epidemicsDespite its claims, the study by Frank Tanser and colleagues1 is not "a formal test of the concurrency hypothesis". The analysis does not tell us whether a woman in this community is more likely to become HIV positive if her partner(s) have concurrent partners, because Tanser and colleagues did not measure this; they measured the future risk a woman faces from the past characteristics of men who live within a few km radius of her. Full-Text PDF Role of concurrency in generalised HIV epidemicsIn their population-based cohort study, Frank Tanser and colleagues1 conclude that they find "no evidence to suggest that concurrent partnerships are an important driver of HIV incidence in this typical high-prevalence rural African population". We believe that this conclusion is unjustified since they have used an inappropriate method to assess this association. Full-Text PDF Role of concurrency in generalised HIV epidemicsFrank Tanser and colleagues1 argue that concurrent partnerships are not an important driver of HIV incidence in KwaZulu Natal, South Africa. They acknowledge that their results suffer from possible attenuation bias because they measure the effects of a community-level exposure (concurrency in men) on an individual disease outcome. However, they do not mention attenuation stemming from measurement error in their main exposure variables, instead assuming that, unlike women, men report their concurrent partnerships accurately. Full-Text PDF