KwaZulu-Natal province of South Africa found that women working at truck stops averaged 22 sex acts per week, with coitus occurring between two to ten times in a 24 hour period [9]. This earlier survey also found that peno-vaginal sex was the only practice reported, with strong cultural sanctions against anal and oral sex. The sex workers in this urban cohort appear to have customers with whom they engage in fewer acts but see more regularly, averaging just three clients per week, and are also engaging in anal and oral sex. Consequently, they have a very different risk profile than what had been anticipated based on the prior studies. This difference in risk behaviour has likely impacted the HIV seroincidence in this cohort. Data are being analysed to determine the specific behavioural risk factors for HIV infection in this cohort. The perceived ease of condom use in this cohort shows that women are significantly more likely to feel that they will be able to use condoms with casual partners than with their steady partners. This difference may explain why condom use has not led to a reduction in HIV risk as these women are still vulnerable due to inconsistent condom use with their steady partners. This observation suggests that condom use may be associated with perceptions of trust; whatever the case, condom use is inconsistent at best in this population. The behavioural risk data collected at baseline will be analysed after the full 24 month follow-up period to see which factors are most predictive of HIV acquisition in this cohort. Based on previous data, the projected annual sero-incidence for this high-risk population was 18.2% [9,12]. The actual seroinci- dence in this cohort has been 7.2% which is a little higher than the national seroincidence estimates of 6.3% for all South African women between the ages of 15 and 49 [14]. The risk of HIV acquisition in the context of the maturing South African HIV epidemic seems to be highly generalized. Hence, our designations of ‘‘high risk’’ are seemingly less relevant in retrospect and perhaps need not be considered as rigorously when designing future studies. Furthermore, unmeasured susceptibility factors may have disproportionately influenced the observed infection rate in this cohort of highly exposed individuals, where the women who were more susceptible had acquired HIV prior to screening for this study. Furthermore, our data support the notion that high-risk cohorts may yield fewer seroconversions over time, and that this attrition should be factored into the design of such cohorts. Women who remain in the cohort at the end of the follow-up, may represent a group enriched with host genetic resistance factors, and this is currently being explored as this group of woman has been recruited into a highly exposed persistently negative cohort for additional study. Conclusion and Significance To the best of our knowledge, this is the first prospective cohort to be assembled in southern Africa for a comprehensive analysis of the behavioural, clinical, and immunological characteristics associated with acute HIV infection. The methods, as well the challenges, of recruiting and successfully retaining a HIV-negative cohort have been described and can provide guidance for others wanting to assemble similar cohorts. Given the characteristics of our cohort, these experiences are particularly relevant to the recruitment and retention of cohorts of urban female sex workers in South Africa for HIV prevention and pathogenesis research. References CAPRISA 002 Study The changing nature of the epidemic must be considered when recruiting high risk HIV-negative cohorts in prevention research, as demonstrated by the lower than anticipated seroincidence in this study. Understanding natural history of HIV infection from as early a point following exposure to HIV as possible is key for the design, development and targeting of new prevention interventions as well as for the treatment of advancing HIV disease. Understanding how to identify those most at risk for HIV infection, whether this is due to high-risk behaviours, or due to the increased risk of infection due to high viral loads during acute or recent infection, or due to the increases risk conferred by concomitant sexually transmitted infections, has recently been suggested as the key concern for prevention science [26]. It is hoped that data from the Acute Infection study will contribute significantly to vaccine development by describing the natural history of HIV infection, the impact of host and viral factors during acute infection on viral load set point, as well as the impact of viral set-point on disease progression. Behavioural data collected before and after acute infection should assist in developing algorithms to identify those most at risk of infection. HIV screening algorithms [27] and clinical data collected during acute infection are potentially fruitful explorations of efficient and effective screening algorithms for identifying acute infection. These specific research areas will be addressed separately in publications from the CAPRISA 002 Acute Infection Study Team. Acknowledgments We would like to acknowledge the co-operation and assistance of Prof Gita Ramjee and her team at the Medical Research Council of South Africa, HIV Prevention Research Unit. The significant contribution of our community research group and community liaison persons to our recruitment and retention is acknowledged. We thank our participants who make a significant personal contribution in HIV prevention research through their continued support and participation in our work. Ethics: The Acute Infection study was reviewed and approved by the research ethics committees of the University of KwaZulu-Natal (E013/04), the University of Cape Town (025/2004), and the University of the Witwatersrand (MM040202). All participants provided written informed consent for screening, enrolment and specimen storage. Author Contributions Conceived and designed the experiments: LM CW CG SAK KM Fv QA. Performed the experiments: LM CW CG KM Fv II. Analyzed the data: AG. Wrote the paper: LM CW CG SAK KM Fv SA AG NB QA. Other: Conceptualized the paper and drove the paper writing and revision process: Fv. Study coordinator and behavioural co-investigator on the Acute Infection Study: Fv. Protocol co-Chair and clinical co-investigator: KM. Assisted in the conceptualization of the paper: AG KM. Commented on succeeding drafts of the paper: AG NB KM II CW LM CG SAK QA. Protocol co-Chair and virology co-investigator: CW. A Research fellow at CAPRISA: SA. Prepared the initial drafts of the paper, conducted the literature review, and assisted with subsequent redrafting of the paper: SA. An immunology co-investigator on the study: LM CG. Head of the laboratory core: LM. The study epidemiologist and a co-investigator: QA SAK. The study statistician: AG. Conducted the statistical analyses and produced the graphs for the paper: AG. The study community liaison officer and co-investigator: NB. Contributed the sections dealing with community: NB. The study clinician: II. Collected the clinical data for the study: II. 1. Costello C, Nelson KE, Suriyanon V, Sennun S, Tovanabutra S, et al. (2005) HIV-1 subtype E progression among northern Thai couples: traditional and non-traditional predictors of survival. Int J Epidemiol 34: 577–584. 2. Deschamps MM, Fitzgerald DW, Pape JW, Johnson WD Jr. (2000) HIV infection in Haiti: natural history and disease progression. AIDS 14: 2515–2521. PLoS ONE | www.plosone.org 7 April 2008 | Volume 3 | Issue 4 | e1954