South Africa is experiencing one of the most rapidly growing HIV epidemics in the world. In 1990 the prevalence of HIV infection among women attending antenatal clinics was less than 1%. At the end of 1999 national antenatal prevalence had reached 23% with provincial prevalences ranging from a low of 7% in the Western Cape to a high of 33% in KwaZulu/Natal . There is some evidence that the rates of infection in KwaZulu/Natal are levelling off .
Many studies have investigated risks factors for HIV infection among adults in sub-Saharan countries and the most important risk factors that have been identified include risky sexual behaviour, the presence of sexually transmitted infections (STI) and lack of male circumcision [2–8]. In the past, particular attention has been paid to sexually transmitted infections that can lead to genital ulcer disease (GUD) such as those caused by Treponema pallidum, Haemophilus ducreyi and herpes simplex virus type 2 (HSV-2), because these infections are known to increase both the susceptibility of uninfected people to HIV and the infectivity of people who are themselves infected with HIV [2,5,9–13]. A recent study undertaken in South Africa among men attending sexually transmitted diseases (STD) clinics found that HSV-2 was the most common cause of GUD with an overall seroprevalence of antibodies to HSV-2 of around 50% .
Most studies of the prevalence of HSV-2 and its role as a risk factor for HIV have been carried out in particular groups that are not representative of the overall population, such as STD or antenatal clinic attendees, blood donors, or men who have sex with men [9,15–19]. Only three population-based surveys have been reported from Africa [20–22]. None of these focused on the association between HIV and HSV-2 infection among youth specifically even though young people, and especially young women, are at particularly high risk of HIV infection in many developing countries .
The purpose of the present study was: (1) to measure the prevalence of HIV and HSV-2 seropositivity by gender and age among 14- to 24-year-olds living in a South African township; and (2) to assess risk factors for HIV infection in these young people, with particular regard to the role of HSV-2 seropositivity. The results are discussed in terms of prevention efforts that are needed to protect young people from becoming infected with HIV.
In August 1999 a population-based, cross-sectional study was conducted in a township northwest of Carletonville, in Gauteng Province, South Africa. The Carletonville district, with a population of 150 000 people, contains the largest gold-mining complex in the world, which houses approximately 70 000 migrant mineworkers  in twelve single-sex hostel complexes around the mineshafts to the south of Carletonville town. The survey was designed to collect data on 800 men and 800 women aged 14 to 24 years living in a nearby township. Households were selected by a two-stage random sampling technique. Index houses were randomly selected from a map obtained from the local municipal offices. Using each index house as a starting point, a systematic sample of households was taken. The sampling scheme was designed to be self-weighting. All men and women aged 14 to 24 years, who slept in the selected households the night before the study team visit were eligible for inclusion in the study. The field workers were able to find the occupants of 89.0% of the sample households and 11.3% of potential participants declined to participate. Eligible participants were transported to a local facility for the interviews and the collection of blood and urine samples. If eligible participants were not at home, the study team made up to three repeated visits before abandoning that household.
The questionnaire used in this study was based on a UNAIDS questionnaire  adapted to the local situation and to the age of subjects enrolled in the sample. The consent form was presented in the language of the respondent, who was invited to take part in the study, and those who agreed were asked to sign the consent form. The interviewers completed the questionnaire during a private interview in the preferred language of the interviewee. Data were collected on background and behavioural characteristics as well as a medical and sexual history. Sexual partnerships were divided into casual partnerships and other partnerships, that are composed of those who were married or living as married. A section of the questionnaire was reserved for information concerning the respondent's last five casual partnerships.
Following the interview, blood and first-catch urine specimens were collected. A single Capillus HIV-1/HIV-2 latex aggregation test (Cambridge Biotech Corporation, Galway, Ireland), which has a sensitivity and specificity of greater than 99.9 and 99.6%, respectively, was subsequently used to screen the sera for HIV infection. A non-treponemal carbon antigen test (Immutrep RPR test; Omega Diagnostics, Alloa, Scotland, UK) and a specific treponemal test (FTA test, Cellognost Syphilis H; Dade Behring, Marburg, Germany) were used for syphilis serology. A qualitative enzyme-linked immunosorbent assay (ELISA) test was used to detect HSV-2 type specific IgG antibodies (MRL Diagnostics, Los Angeles, California, USA). Neisseria gonorrhoeae- and Chlamydia trachomatis-specific DNA sequences were detected in the urine samples by using ligase chain reaction (LCR) test (Abbott Laboratories, North Chicago, Illinois, USA). Treatment of symptomatic and asymptotic STI's were subsequently offered to all participants prior to destruction of all identifiers and performance of HIV testing. Any participants who wished to know their HIV status were offered a separate ELISA test with pre- and post-test counselling to be arranged through the normal clinic channels.
Laboratory results and data generated from questionnaires were entered twice into a database (Microsoft Access, Redmond, Washington, USA) by different people. The two entries were compared and discrepancies were corrected. The data were then extensively checked for inconsistencies. The files were then imported into the Statistical Package for Social Sciences (SPSS 8.0 for Windows, Chicago, Illinois, USA) and prepared for statistical analysis.
Five months after completion of the survey, a validation study was conducted among a random sample of young people who participated in the main study. This sample was stratified by gender and by whether or not they had ever had sexual intercourse. The aim was to verify the age at first sex and the number of lifetime partners at the time of the survey interview. Of the 120 selected participants, 106 (88.3%) were traced and interviewed using an in-depth, semi-structured topic guide.
A total of 723 men and 784 women agreed to participate in the survey. In order to assess risk factors for HIV infection, the analysis was restricted to men and women who reported ever having had penetrative vaginal intercourse. These individuals are referred to below as ‘sexually active'.
The following risk factors for HIV infection were explored in the univariate analysis. Background characteristics: current age, marital status, whether respondent had living children, living in a squatter settlement, was born in the Carletonville area, had consumed alcohol in the last month, or was currently in secondary school. Sexual practices: age at first sexual intercourse, number of lifetime sexual partners, age difference between respondent and partners, number of current relationships, condom use, having had forced sex, having had sex with married people, having exchanged money for sex (men), and, for women, having had sex with mineworkers. Health issues and STI's: symptoms of STI's in the last year, circumcision status and penile hygiene (men), dry sex (women), serology status for HSV-2 and syphilis (RPR and FTA), and detection of specific DNA sequence of N. gonorrhoeae and of C. trachomatis in urine.
Computations were carried out ignoring clustering. Means and proportions were calculated with 95% confidence intervals (CI). For non-normally distributed parameters, medians were calculated with the inter-quartile range (IQR) and significance testing was carried out using the Kruskall–Wallis test. Medians were compared using the median test. Median and IQR of age at first sexual intercourse and of duration of partnership were computed using survival analysis. Duration of partnerships among men and women were compared using the log rank test.
We used 95% CI of the odds ratio (OR) and χ2 tests to investigate the statistical significance of the association between HIV infection and potential risk factors. Continuous variables were categorized. All risk factors associated with HIV infection at a significance level of 0.25 or less were entered into a forward stepwise selection, multiple logistic regression model. Two models are presented. Model 1 was constructed without HSV-2 serology and model 2 with HSV-2 serology. All potential interactions involving age, HSV-2 serostatus and number of lifetime partners were tested in models 1 and 2.
We compared HIV prevalence among women aged 16–18 years with the HIV prevalence among their casual partners. For this purpose we first estimated the prevalence of HIV among the casual partners of these women by taking the age distribution of their most recent casual partner as reported by the women, and then combining this with the age-specific HIV prevalence of men in the sample. In this estimate, we assumed that the 4.9% (eight of 162) of casual partners above the age of 24 years [with a mean (median) age of 28.7 years (27.5 years)] have the same HIV prevalence (40.0%) as the 24-year-old men in the sample. The male-to-female transmissibility of HIV per-partnership is defined as the probability per-partnership of transmission from an infected man to his female partner. Based on the HIV status of women aged 16–18 years, their number of lifetime partners and the estimate of prevalence among their casual partners, the male-to-female transmissibility of HIV per-partnership was estimated using a Bernoulli model [26,27] and the maximum likelihood method. The IQR was estimated using the non-parametric resampling bootstrap technique  applied to the sample of women aged 16–18 years and to their partners (see Appendix).
Ethical approval for the project was obtained from the committee for Research on Human Subjects of the University of Witwatersrand, Johannesburg, on 10 March 1997 (Protocol number M 970235).
The male-to-female sex ratio was 0.92 (723 of 784) (Table 1). The median age was 18 years for men (IQR, 16–21 years) and 19 years for women (IQR, 16–21 years). Approximately half (53.0%) of the sample was born in the Carletonville district, 26.6% in another urban area in South Africa, 16.1% in a rural area in South Africa and 4.4% in another country. The majority of the participants were either students or scholars (59.7%), or unemployed (31.4%) and three men were mineworkers. Among those aged 14 to 16 years, 94.1% of the men and 85.2% of the women were students or scholars at the time of the survey. Among those aged 17 to 18 years, the corresponding figures were 84.0 and 76.5%, respectively. Background characteristics of sexually active men and women are reported in Table 1.
The median age at first sex was 16.6 years for men (IQR, 15.2–18.2 years) and 17.2 years for women (IQR, 16.0–18.5 years). The first decile for age at first sex was 13.7 years for men and 15.0 for women. At the time of the interview, 77.6% (560 of 722) of the men and 79.4% (622 of 783) of the women reported having had sex. The majority of men (96.5%) and women (86.5%) had never been married. Of the men, 5.8% said that they had had more than 11 lifetime sexual partners whereas among the women, 7.8% said that they had had more than four partners. The proportion of men and women who lived in the Carletonville area at the time of first sex was 85.5 and 75.7%, respectively. From the age of the participants and the reported age at first sex, we estimated that the proportion of young women aged 15 to 19 years who were sexually active increased significantly between 1994 and 1999 (56.9 versus 67.5%, P = 0.0032). This increase was not significant among men in the same age group (65.9 versus 68.4%, P = 0.51).
Sexual behaviour characteristics of sexually active men and women are reported in Table 2. The mean (median) number of lifetime partners was 4.7 (3) and 2.6 (2), respectively. Of the women, 9.0% reported having had sexual partners who were at least 10 years older than themselves. The median number of sexual acts in the last month was one for both men and women. Of those who had at least one current casual relationship, 28.3% of men and 13.7% of women had more than one current relationship. Forty one percent of men and 42.5% of women reported never having used condoms. Of the women, 15.8% reported ever having had sex against their will.
We considered the last five casual relationships, 1578 for men and 1345 for women, as reported by the respondents. Of sexually active men and women, 80.5 and 95.6%, respectively had five or fewer casual relationships. Women generally had casual partners who were older than themselves (median age difference of 2 years; IQR, 0–5 years) whereas men generally had casual partners who were younger than themselves (median age difference of 3 years; IQR, 1–5 years). The median duration of relationships reported by women (12 months; IQR, 4–48 months) was longer than the duration of relationships reported by men (8 months; IQR, 2–37 months) (P < 0.0001). The median number of times that sexual intercourse occurred per relationship for women (6; IQR, 3 to > 20) was significantly higher than that for men (3; IQR, 1–7) (P < 0.0001). The casual partners of women were more likely to be married than were the casual partners of men (10.8 versus 2.4%, respectively, P < 0.0001). Of the relationships reported by women, 7.8% involved mineworkers. The casual partners of women living in squatter settlements were more likely to be mineworkers than were the casual partners of other women (12.4 versus 3.9%, respectively, P < 0.0001). Condoms were never or rarely used in 68.8% of the relationships reported by men and in 75.6% of relationships reported by women. However, the situation is improving: whereas condoms were never or rarely used in 80.2% of relationships that started between 1993 and 1995, this had fallen to 69.3% in relationships that started between 1996 and 1998 (P < 0.0001).
Prevalence of HIV and HSV-2 seropositivity
The overall prevalence of HIV was 22.4% (338 of 1507) and of HSV-2 seropositivity was 35.4% (533 of 1489) with higher rates among women than among men. The prevalence of HIV infection was 9.4% (68 of 723) among men and 34.4% (270 of 784) among women and of HSV-2 was 17.0% (122 of 718) among men and 53.3% (411 of 771) among women. The comparison of HIV prevalence and HSV-2 seroprevalence by age among men and women shows that in both genders and in all age groups HSV-2 seroprevalence exceeded HIV prevalence (Figs 1a, b). The prevalence of HIV and HSV-2 seropositivity increased rapidly with age, especially among women. The prevalence of HIV increased from 6.1% (95% CI, 1.2–11.0) among women aged 14 to 15 years to 66.7% (95% CI, 56.0–77.4) among women aged 24 years, and the prevalence of HSV-2 seropositivity rose from 18.6% (95% CI, 11.4–25.8) to 94.3% (95% CI, 87.6–100). Among HIV-seropositive individuals, 64.7% of men and 90.7% of women were also HSV-2 positive.
Among those who said that they had never had sex, 13 of 323 (4.0%) were HIV-positive and 26 of 319 (8.2%) were HSV-2 positive representing 3.8% (13 of 338) and 4.9% (26 of 533) of all HIV and HSV-2 positive individuals, respectively.
The increase in prevalence of HIV and of HSV-2 seropositivity by number of lifetime partners was more marked among women than among men (Fig. 2a, b). The HIV prevalence among men who said that they had never had sex was 1.2% (95% CI, 0.0–3.8%) and reached 18.8% (95% CI, 4.7–32.8%) among men who reported having had eight to 10 lifetime partners. Among women who reported having had zero, one, two and three or more lifetime partners, the prevalence of HIV was 6.8, 19.9, 42.7 and 56.1%, respectively.
Among sexually active men and women the prevalence of HIV was 11.8 and 41.6%, respectively, and the median (IQR) number of lifetime partners was 3 (2–4) and 2 (1–3), respectively. If we consider only sexually active women aged 16 to 18 years, the prevalence of HIV was 21.1% (95% CI, 15.9–26.2%), the median (IQR) number of sexual partners was 1 (1–2) and the mean 1.7 (95% CI, 1.6–1.9). Among these sexually active women, 3.0% (95% CI, 0.0–6.2%) declared ever having had a casual partnership with a mineworker. HIV prevalence among those who declared casual partnerships only with men living in the township did not differ from other women of the same age group who declared no casual partnerships with miners (20.0 versus 20.4%, P = 1.0). Using the method described above, we estimated that the prevalence of HIV among the 162 most recent casual partners of these women is 13.4%. It appears, therefore, that the prevalence of HIV among the partners of women aged 16 to 18 years is lower than among the women themselves. The male-to-female per-partnership transmissibility (IQR) from men to women aged 16 to 18 years is estimated to be 0.92 (0.77–1) per partnership. If women under-report their number of lifetime partners by a factor of 2, say, then this transmissibility is estimated to be 0.49 (0.39–0.56) (see Appendix).
Risk factors for HIV status of sexually active men and women
The HIV prevalence amongst married men was found to be more than four times greater (44.0%/10.0%) than the prevalence amongst men who had never been married (OR, 7.1; 95% CI, 3.0–16.3) (Table 1). Being married was also a risk factor for women but the OR was smaller (OR, 2.9; 95% CI, 1.8–4.4). About four out of every ten women had at least one living child and the prevalence of HIV among these women was higher than among those without children (54.8 and 32.9%, respectively; P < 0.0001). The prevalence of HIV was higher among men and women born outside the Carletonville area than it was among those born in the Carletonville area. It was also higher among those not attending secondary school. Almost half of the men, but only 16.2% of the women, drank alcohol at least once a month, and those who consumed alcohol were more likely to be HIV infected than those who did not.
HIV prevalence was lower amongst those who had sex for the first time at or before 15 years of age for men and for women (Table 2). However, this is probably due to the relationship between age and age at first sex in our sample, which is composed of sexually active people. When adjusted for age, the OR related to age at first sex became non significant (men: adjusted OR, 1.5; 95% CI, 0.8–2.7; women: adjusted OR, 0.8; 95% CI, 0.5–1.3).
The likelihood of being infected with HIV increased with the number of sexual partners. Compared to those who had one casual partner, the odds ratio for HIV infection for those who had two partners was 1.1 for men (95% CI, 0.4–2.7) and 3.0 for women (95% I, 1.9–4.7). For those who had three or more partners the odds ratio was 2.1 for men (95% CI, 1.0–4.3) and 6.0 for women (95% CI, 3.9–9.2). For men the prevalence of HIV was not significantly associated with the number of current relationships or whether or not they had at least one casual partner who had other sexual partners. Both men and women who used condoms during their first sexual encounter were less likely to be infected with HIV than those who did not. Women who reported having had sex with mineworkers were more likely to be infected with HIV than those who said that they had not.
Health issues and STI's
Self-reported symptoms of STI's in the last year were common in both sexes (18.0% of men and 31.9% of women reported having had a discharge or genital pain in the last year) and HIV prevalence was higher amongst those who reported such symptoms (Table 3). Circumcision was not common (only 10.1% of men were circumcised) and median (IQR) age at circumcision was 16 years (12–19 years). No protective effect of circumcision on HIV prevalence was shown. The prevalence of HIV was lower among the 21 sexually active men who were circumcised before their first sexual encounter than among the 535 other sexually active men (4.8 versus 12.1%) but the difference was not significant (P = 0.50). The few men (12.7%) who washed their penis with soap less than once a day had a higher prevalence of HIV than among those who did wash it at least once a day. The highest odds ratios for men were related to HSV-2 serology and syphilis serology. Men who were HSV-2 seropositive were seven times (prevalence ratio, 38.7%/5.2%) more likely to be HIV positive than men who were HSV-2 seronegative. The association between positive urine tests for N. gonorrhoeae or C. trachomatis and HIV status was not significant.
For women the association between syphilis and HIV was not significant whereas the association between N. gonorrhoeae and HIV was significant. For women, being infected with HSV-2 was strongly associated with HIV infection. No significant effect of dry sex or of time between menarche and first sex was found on HIV status.
Multivariate analysis of risk factors
In the multivariate analysis, only a few factors were significantly associated with HIV (Table 4). For men, in model 2, six factors were significantly associated with HIV namely HSV-2 serostatus, age, history of discharge, genital pain or sores in the last year, ever having had sex against his will, not attending secondary school and washing the penis with soap less than once a day. The most significant factor associated with HIV status was HSV-2 serostatus. With the exception of HSV-2 serostatus, model 1 and model 2 are composed with the same risk factors. For women, in model 2, four factors were significantly associated with HIV namely HSV-2 serostatus, age, number of sexual partners and type of housing. As for men, the most significant factor was HSV-2 serostatus. In comparison with model 2, model 1 included two additional factors namely reporting sexual partnership with a mineworker and reporting at least one casual partnership involving 20 or more sexual acts. Interactions tested did not reach statistically significant levels. When adjusted for factors composing model 1, the OR related to syphilis status was non-significant (men: adjusted OR, 3.0; 95% CI, 0.7–12.6; women: adjusted OR, 1.0; 95% CI, 0.4–2.4) and the OR related to HSV-2 status was close to values in model 2 (men: adjusted OR, 5.3; 95% CI, 2.7–10.3; women: adjusted OR ,8.3; 95% CI, 4.8–14.3).
The prevalence of HIV infection detected among people aged 14 to 24 years recorded in this study was extremely high. In South Africa, the prevalence of HIV infection has increased rapidly over the last few years. In Gauteng, the province within which Carletonville falls, the prevalence of HIV infection among women aged 15 to 49 years attending antenatal clinics in the public sector, increased from 0.7% in 1990, to 2.5% in 1992 and 15.5% in 1996 . Although surveillance studies have been conducted at several sentinel sites in South Africa, Carletonville is one of only a few sites at which a large-scale, population-based study has been conducted. Further research is urgently needed to assess the situation in other urban and rural centres in South Africa, to determine the likelihood that equally high infection rates have been reached, and to plan interventions.
The results presented here are from a large, population-based, cross-sectional study conducted in August 1999 in which the refusal rate was low. The quality of the data on reported sexual behaviour was reasonably good as assessed by a validation study among 106 randomly selected respondents. Of those people who indicated in the survey that they had not had sex, 11 of 54 (20.4%) subsequently said in the validation study that they had experienced sex at the time of the survey. Among those who reported having had at least one lifetime partner at the time of the survey, the median of the paired differences in the number of lifetime partners reported in the survey and the validation study was 0 (IQR, -1 to 1). Among these people, seven of 27 women (25.9%) and six of 21 men (28.9%) reported a higher number of lifetime partners in the subsequent validation study. These results indicate that there was some inconsistent reporting of lifetime partners by both sexes.
The main risk factors for HIV were related to STI's, sexual behaviour and age. Multivariate risk factor analysis showed that reporting symptoms of STI's in the last year was an independent risk factor for HIV amongst men. Syphilis serological status did not appear to be independently associated with HIV but the power of the analysis of this association was small because of the low syphilis prevalence in the studied population. Among women, the age at first sex and having been forced to have sex, which appeared to be relatively common in this population, were not independent risk factors for HIV infection. By contrast, the number of lifetime partners was a strong and independent predictor of HIV status among women. The significant increase in HIV prevalence as a function of the number of lifetime partners indicates the high vulnerability of young women to HIV infection.
We found that women being married or living as married, and women and men reporting casual partnerships with 20 or more sexual acts are more at risk of HIV infection. A likely interpretation is that the per-partnership transmission of HIV from males-to-females and from females-to-males depends on the number of sexual acts in the relationship. Having had a sexual relationship with a mineworker was a significant independent risk factor only when not taking account of HSV-2. It is likely that sexual relationship with mineworkers, the vast majority of whom live in single-sex hostels without their wives or families, who are relatively well paid, and whose HIV prevalence is 28.5% , could have played a role in the introduction of HIV in this township. But the low proportion of young township women reporting ever having had a sexual relationship with a mineworker indicates that the current high HIV prevalence in the township is unlikely to be sustained solely by sexual mixing with mineworkers.
In this study a strong, independent association between HIV and HSV-2 serological status was detected. The high seroprevalence of HSV-2, the association between HIV infection and HSV-2 seroprevalence, and the fact that HSV-2 infection is a cause of genital lesions indicate that HSV-2 infection could play a major role in the spread of HIV in the population of this study. A recent study conducted in South Africa drew attention to the likely impact of genital herpes on HIV infection . Our study supports those previous findings. Although HSV-2 has been used as a marker of sexual risk behaviour , the relationship between HIV and HSV-2 infection is complex and poorly understood. Whereas it is highly likely that HSV-2 facilitates infection with HIV, being infected with HIV may also facilitate the spread of HSV-2 [4,9,14,18–20,22,31–36]. Further studies, in particular longitudinal studies involving available treatments for HSV-2, are needed to elucidate the nature of the relationship between infection with HSV-2 and HIV, especially among youth, in order to understand the spread of HIV in sub-Saharan Africa and the role played by HSV-2 infection.
The finding that poor penile hygiene was associated with HIV infection is important and should help in the design of interventions, even though most of the men in this study already washed their penis at least once a day. The effect of post-coital cleansing on HIV transmission should be investigated.
The prevalence of HIV infection among sexually active women between the ages of 16 and 18 years was high (21.1%) even though they only reported a median of one lifetime partner (upper quartile 2) and the best estimate of HIV prevalence among their casual partners was 13.4%. It is clear from our data that the high prevalence of HIV infection among young women is not simply due either to very high rates of infection amongst their partners or to very high levels of sexual activity. Even if there is under-reporting of sexual activity, as is likely to be the case among women in particular, the validation study indicates that it is unlikely that such under-reporting would be sufficient to substantially change these arguments.
Rather, the high rates of HIV infection among these young women are suggestive of high male-to-female transmissibility. A recent review of European and American studies found estimates of heterosexual, male-to-female transmissibility ranging from 0.1 to 0.3 per partnership . Our estimates of transmissibility are substantially higher than those reported in developed countries and several factors could explain this difference. A recent study has shown that viral load is an important predictor of the risk of heterosexual transmission of HIV-1 in Uganda  and viral loads in South Africa may be higher than in Europe and America. Alternatively, the high prevalence of genital herpes  in our sample might contribute to the high transmissibility of HIV. Further studies are needed to explore infection rates in young women and these should ideally include measurements of the viral load in the HIV-positive partners of these women, including mineworkers. Finally, this high male-to-female transmissibility could be explained by under-reporting of lifetime partners by women but even if we assume that women under-report by a factor of two, the estimate of per partnership male-to-female transmissibility is still substantially higher than that reported in developed countries.
Reducing HIV transmission in this population is a major challenge. The survey data reported here were collected as part of an intervention to control STI's and ultimately HIV–AIDS in a major mining centre. At the present time there is insufficient evidence concerning the extent to which HSV-2 increases the transmission of HIV to recommend mass treatment of genital herpes. Moreover, genital herpes, a viral STI, can only be treated symptomatically or suppressed by using specific antiviral drugs at considerable cost, and no HSV-2 vaccine is currently commercially available. Therefore, the best options for reducing transmission includes prompt treatment of curable STI's, communication campaigns aimed at alerting the population to the relatively mild manifestations of genital herpes and the need to abstain from sexual contact while lesions persist. In addition it is important to find ways to persuade young people to reduce their number of lifetime partners and to substantially increase condom use. Our evidence for an increase in reported condom use among young people between 1992–1995 and 1996–1999 is encouraging but rates of condom use are still disappointingly low. We believe that condom use should be promoted as strongly as possible, and that only a high rate of consistent condom use will have an effect on the spread of HIV and other STI's in this population. As most young people in this study were still at school when they had their first sexual experience, attempts to limit the future spread of HIV should involve not only prevention in the community but also the development, evaluation and implementation of effective school-based interventions as an essential part of the school curriculum.
We thank all those who supported this work and in particular the Carletonville community and the Mothusimpilo intervention team: Solly Moema, Zodwa Mzaidume and Bareng Rasego. Very special thanks go to Reathe Taljaard and Gaph Pathedi who helped to make the survey possible. Mrs. Venessa Tshabalala-Maseko, Mr. F. Radebe, Mr. D. Mabaso and Mr S. Khumalo provided excellent technical assistance in regard to the laboratory testing. We thank Lewis Ndhlovu, Population Council, South Africa and Jamie Robinson, Wellcome Research and Development, UK for their invaluable assistance and support.
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In a Bernoulli model, the probability of remaining uninfected is (1− T)n, where n is the number of partners and T is the per-partnership transmission so that the probability of infection is 1−(1− T)n.
Let τ be the male-to-female transmissibility of HIV per-partnership, and P m be the prevalence of HIV among the pool of men who are the casual partners of the women. Under the hypothesis that women choose their casual partners randomly, the probability of HIV infection (P w) for a woman who has had n non-spousal partners is EQUATION Applying equation (1) to women aged 16 to 18 years; the maximum likelihood estimate of τ P m is 0.124. With P m = 13.4% we obtain a value for the transmissibility of τ = 0.92, and for the IQR by the bootstrap technique of 0.77–1. If we assume that women under-report their number of lifetime partners by a factor of 2, the estimate of τ P m is 0.0658. With P m = 13.4% we obtain a value for the transmissibility of τ = 0.49, and a IQR of 0.39–0.56.