In several high-income countries, human immunodeficiency virus (HIV) continues to be an infection that disproportionately affects men who have sex with men (MSM) . After the earliest years of the epidemic, the HIV-1 incidence among MSM strongly declined, mainly because of decreasing levels of sexual risk behaviour [2–5]. Throughout the 1990s, the lower incidence remained relatively constant in this group, but the 1996 introduction of effective combination antiretroviral therapy (cART) has completely changed the HIV-1 epidemic. cART improved the quality of life of HIV-infected individuals and dramatically decreased the number of people dying of AIDS [6,7]. In response to these beneficial effects, increases in sexual risk behaviour among MSM have been observed in several countries [8–11], including the Netherlands [12,13]. Sexually transmitted infections (STIs) have likewise increased [8,10,12,14,15]. Some studies suggest that MSM, perceiving less HIV/AIDS threat since the introduction of cART, changed their sexual behaviour from protected to unprotected sex [16,17]. HIV infection was no longer seen as a death sentence but as a manageable chronic disease. All these signals were highly suggestive for a re-emerging HIV-1 epidemic among MSM [18,19]. Recent surveillance data show an increase in new HIV diagnoses among MSM in several affluent countries [20–23], but these might reflect increases in HIV testing. Available data from the few existing observational cohorts of HIV-negative MSM, which yield direct estimates of the HIV-1 incidence rate, showed a decreased or stabilized incidence [24–26].
For prevention, it is important to monitor trends in HIV-1 incidence continuously and to be aware of possible changes in risk factors for HIV-1 infection over time. The ongoing Amsterdam Cohort Studies (ACS) among MSM, which recently achieved 25 years of follow-up, offers the unique opportunity to study these questions. We therefore used ACS data to investigate the long-term trends in HIV-1 incidence rates and sexual risk behaviour. In addition, we identified risk factors for HIV-1 seroconversion and explored whether these have changed over time.
Study population and study procedure
The ACS among MSM (www.amsterdamcohortstudies.org) is an open, prospective cohort study that started in 1984 to investigate the epidemiology, pathogenesis, and prevention of HIV/AIDS among HIV-negative and positive men . Until 1995, men of all age groups were eligible to participate if they lived in or around Amsterdam and had had at least two male sexual partners in the previous 6 months. In the period 1995–2004, only men aged 30 or less with at least one male sexual partner in the previous 6 months could enter the study. In 1996, follow-up was terminated for HIV-negative MSM aged above 35 who had entered the study before 1995. From 2005 onwards, however, men of all age groups with at least one male sexual partner in the previous 6 months could participate. Recruitment entailed ‘convenience sampling’ (brochures/advertisements in the STI clinic and other MSM meeting places) and ‘chain referral sampling’ (participants recruiting other participants). The ACS has been carried out in accordance with the ethical principles set out in the Declaration of Helsinki. Participation is voluntary, and written informed consent is obtained from every participant at intake.
Amsterdam Cohort Studies participants visit the Public Health Service (PHS) of Amsterdam every 3–6 months to complete self-administered questionnaires and to give blood for HIV-1 testing and storage. HIV-1 antibodies have been prospectively detected using consecutive generations of commercially available screening assays (obtained from Abbott Laboratories, Abbott Park, Illinois, USA; Organon International, The Netherlands, and bioMérieux, France) and confirmed by Western blot analyses (Genelabs Diagnostics, Singapore). In addition, participants who seroconvert are questioned in detail on the assumed source of HIV-1 infection. In the present study, we included MSM who were HIV-1-negative at ACS entry and had at least two ACS visits between October 1984 and December 2009.
Baseline characteristics, including nationality (Dutch vs. non-Dutch), educational level (at least college degree vs. no college degree), and age at study entry were collected using standardized written questionnaires. Additionally, information about sexual behaviour and STI in the preceding 6 months was collected every 6 months using standardized self-administered questionnaires. Sexual behaviour variables included number of sexual partners, having had anal intercourse (both insertive and receptive) (yes/no), and condom use during anal intercourse, measured on a five-point scale ranging from always to never, all referring to the preceding 6 months. Since 1991, sexual behaviour variables have been asked separately for steady and casual partners. We defined unprotected anal intercourse (UAI) as not or not always having used a condom while practising (receptive or insertive) anal sex, again differentiating between UAI with steady or casual partners from 1991 onwards. STI variables included self-reported episodes of infectious syphilis and gonorrhoea.
Variables used to investigate the source of HIV-1 infection have been described in detail elsewhere . In short, data until 2000 about whether the source of HIV-1 infection was allocated to the steady or the casual partner were used from the study of Davidovich et al. in which both data from self-reported questionnaires and structured postseroconversion interviews were used. Data were updated, primarily using structured postseroconversion interviews as these were previously found to be the more valid source of information for two specific reasons: the interviews allowed the clarification of vague replies, and during the interviews participants sometimes acknowledged having had UAI with a particular partner and having withheld that information in the questionnaire. The interviews obtained information on UAI in the 6 months preceding the first positive test result, separately for steady and casual partners, also including the efficacy of condom use (condom breakage or slip-off). On the basis of this information, the source of infection was allocated to the steady or casual partner. All cases in which the source of infection could not be allocated, because unprotected sex was reported with both types of partners or no sex was reported with any type of partner, were excluded.
To investigate trends in HIV-1 incidence, the date of seroconversion was defined as the midpoint between the last HIV-1-negative and the first HIV-1-positive ACS visit. Individual follow-up time was calculated from the date of ACS entry until seroconversion, the end of HIV-negative follow-up, or end of study period (i.e. 31 December 2009). The observed HIV-1 incidence rate per calendar year among all MSM was calculated using person-time techniques. Additionally, incidence rate curves were modelled using Poisson regression with calendar year as a continuous variable allowing smoothly varying trends via restricted cubic splines . Age was included in the model as a continuous time-dependent covariate, again modelled using restricted cubic splines. We allowed an interaction between calendar year and age to investigate whether HIV incidence rate curves differed for various ages.
Trends in sexual risk behaviour (UAI, receptive UAI, insertive UAI) were studied among MSM practising anal intercourse by comparing five sequential time periods (1984–1987: no therapy; 1987–1992: mono-therapy; 1992–1996: pre-cART; 1996–2003: cART; 2003–2009: long-term cART) using a univariate logistic regression model via the generalized estimating equation (GEE) method, with an exchangeable covariance matrix to adjust for intra-individual correlations.
To identify risk factors for HIV-1 seroconversion, Poisson regression analysis was used. All variables subject to change were treated as time-dependent variables. HIV-1 status at every visit was linked to the risk behaviour reported at that visit, as HIV seroconversion is assumed to have occurred in the preceding 6 months. However, as it is possible that infection occurred earlier, an additional sensitivity analysis was conducted in which we assigned the risk behaviour reported at the last HIV-negative visit to seroconversion. For nonseroconverters we also extrapolated the behaviour from a preceding visit to the current visit. Any visit above 12 months following the previous visit was excluded from the risk factor analyses. Also excluded were seroconverters (n = 17) who had no behavioural data at their first positive visit nor at their last negative visit. For 58 seroconverters who had no behavioural data on the first positive visit, we extrapolated the behavioural data of the last negative visit to the first positive visit, assuming consistent behaviour over 1 year's time. We tested for differences between these 58 seroconverters and the other included seroconverters, using Mann–Whitney U test and chi-square test. We built two multivariate models: without a distinction between steady and casual partners (1984–2009), and with a distinction between steady and casual partners (1991–2009). All multivariate models included age, calendar time, and all variables on sexual risk behaviour. Changes in risk factors for acquiring HIV-1 infection over time were analysed by means of interactions between calendar time and the potential risk variables. In the final models, a P-value of 0.05 or less was considered to be statistically significant.
The source of HIV-1 infection was studied among seroconverters using logistic regression analyses, with the dichotomous outcome of steady vs. casual partner. The effects of calendar year, age, and their interaction were modelled using restricted cubic splines. Results were back-transformed into the proportion scale, yielding the proportion infected by steady partners as a function of calendar year and age. All statistical analyses were performed using the statistical packages SPSS version 15.0 (SPSS Inc., Chicago, Illinois, USA), STATA version 9.2 (STATA Corp., College Station, Texas, USA), and R version 2.9.0.
In total, 1642 MSM were HIV-1-negative at ACS entry and met our inclusion criterion of at least two study visits in the time period between October 1984 and December 2009 (Table 1). Their median age at study entry was 28.8 years [interquartile range (IQR) 24.8–35.9]; 81% was of Dutch nationality, and 55% had a college degree. The median number of visits was 15 (IQR 7–29); the median follow-up time was 6.2 years (IQR 2.3–11.5), and the median time between visits was 173 days (IQR 91–184).
Over 25 years, with a total of 11 223 person-years of follow-up, 217 of 1642 MSM seroconverted for HIV-1. The median age at seroconversion was 33.9 years (IQR 29.8–39.7), and the majority of the seroconverters (86.5%) were of Dutch nationality. Figure 1 shows the observed HIV-1 incidence rate per year for all MSM, and the fitted HIV-1 incidence rate per year for different ages. In the early years of the ACS, the observed incidence rate strongly decreased from 8.6/100 person-years [95% confidence interval (CI) 6.18–11.79] in 1985 to 1.3/100 person-years (95% CI 0.52–2.70) in 1992. It briefly remained relatively stable, ranging from 0.2/100 person-years (95% CI 0.01–1.12) in 1993 to 1.4/100 person-years (95% CI 0.64–2.63) in 1996. Since 1996, when cART became widely available, the trend in observed HIV-1 incidence shows a slight nonsignificant increase reaching 2.0/100 person-years (95% CI 1.00–3.75) in 2009. Interaction between age and calendar time was not statistically significant; thus the HIV-1 incidence rate curves were similar for MSM of different ages. Restricting the analysis to the period from 1996 onwards, the HIV-1 incidence rate slowly increased, although the effect was not statistically significant [with calendar year included as a linear covariate, the incidence rate ratio (IRR) was 1.04 per calendar year; 95% CI 0.99–1.09, P = 0.14].
Trends in sexual risk behaviour
Among HIV-1-negative MSM practising anal sex, the percentage of men practising UAI in the preceding 6 months decreased from 78% in 1984 to 33% in 1988. After that, it increased to 38% in 1995 and subsequently continued to increase, reaching 55% in 2009 (Fig. 2). MSM were more likely to have engaged in UAI in the time periods after the introduction of cART compared to the period just before (1992–1996), with odds ratios (ORs) being 1.4 (95% CI 1.16–1.56) and 1.5 (95% CI 1.33–1.79) (overall P < 0.01) for 1996–2003 and 2003–2009, respectively. Similar increasing trends from 1996 onwards were observed (all P < 0.01), for UAI with steady and casual partners (Fig. 2). Additionally, over the whole period studied, the proportion of UAI with steady partners was higher than the proportion of UAI with casual partners (respectively 60 vs. 26% in 2009). Moreover, HIV-1-negative MSM in the ACS had higher levels of insertive UAI than receptive UAI. This was especially true for casual partners (data not shown).
Risk factors for HIV-1 seroconversion
The population we studied to identify risk factors for HIV-1 seroconversion consisted of 1575 MSM. There were no significant differences between the 58 seroconverters and the other included seroconverters in any of the demographic variables. For the time period 1984–2009, with in total 9193 person-years of follow-up, multivariate analysis revealed that men who had a lower educational level (meaning no college degree) were twice more likely to become infected with HIV-1. Furthermore, men who reported more than five sexual partners (IRR 2.5, 95% CI 1.58–4.08), receptive UAI (IRR 4.1, 95% CI 2.37–6.96), and a history of gonorrhoea (IRR 5.8, 95% CI 2.49–13.71), all in the preceding 6 months, were also at increased risk for HIV-1 infection (Table 2).
In the time period 1991–2009 (Table 3), distinguishing between steady and casual partners, men who reported more than five casual partners in the preceding 6 months were almost twice more likely to become infected with HIV-1 than those with no casual partners, and men who reported receptive UAI with casual partners in the preceding 6 months were six times more likely to become infected compared with those reporting no or protected anal intercourse. A history of gonorrhoea in the preceding 6 months (IRR 6.0, 95% CI 2.28–15.66) was again associated with an increased risk for HIV-1 seroconversion.
In both models, there were no significant interactions between risk factors for acquiring HIV-1 infection and calendar year. Furthermore, sensitivity analyses that included behaviour reported at the last HIV-1-negative visit revealed the same direction effects of the risk factors, although gonorrhoea was no longer associated with seroconversion.
Reported source of HIV-1 infection
As the source of HIV-1 infection could not be attributed in 37 cases, this analysis included 180 of the 217 MSM who seroconverted. During the study period, for 74% (134) of the seroconverters, the most likely source of infection was allocated to a casual partner, and for 26% (46) it was allocated to the steady partner.
For younger seroconverters, the proportion infected by steady partners was less than 0.4 (95% CI 0.01–0.74) between 1985 and 1989. However, in the period 1990–2001, the chance to be infected by a steady partner increased, reaching the highest value of 0.8 (95% CI 0.39–0.96) in 1995 (Fig. 3a). From 2002 onwards, young MSM again had a small chance to be infected by a steady partner (Fig. 3a,b). For older seroconverters, the proportion infected by steady partners was less than 0.5 at the beginning of the epidemic, but it appeared to increase from 1995 to 2009 (Fig. 3c–e).
To our knowledge, this is the first study in recent years to document the HIV-1 incidence rate and risk factors for HIV-1 transmission among MSM in an observational cohort with 25 years of follow-up. In latest years the HIV-incidence remained relatively stable, although there is a suggestion toward a slight increase from 1.4/100 person-years in 1996 to 2.0/100 person-years in 2009. This finding is in line with previous findings of a slight, nonsignificant increase in HIV incidence among young ACS participants from 1999 through 2005 . As mentioned earlier, national and international studies have been showing increases in STIs and new diagnoses of HIV-1 infection among MSM. A study by Sullivan et al. showed that these reports of increasing HIV-1 infections are not fully explained by increases in HIV testing. All this, together with our findings, including the increase in sexual risk behaviour since 1996, provides evidence for ongoing substantial HIV-1 transmission among MSM. Interestingly, this accords with the outcomes of a recently published study that used mathematical models to estimate the HIV incidence among MSM in the Netherlands .
In contrast with the increasing trend in Amsterdam, a fairly comparable cohort study in Sydney, Australia, found that the HIV incidence rate among MSM decreased nonsignificantly from 1.7/100 person-years in 2002 to 0.4/100 person-years in 2006 . The difference between the Amsterdam and Sydney findings might be explained by different inclusion criteria and recruitment procedures used by the two cohorts. Additionally, it might be partly explained by differences in HIV testing rates. Mathematical models have shown that a higher testing rate can reduce the HIV incidence , because HIV-infected individuals can use cART and thereby decrease their infectiousness . In Sydney, about 95% of the MSM know their HIV status , whereas in the Netherlands lower awareness of the HIV status was observed: 88% in 2007 and much lower in the preceding years . The proportion of Dutch MSM being unaware of their HIV status might have contributed to the ongoing HIV-1 transmission in Amsterdam. Previously, we demonstrated that HIV testing uptake among MSM increases with use of an opt-out strategy . Increasing the testing rate by means of this strategy will increase the percentage of MSM aware of their HIV status, perhaps leading to a reduction in HIV-1 transmission. Furthermore, in view of the fact that HIV-1 is more transmissible during primary infection , frequent testing should be encouraged to detect acute infections, especially among MSM engaging in high-risk behaviour.
The most important risk factors for HIV-1 infection for the whole study period were receptive UAI (especially with casual partners), a larger number of sexual partners, and lower educational level. A history of gonorrhoea in the preceding 6 months was also associated with seroconversion of HIV-1. These risk factors were comparable with those found in the past [5,34,35] and more recently [15,36]. The fact that gonorrhoea reported at the last HIV-negative visit in sensitivity analysis was not related to HIV-1 seroconversion, whereas gonorrhoea reported at the first positive visit is, suggests that seroconversion occurred closer to the first positive visit.
We demonstrated that MSM contract HIV-1 infection mostly from casual partners. This finding together with the risk factor analyses confirms that HIV-1 transmission continues to be predominantly driven by casual partners. However, we found that in recent years a growing proportion of the infections among older MSM are acquired from steady partners. It could be that, in general, older MSM are more likely to have a steady partner who is also older and therefore has a higher probability of being HIV-infected. Therefore, even when sexual risk behaviour is equivalent to those of younger MSM, the probability of HIV exposure will be higher. It could also be that, in recent years, older MSM practiced high-risk behaviour more often with their steady partner than with casual partners or they simply had fewer casual partners than younger MSM. Another explanation is that older MSM and those in longer relationships often break negotiated safety rules (i.e. the negotiated practice of UAI within steady relationships of concordant HIV-negative serostatus) due to changes in sexual preferences, relationship context, and other reasons. However, since the sample size for analysing the source of HIV-1 infection was small, CIs are wide. Moreover, as analysis is based on self-report, phylogenetic analysis of the HIV strains of the source and index cases would have been the best option to investigate whether indeed older MSM today contract HIV-1 infection to a greater extent from steady partners than from casual partners. Unfortunately, this was not feasible in our study.
There were some study limitations. First of all, MSM participating in ACS tend to be strongly identified as homosexual and, as such, may not be representative of MSM in general. However, trends in sexual behaviour among Dutch MSM presented by the Schorer (Netherlands' institute for homosexuality, health and well being) are in line with our findings . Second, inclusion criteria (age at entry and number of sexual partners) have changed during the study period, which could have affected the presented trends in the study. However, results were all adjusted for age, and trends in HIV-1 incidence were similar for different ages. Additionally, to exclude a potential impact of the change in number of partners at study entry, we conducted a sensitivity analysis (data not shown), excluding MSM with only one partner at study entry, resulting in a comparable trend in HIV-1 incidence. Also, the behavioural trends are similar for those with steady partners and with casual partners, indicating that the rise in UAI is not only due to higher numbers of steady partners. So despite these limitations, the results of our study stress the need for ongoing monitoring and developing new and more effective interventions targeted at specific groups of MSM.
In conclusion, followed by increases in sexual risk behaviour from 1996 onwards, HIV-1 continues to spread among MSM. Receptive unprotected anal intercourse with casual partners remains the strongest risk factor for acquiring HIV-1 infection over time. Casual partners appear to remain the dominant source of HIV-1 infection, but trends suggest that with increasing age, a growing proportion of HIV-1 infection is acquired from steady partners. Therefore, in order to minimize further spread of HIV-1, we recommend targeted prevention messages for MSM. There is no doubt that prevention should continue to focus on their sexual behaviour with casual partners. However, our findings also suggest the need to pay specific attention to prevention measures regarding sexual behaviour with steady partners.
The authors would like to thank all the personnel of the ACS (currently Marc van Wijk and Marjolein Martens) for their contribution in data collection; Bart Maertzdorf and Anneke Krol for data management; and Lucy Philips for editing the final manuscript. Furthermore we gratefully acknowledge all the study participants for their co-operation and participation, which made this study possible.
Author contributions: I.J. analysed and interpreted the data, and wrote the draft manuscript; R.G. gave substantial contributions to the analyses and interpretation of the data. U.D., S.J., M.P., and R.C. contributed to the analyses and interpretation of data. I.S. designed and supervised the overall study, and contributed to the analyses and interpretation of data. All authors contributed to the final version of this manuscript.
Sponsorship: The Amsterdam Cohort Studies on HIV infection and AIDS, a collaboration between the Public Health Service of Amsterdam, the Academic Medical Center of the University of Amsterdam, Sanquin Blood Supply Foundation and the University Medical Center Utrecht, are part of the Netherlands HIV Monitoring Foundation and financially supported by the Netherlands National Institute for Public Health and the Environment.
1. van Griensven F, de Lind van Wijngaarden JW, Baral S, Grulich A. The global epidemic of HIV infection among men who have sex with men
. Curr Opin HIV AIDS 2009; 4:300–307.
2. Winkelstein W Jr, Samuel M, Padian NS, Wiley JA, Lang W, Anderson RE, et al
. The San Francisco Men's Health Study: III. Reduction in human immunodeficiency virus transmission among homosexual/bisexual men, 1982–86. Am J Public Health 1987; 77:685–689.
3. Winkelstein W Jr, Wiley JA, Padian NS, Samuel M, Shiboski S, Ascher MS, et al
. The San Francisco Men's Health Study: continued decline in HIV seroconversion rates among homosexual/bisexual men. Am J Public Health 1988; 78:1472–1474.
4. van Griensven GJ, de Vroome EM, Goudsmit J, Coutinho RA. Changes in sexual behaviour and the fall in incidence of HIV infection among homosexual men. BMJ 1989; 298:218–221.
5. Page-Shafer K, Veugelers PJ, Moss AR, Strathdee S, Kaldor JM, van Griensven GJ. Sexual risk behavior and risk factors for HIV-1 seroconversion in homosexual men participating in the Tricontinental Seroconverter Study, 1982–1994. Am J Epidemiol 1997; 146:531–542.
6. Palella FJ Jr, Delaney KM, Moorman AC, Loveless MO, Fuhrer J, Satten GA, et al
. Declining morbidity and mortality among patients with advanced human immunodeficiency virus infection. HIV Outpatient Study Investigators. N Engl J Med 1998; 338:853–860.
7. Detels R, Munoz A, McFarlane G, Kingsley LA, Margolick JB, Giorgi J, et al
. Effectiveness of potent antiretroviral therapy on time to AIDS and death in men with known HIV infection duration. Multicenter AIDS Cohort Study
Investigators. JAMA 1998; 280:1497–1503.
8. Chen SY, Gibson S, Katz MH, Klausner JD, Dilley JW, Schwarcz SK, et al
. Continuing increases in sexual risk behavior and sexually transmitted diseases among men who have sex with men
: San Francisco, Calif, 1999–2001, USA. Am J Public Health 2002; 92:1387–1388.
9. Katz MH, Schwarcz SK, Kellogg TA, Klausner JD, Dilley JW, Gibson S, et al
. Impact of highly active antiretroviral treatment on HIV seroincidence among men who have sex with men
: San Francisco. Am J Public Health 2002; 92:388–394.
10. Rietmeijer CA, Patnaik JL, Judson FN, Douglas JM Jr. Increases in gonorrhea and sexual risk behaviors among men who have sex with men
: a 12-year trend analysis at the Denver Metro Health Clinic. Sex Transm Dis 2003; 30:562–567.
11. Zablotska IB, Crawford J, Imrie J, Prestage G, Jin F, Grulich A, et al
. Increases in unprotected anal intercourse with serodiscordant casual partners among HIV-negative gay men in Sydney. AIDS Behav 2009; 13:638–644.
12. Stolte IG, Coutinho RA. Risk behaviour and sexually transmitted diseases are on the rise in gay men, but what is happening with HIV? Curr Opin Infect Dis 2002; 15:37–41.
13. Hospers HJ, Dörfler TT, Zuilhof W. Increase of young MSM with high sexual risk behaviour.Schorer Monitor
2008; 45–47 [Article in Dutch].
14. Stolte IG, Dukers NH, de Wit JB, Fennema JS, Coutinho RA. Increase in sexually transmitted infections among homosexual men in Amsterdam in relation to CART. Sex Transm Infect 2001; 77:184–186.
15. van der Bij AK, Stolte IG, Coutinho RA, Dukers NH. Increase of sexually transmitted infections, but not HIV, among young homosexual men in Amsterdam: are STIs still reliable markers for HIV transmission? Sex Transm Infect 2005; 81:34–37.
16. Dilley JW, Woods WJ, McFarland W. Are advances in treatment changing views about high-risk sex? N Engl J Med 1997; 337:501–502.
17. Stolte IG, Dukers NH, Geskus RB, Coutinho RA, de Wit JB. Homosexual men change to risky sex when perceiving less threat of HIV/AIDS since availability of highly active antiretroviral therapy: a longitudinal study. AIDS 2004; 18:303–309.
18. Jaffe HW, Valdiserri RO, De Cock KM. The reemerging HIV/AIDS epidemic in men who have sex with men
. JAMA 2007; 298:2412–2414.
19. Bezemer D, de Wolf F, Boerlijst MC, van Sighem A, Hollingsworth TD, Prins M, et al
. A resurgent HIV-1 epidemic among men who have sex with men
in the era of potent antiretroviral therapy. AIDS 2008; 22:1071–1077.
20. Defraye A, Crucitti T, Ducoffre G, Mak R, Sasse A. Incidence of Neisseria gonorrhoeae
infections in Belgium: trends
2000–2006. Rev Med Brux 2009; 30:93–98.
21. Rice B, Nardone A, Gill N, Delpech V. Continuing high levels of HIV diagnoses in men who have sex with men in the United Kingdom.Euro Surveill
22. Centers for Disease Control and Prevention. Trends in HIV/AIDS diagnosis among men who have sex with men 33 states, 2001–2006.MMWR Morb Mortal Wkly Rep
23. Sullivan PS, Hamouda O, Delpech V, Geduld JE, Prejean J, Semaille C, et al
. Reemergence of the HIV epidemic among men who have sex with men
in North America, Western Europe, and Australia, 1996–2005. Ann Epidemiol 2009; 19:423–431.
24. Dukers NH, Fennema HS, van der Snoek EM, Krol A, Geskus RB, Pospiech M, et al
. HIV incidence
and HIV testing behavior in men who have sex with men
: using three incidence sources, The Netherlands, 1984–2005. AIDS 2007; 21:491–499.
25. Scheer S, Kellogg T, Klausner JD, Schwarcz S, Colfax G, Bernstein K, et al
. HIV is hyperendemic among men who have sex with men
in San Francisco: 10-year trends
in HIV incidence
, HIV prevalence, sexually transmitted infections and sexual risk behaviour
. Sex Transm Infect 2008; 84:493–498.
26. Jin F, Prestage GP, McDonald A, Ramacciotti T, Imrie JC, Kippax SC, et al
. Trend in HIV incidence
in a cohort of homosexual men in Sydney: data from the Health in Men Study. Sex Health 2008; 5:109–112.
27. Davidovich U, de Wit J, Albrecht N, Geskus R, Stroebe W, Coutinho R. Increase in the share of steady partners as a source of HIV infection: a 17-year study of seroconversion among gay men. AIDS 2001; 15:1303–1308.
28. Durrleman S, Simon R. Flexible regression models with cubic splines. Stat Med 1989; 8:551–561.
29. Xiridou M, Geskus R, de Wit J, Coutinho R, Kretzschmar M. The contribution of steady and casual partnerships to the incidence of HIV infection among homosexual men in Amsterdam. AIDS 2003; 17:1029–1038.
30. Quinn TC, Wawer MJ, Sewankambo N, Serwadda D, Li C, Wabwire-Mangen F, et al
. Viral load and heterosexual transmission of human immunodeficiency virus type 1. Rakai Project Group. N Engl J Med 2000; 342:921–929.
31. Jin FY, Prestage G, Law MG, Kippax S, van de Ven, Rawsthorne P, et al.Predictors of recent HIV testing in homosexual men in Australia
. HIV Med
32. Heijman RL, Stolte IG, Thiesbrummel HF, van Leent E, Coutinho RA, Fennema JS, et al
. Opting out increases HIV testing in a large sexually transmitted infections outpatient clinic. Sex Transm Infect 2009; 85:249–255.
33. Hollingsworth TD, Anderson RM, Fraser C. HIV-1 transmission, by stage of infection. J Infect Dis 2008; 198:687–693.
34. van Griensven GJ, Tielman RA, Goudsmit J, van der Noordaa J, de Wolf F, de Vroome EM, et al
. Risk factors and prevalence of HIV antibodies in homosexual men in the Netherlands. Am J Epidemiol 1987; 125:1048–1057.
35. Kuiken CL, van Griensven GJ, de Vroome EM, Coutinho RA. Risk factors and changes in sexual behavior in male homosexuals who seroconverted for human immunodeficiency virus antibodies. Am J Epidemiol 1990; 132:523–530.
36. Li A, Varangrat A, Wimonsate W, Chemnasiri T, Sinthuwattanawibool C, Phanuphak P, et al
. Sexual behavior and risk factors for HIV infection among homosexual and bisexual men in Thailand. AIDS Behav 2009; 13:318–327.