This study describes trends in HIV incidence, injecting drug use and sexual risk behaviour among DU of the ACS from 1986–2011. The major findings are declining trends in HIV incidence, injecting and sexual risk behaviour. In addition, STI screening performed among participants of the ACS with a study visit in 2010–2011 demonstrates a low STI prevalence. Although prevalence of unprotected sex is substantial, it is associated with having sex with a steady partner and, of interest, such prevalence is less in HIV-infected participants.
The decreasing trend in HIV incidence presented here is in line with other longitudinal studies and surveillance systems on drug using populations in high-income countries 
. However, many areas of the world report an increasing HIV-incidence rate among DU 
. This epidemiologic discrepancy could be a result of inequalities in access to harm-reduction programmes and treatment services 
. Coverage of HIV treatment and prevention services is highest in Western Europe, reaching 61% of the injecting DU 
. As one of the first countries in Western Europe, the Netherlands initiated harm-reduction programmes in the 1980s 
. The declining trend in the use of needle exchange, as observed in the ACS, was confirmed by a reduction in the absolute number of exchanged needles per calendar year in Amsterdam, which peaked in 1992 with 1,100,000 needles, whereas since 2007 about 150,000 needles per year were exchanged. A study to evaluate the effect of needle exchange programmes and opiate substitution therapy on HIV incidence among DU of the ACS found that the combination of these approaches was associated with a lower risk for acquiring HIV and hepatitis C infection 
. Interestingly, phylogenetic analysis indicated that before 2002, 37 out of 47 cases who acquired HIV in the ACS were infected by subtype B virus strains specific for DUs, whereas after 2002 all four new HIV infections were unspecific for DUs. This might relate to the change in injecting risk behaviour 
In addition to the effect of harm-reduction programmes on reducing transmission through needles, injecting drug use seems to be out of fashion in the Netherlands 
. According to data on young DU (aged 18–30) in Amsterdam, the proportion of individuals reporting a history of injection was 88% between 1985 and 1989 and declined to 31% between 2000 and 2004 
. On a broader level, new injecting DU constitute less than 10% of all injecting DU in 10 European countries 
. The Netherlands appears to have the lowest rate of initiation of injecting among DU (2.1/100 PY) in Europe 
Another explanation for the declining trends in risk behaviour could be the aging population of the ACS. American studies support the finding that DU older than 50 years inject drugs 
or share needles 
less often than younger users. In addition, when comparing sexual risk behaviours among older and younger DU, older DU were less likely to have had sex in the past month 
. Moreover, our analysis of the recent visitors revealed that a large number of participants reported zero sexual partners in the past 6 months. However, aging DU that do have sex still engage in high-risk sexual practices, such as inconsistent condom use 
. Selective loss to follow-up of high-risk participants could be another reason for the observed declines in risk behaviour over time. We demonstrated in a sensitivity analysis, however, that there were comparable declines among participants with a visit in 2010 as compared to the total DU population in the ACS. In addition, our findings are in line with national surveillance programmes showing that diagnoses of HIV, acute HBV and HCV infection are rarely reported in DU 
A previous study of the ACS in DU described that sources of HIV transmission changed from mainly related to injecting risk behaviour before 1996 to mainly related to unprotected sex after 1996 
. This change is of importance not only for DU populations but for others as well, since DU have the potential to serve as a bridge for sexual HIV transmission to the wider community 
. Of interest, in contrast to observations among men who have sex with men 
, no increase in sexual risk behaviour was found among HIV-infected DU of the ACS who initiated cART 
In a previous study among DU of the ACS between 1985 and 2005, we found a decline in HIV incidence and injecting, but not in sexual risk behaviour 
. However, our data suggest a gradually decreasing proportion of any unprotected sex since 2004, accompanied by a low STI prevalence. Still, the prevalence of unprotected sex is substantial, but our results among the recent visitors demonstrate that unprotected sex is mainly done with a steady partner and is less common in HIV-infected participants. Furthermore, the recent visitors of the ACS show a low STI prevalence (2.5%), all diagnosed with CT. A CT screening in 2008 among young people (aged 15–29 years old) living in Amsterdam found a CT prevalence of 3.6% 
. Data from drug treatment centres and other cohort studies from the United Kingdom and the United States all showed higher prevalences of CT and NG 
. This comparison of prevalences suggests that there is a low transmission rate of STI among DU in Amsterdam. These findings may indicate that there is no major risk for sexual HIV transmission among DU.
Due to the extension of the European Union, sex trafficking has become easier. A recent study among CSW who had migrated from eastern Europe to London found higher prevalences (although not significant) of HIV, CT, NG and syphilis in CSW from eastern Europe as compared to CSW from the United Kingdom 
. In contrast, these migrants less commonly reported a history of drug use.
The current study has several limitations. First, our results can not be generalised to younger DU and those followed in regions with no or limited access to comprehensive harm-reduction programmes. Second, data on drugs and sexual risk behaviour were self-reported. Consequently, data could be influenced by socially desirable answers and therefore may underestimate true risk behaviour. However, STI screening and self-reported STI showed comparable prevalences, which suggests accuracy in reporting STI history, which has also been described before 
. Third, to increase uptake for the STI screening among recent visitors we chose to use self-swabs. Unfortunately these self-swabs could not be analysed for TV. Other studies reported high prevalence of trichomoniasis among female DU, varying from 8.6% to 43% 
. We were only able to test for TV in the 7 high-risk participants. Furthermore, participants at high risk for STI and those who reported clinical symptoms were referred to the outpatient STI clinic of the Public Health Service of Amsterdam where more extensive testing occurred (including for TV).
Fourth, to confirm our findings regarding the low prevalence of STIs, the STI screening should be repeated and more data on STI prevalence among DU from outside our cohort is needed.
To conclude, we documented a continuing very low HIV-incidence rate accompanied by a low injecting risk behaviour among DU of the ACS. Prevalence of unprotected sex was substantial, but was mainly associated with having a steady partner and was less common in HIV-infected participants. Taken together with a low STI prevalence among the recent visitors, our findings indicate a low transmission risk of HIV and STI. These results suggest that DU no longer play a significant role in the spread of HIV in Amsterdam.