< Supplementary ﬁgures and appendix are published online only. To view these ﬁles please visit the journal online (http://sti. bmj.com). 1Department of Infectious Disease Epidemiology, Imperial College London, UK 2HIV Monitoring Foundation, Amsterdam, Netherlands 3Academic Medical Centre, University of Amsterdam, The Netherlands Correspondence to Dr Timothy B Hallett, Department of Infectious Disease Epidemiology, Imperial College London, St Mary’s Campus, Norfolk Place, London W2 1PG, UK; timothy.hallett@imperial.ac.uk TBH and CS contributed equally to this paper. Accepted 20 May 2010 Published Online First 18 July 2010 This paper is freely available online under the BMJ Journals unlocked scheme, see http://sti. bmj.com/site/about/unlocked. xhtml Sex Transm Infect 2011;87:17e21. doi:10.1136/sti.2010.042622 Epidemiology Estimating the risk of HIV transmission from homosexual men receiving treatment to their HIV-uninfected partners Timothy B Hallett,1 Colette Smit,2 Geoff P Garnett,1 Frank de Wolf1,2,3 ABSTRACT Objective To determine how the risk of HIV transmission from homosexual men receiving antiretroviral treatment is related to patterns of patient monitoring and condom use. Methods A stochastic mathematical simulation model was developed of cohorts of men in the Netherlands who have sex with men (MSM), deﬁning the parameters of the model using observational cohort data. The model incorporates viral load trends during ﬁrst-line treatment, patient monitoring and different scenarios for the way in which condom use may depend on recent viral load measurements. The model does not include the effect of sexually transmitted infections on HIV transmission. Results For MSM receiving treatment, the risk of transmitting HIV to their long-term partner is 22% (uncertainty interval: 9e37%) if condoms are never used. With incomplete use (in 30% of sex acts) the risk is reduced slightly, to 17% (7e29%). However, the risk is as low as 3% (0.2e8%) when men receiving treatment use condoms only 6 months beyond their last undetectable viral load measurement. The risk is further reduced when 3 months is the time period beyond which condoms are used. Conclusions When condom use by HIV-infected men receiving combination treatment with antiretroviral agents is based on their last viral load measurement, the transmission risk is much lower than with incomplete condom use. The key message for patients is that although always using condoms during treatment is the best way to protect partners from the risk of HIV transmission, when such use cannot be achieved, the second best strategy is to use condoms whenever the last undetectable viral load was measured more than 3 months ago. INTRODUCTION Since the mid-1990s, people infected with HIV in industrialised countries have been able to access combination antiretroviral treatment (cART),1 2 which has substantially decreased HIV-associated mortality in these populations.3 More recently, this trend has been repeated in many developing coun- tries, along with signs of decreasing HIV mortality.4 In both settings, there is a trend for earlier initiation of treatment5 and increasing debate about whether starting treatment earlier (or even immediately on HIV diagnosis) is advisable in order to improve clinical outcomes and reduce HIV transmission.6 7 As the population receiving treatment grows its potential contribution to HIV epidemic increases. Therefore, we expect that the infectiousness and the sexual behaviour of those receiving treatment will become key determinants of the trajectory of the global epidemic in the coming years. The rate of HIV transmission from infected people is closely tied to their plasma viral load, which cART can reduce to very low levels.8 On this evidence, in 2008 the Swiss National AIDS Commission (EKAF) suggested that there was effectively no risk of sexual transmission from patients receiving cART, provided that they (i) follow the guidelines for antiretroviral treatment strictly, (ii) have no detectable viral load when measured in the past 6 months and (iii) have no other sexually transmitted infection (STI).9 10 The implication that such patients need not use condoms with their sexual partners was warmly welcomed by patient groups and activists.11 In response, many argued that this was not a helpful public health message. Although the chance of transmission from men receiving treat- ment is likely to be low, it is unlikely to be zero, since at least one transmission event from a man receiving successful cART has occurred,12 and over many sex acts among many men, a small rate of transmission could translate into a large number of new infections.13 14 A simple model was used to argue this point,13 but it did not estimate the absolute risk of transmission from men receiving treatment; nor did it account for condom use being conditional on a recent viral load measurement, as the EKAF statement recommended.9 In this paper, using uniquely detailed data on the trends in viral loads among men receiving treat- ment17 and a new stochastic mathematical model, we estimate the risk of HIV transmission from men in the Netherlands who have sex with men (MSM) and determine how this is inﬂuenced by various patterns of condom use (including those based on the Swiss recommendations) and schedules of viral load monitoring. METHODS Following earlier work,18 19 we developed a stochastic individual-based simulation model of viral load trends, HIV transmission risk and patient monitoring among cohorts of MSM in the Netherlands. The vast range of possibilities for the evolution of viral load over time was conceptualised as three distinct trajectories (ﬁgure 1). Data from the ATHENA observational cohort of HIV-infected individuals in the Netherlands17 were used to evaluate the parameters (table 1, with further detail provided in the online technical appendix). The risk of transmission is characterised as the probability that a man receiving treatment will 17