National Centre in HIV Epidemiology and Clinical Research, The University of New South Wales, Sydney, Australia.
Received 2 December, 2009
Accepted 19 January, 2010
Serosorting is an increasingly common practice among gay men whereby the selection of sexual partners is based on concordance of HIV serostatus [1–3]. Serosorting occurs in order to facilitate unprotected anal intercourse without risk of HIV transmission. Evidence suggests that disclosure of HIV serostatus is increasingly influential not only on the formation of casual partnerships among gay men (serosorting), but also on the sexual position that each man is likely to take in a coital act (known as strategic positioning) and the likelihood of using a condom (known as negotiated safety) [3–9]. Although these inter-related strategies appear to be sophisticated measures for reducing risk, in some circumstances they may lead to an increased risk of HIV transmission in a population [10,11]. Serosorting, when coupled with actual positioning and condom behavior, may result in an increased risk of HIV acquisition if a moderate proportion of the HIV-infected population is undiagnosed. This undiagnosed proportion may lead to the formation of partnerships that are thought to be concordant but are actually discordant. In partnerships that are thought to be concordant, HIV-negative men are more likely to take a receptive role (which is more risky in a discordant partnership) and less likely to use condoms.
Cassels et al.  developed a detailed compartmental mathematical transmission model in order to demonstrate that ‘under realistic scenarios of sexual behavior and testing for men who have sex with men (MSM) in the USA, serosorting can be an effective harm reduction strategy’ . However, they failed to consider how strategic positioning and negotiated safety behaviors change with or without disclosure of HIV status and therefore their assertions about the benefits of serosorting are overstated. Additionally, they assumed that all men test for HIV 1–2 times per year. This is much greater than the best available data on actual testing rates in the USA . It is estimated that between 25% [14–16] and 48%  of all HIV infections in the USA are undiagnosed, which implies that testing rates are considerably lower than 1–2 times per year for all men. If Cassel's et al.  used more conservative testing rates in line with rates that are consistent with levels of undiagnosed infections then based on their model-derived Figure 4, serosorting would have been shown to result in higher equilibrium prevalence. The authors also state that, with serosorting, 22% of contacts are apparently serodiscordant, and 50% without. With a prevalence of 16%, their model would require partner acquisition rates among HIV-positive men to be much greater than among HIV-negative men. Lastly, we question the relevance of using equilibrium prevalence (that is, no epidemiological or behavioral change) to gain insight into the effect of a recent phenomenon on an epidemic where there is increasing incidence.
Using more realistic assumptions, the model of Cassel et al.  would yield very different conclusions, such that serosorting is not likely to reduce risk of HIV acquisition among gay men in the USA but in fact is likely to increase risk in most contexts. Serosorting, strategic positioning and negotiated safety cannot be decoupled. It has been demonstrated that accounting for behavior change is integral because the risk reduction strategies adopted by negative men vary with the level of knowledge of their partner's serostatus [11,18]. There is insufficient evidence to measure the true effectiveness of serosorting as a harm-reduction strategy. Modeling can be useful to inform our understanding when realistic behavioral inputs are used. The benefit of serosorting is highly context specific and depends upon the frequency of testing, accurate disclosure of serostatus and behavior. Our position, based on this study and others in the literature, is that serosorting has a real potential to increase risk and should not be promoted as a public-health strategy.
1. Suarez T, Miller J. Negotiating risks in context: a perspective on unprotected anal intercourse and barebacking among men who have sex with men: where do we go from here? Arch Sex Behav 2001; 30:287–300.
2. Suarez TP, Kelly JA, Pinkerton SD, et al
. Influence of a partner's HIV serostatus, use of highly active antiretroviral therapy, and viral load on perceptions of sexual risk behavior in a community sample of men who have sex with men. J Acquir Immune Defic Syndr 2001; 28:471–477.
3. Mao L, Crawford JM, Hospers HJ, et al
. ‘Serosorting’ in casual anal sex of HIV-negative gay men is noteworthy and is increasing in Sydney, Australia. AIDS 2006; 20:1204–1206.
4. Elford J, Bolding G, Sherr L, Hart G. High-risk sexual behaviour among London gay men: no longer increasing. AIDS 2005; 19:2171–2174.
5. Elford J, Ibrahim F, Bukutu C, Anderson J. Sexual behaviour of people living with HIV in London: implications for HIV transmission. AIDS 2007; 21(Suppl 1):S63–S70.
6. Truong HM, Kellogg T, Klausner JD, et al
. Increases in sexually transmitted infections and sexual risk behaviour without a concurrent increase in HIV incidence among men who have sex with men in San Francisco: a suggestion of HIV serosorting? Sex Transm Infect 2006; 82:461–466.
7. Golden MR, Wood RW, Buskin SE, Fleming M, Harrington RD. Ongoing risk behavior among persons with HIV in medical care. AIDS Behav 2007; 11:726–735.
8. Elford J. Changing patterns of sexual behaviour in the era of highly active antiretroviral therapy. Curr Opin Infect Dis 2006; 19:26–32.
9. Xia Q, Molitor F, Osmond DH, et al
. Knowledge of sexual partner's HIV serostatus and serosorting practices in a California population-based sample of men who have sex with men. AIDS 2006; 20:2081–2089.
10. Butler DM, Smith DM. Serosorting can potentially increase HIV transmissions. AIDS 2007; 21:1218–1220.
11. Wilson DP, Regan DG, Heymer KJ, Jin F, Prestage G, Grulich A. Serosorting may increase the risk of HIV acquisition among men who have sex with men
. Sex Transm Dis
12. Cassels S, Menza TW, Goodreau SM, Golden MR. HIV serosorting as a harm reduction strategy: evidence from Seattle, Washington. AIDS 2009; 23:2497–2506.
13. Brewer DD, Golden MR, Handsfield HH. Unsafe sexual behavior and correlates of risk in a probability sample of men who have sex with men in the era of highly active antiretroviral therapy. Sex Transm Dis 2006; 33:250–255.
14. Branson B. Current HIV epidemiology and revised recommendations for HIV testing in health-care settings. J Med Virol 2007; 79(Suppl 1):S6–S10.
15. Branson BM, Handsfield HH, Lampe MA, et al
. Revised recommendations for HIV testing of adults, adolescents, and pregnant women in health-care settings. MMWR Recomm Rep 2006; 55:1–17, quiz CE1-4.
16. Stekler JD, Golden MR. Learning from the missed opportunities for HIV testing. Sex Transm Infect 2009; 85:2–3.
17. Glynn MK, Rhodes P. Estimated HIV prevalence in the United States at the end of 2003
. In: 2005 National HIV Prevention Conference
; 14 June 2005. p. 205.
18. Jin F, Crawford J, Prestage GP, et al
. Unprotected anal intercourse, risk reduction behaviours, and subsequent HIV infection in a cohort of homosexual men. AIDS 2009; 23:243–252.