1. www.aids2014.org Does HIV serosorting increase syphilis prevalence among men who have sex with men in Western Europe? - A mathematical modelling study - A. Lucht 1, M. Shah 2, M. Kretzschmar 3,4 1 Labor Krone, Siemensstraße 40, D-32105 Bad Salzuflen, Germany 2 Johns Hopkins University, Division of Infectious Diseases, Baltimore, MD, US 3 University Medical Centre Utrecht, Julius Centre for Health Sciences & Primary Care, Utrecht, Netherlands, 4 Centre for Infectious Diseases Control, RIVM, Bilthoven, Netherlands Abstract no. TUPDC0106

2. www.aids2014.org Background Serosorting definition: –“restriction of sexual partnerships to those perceived to be of the same HIV serostatus”. Serosorting is sometimes used as an HIV risk reduction strategy among men who have sex with men (MSM). Some studies suggest serosorting may reduce HIV transmission However, serosorting may increase transmission of other STIs such as syphilis Syphilis prevalence is high and increasing among MSM in several Western European countries

3. www.aids2014.org Study Aims and Methods Primary Study Aim: –To assess the impact of serosorting on syphilis prevalence among MSM in Western Europe (UK, Netherlands, and Germany) using a deterministic S-I-R-S mathematical model Secondary outcomes: –To examine the impact of HIV prevalence and sexual risk behaviors on the impact of the serosorting effect Differential Equations describing rates of each compartment Legend: S-susceptible, I = infectious, R = resistant.

4. www.aids2014.org Results and Conclusions CONCLUSIONS HIV serosorting increases syphilis prevalence among MSM compared to scenarios without serosorting –Effect is more pronounced in HIV-positive compared to HIV-negative –Even small proportions of serosorting among MSM leads to increases in syphilis prevalence The size of the serosorting effect on syphilis prevalence is increased by: –higher HIV prevalence in the MSM population, –higher numbers of sexual partnerships of HIV-positive and HIV-negative MSM, –Increased probability of syphilis transmission per partnership (e.g. without condom usage) RESULTS: Effect of serosorting on syphilis prevalence among HIV-positive MSM Base-Case syphilis prevalence: Base-case Serosorting 4-12% HIV- 7- 37% HIV+ HIV+: 3.7-4.0% HIV-: 1.0-2.0% (range is across UK, Germany, Netherlands)