Assessing the drivers of syphilis among men who have sex with men in Switzerland reveals a key impact of screening frequency: A modelling study

Abstract

Over the last decade, syphilis diagnoses among men-who-have-sex-with-men (MSM) have strongly increased in Europe. Understanding the drivers of the ongoing epidemic may aid to curb transmissions. In order to identify the drivers of syphilis transmission in MSM in Switzerland between 2006 and 2017 as well as the effect of potential interventions, we set up an epidemiological model stratified by syphilis stage, HIV-diagnosis, and behavioral factors to account for syphilis infectiousness and risk for transmission. In the main model, we used ‘reported non-steady partners’ (nsP) as the main proxy for sexual risk. We parameterized the model using data from the Swiss HIV Cohort Study, Swiss Voluntary Counselling and Testing center, cross-sectional surveys among the Swiss MSM population, and published syphilis notifications from the Federal Office of Public Health. The main model reproduced the increase in syphilis diagnoses from 168 cases in 2006 to 418 cases in 2017. It estimated that between 2006 and 2017, MSM with HIV diagnosis had 45.9 times the median syphilis incidence of MSM without HIV diagnosis. Defining risk as condomless anal intercourse with nsP decreased model accuracy (sum of squared weighted residuals, 378.8 vs. 148.3). Counterfactual scenarios suggested that increasing screening of MSM without HIV diagnosis and with nsP from once every two years to twice per year may reduce syphilis incidence (at most 12.8% reduction by 2017). Whereas, increasing screening among MSM with HIV diagnosis and with nsP from once per year to twice per year may substantially reduce syphilis incidence over time (at least 63.5% reduction by 2017). The model suggests that reporting nsP regardless of condom use is suitable for risk stratification when modelling syphilis transmission. More frequent screening of MSM with HIV diagnosis, particularly those with nsP may aid to curb syphilis transmission.

Triggers of Change in Sexual Behavior Among People With HIV: The Swiss U = U Statement and COVID-19 Compared

“…Unlike condom use among MSM and HET, which decreased steadily over the study period, syphilis incidence stabilized in 2014 and decreased in COVID-19 times when frequency of occasional partners also decreased. While claiming causation in this case is out of the scope of this manuscript, this observation is consistent with previous studies that underscored occasional sexual partnership regardless of condom use a key risk factor for syphilis transmission [13,14].…”

Risk Factors and Incidence of Sexually Transmitted Infections in the Swiss HIV Cohort Study

“…Several reports described increasing trends in STIs incidence among PWH [ 3 , 5 ]. For example, in the Swiss HIV Cohort Study (SHCS), syphilis incidence increased from 30.1 to 59.2 per 1000 patient-years between 2006 and 2017 [ 5 , 6 ].…”

“…Improved screening methods, intensified testing campaigns, accessibility to point-of-care testing, but also increased transmission rates in at-risk populations contribute to the rise in diagnosed STIs [ 7 ]. Among PWH, increases in condomless sex and a greater number of sexual partners, as well as increased testing frequencies are associated with higher rates of STIs diagnoses [ 5 , 6 , 8–10 ]. There is an interplay between human immunodeficiency virus (HIV) and STIs: HIV-associated ulcerative lesions can increase the acquisition of STIs and on the other hand STIs promote local inflammation in the genital tract facilitating HIV shedding and increasing HIV-risk transmission [ 11 ].…”

Social and behavioural determinants of syphilis: Modelling based on repeated cross-sectional surveys from 2010 and 2017 among 278,256 men who have sex with men in 31 European countries

“…Some countries, therefore, explicitly recommend biannual syphilis-screening in multi-partners MSM. 30 , 31 …”