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

Author summary Syphilis, one of the most common sexually transmitted infections, remains a major public health problem. Over the last decade, a rising number of diagnoses especially in men-who-have-sex-with-men (MSM) was observed in Western Europe and Northern America. In Switzerland, the number of syphilis diagnoses in MSM tripled between 2006 and 2017. In this study, we used a mathematical model to assess the drivers of this increase among MSM in Switzerland. Our model could reproduce the increase in syphilis diagnoses in both MSM with and without HIV diagnosis between 2006 and 2017. Based on this model we found that MSM with HIV diagnosis have an over 45 times higher syphilis incidence than MSM without HIV diagnosis. Furthermore, we found that reported sex with non-steady partners is a useful proxy of behavioral risk. Considering counterfactual scenarios, we showed that increasing the screening frequency for syphilis among MSM with HIV diagnosis and with non-steady partners from once a year to twice per year can reduce syphilis incidence by 63.5% to 99.2%. 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