Assessing the drivers of syphilis among men who have sex with men in Switzerland reveals a key impact of screening frequency: A modelling study. PLoS Computational Biology
Balakrishna et al. aimed to identify the drivers of syphilis transmission in MSM in Switzerland between 2006 and 2017 as well as the effect of potential interventions such as increased screening rates for syphilis.
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 increased from 168 cases in 2006 to 418 cases in 2017. In this study, they used a mathematical model to assess the drivers of this increase among MSM in Switzerland stratified by syphilis stage, HIV-diagnosis, and behavioral factors to account for syphilis infectiousness and risk for transmission. They 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.
Their model could reproduce the increase in syphilis diagnoses in both MSM with and without HIV diagnosis between 2006 and 2017. Based on the model, they found that MSM with HIV diagnosis have an over 45 times higher syphilis incidence than MSM without HIV diagnosis. Furthermore, they found that reported sex with non-steady partners is a useful proxy of behavioral risk. Considering counterfactual scenarios, they 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%.
In conclusion, their model suggests that reporting non-steady partners regardless of condom use is suitable for risk stratification when modelling syphilis transmission. More frequent screening of MSM with HIV diagnosis, particularly those with non-steady partners may aid to curb syphilis transmission.