Why is syphilis rising in Europe? Multi-level modelling of alternative hypotheses in 31 countries

Background Syphilis transmission has increased markedly over the past two decades in Europe, concentrated in men who have sex with men. We test alternative potential social and behavioral individual- and population-level determinants of this resurgence. Methods Two rounds of the cross-sectional European Men who have sex with men Internet Survey (EMIS 2010 and 2017, n = 272,902) were used to fit multi-level linear probability models to evaluate determinants of the incidence of self-reported syphilis, capturing risky sexual behaviours and pre-exposure prophylaxis use, among others, adjusting for potential sociodemographic confounders. Results Self-reported syphilis incidence rates rose by about 1.8 percentage points (within the last 12 months) and 3.9 (within the last 5 years) between the 2010 and 2017 waves, after adjusting for sociodemographic factors. HIV status was a major risk factor for syphilis infection (27.6 ppt higher incident rate, 95%CI: 24.7 to 30.5). A dose-response relationship was observed between greater numbers of condomless non-steady partners and syphilis infection, with more than 10 partners estimating increases in the probability of diagnosis of over 25 ppt (11-20 partners vs none: 24.5 ppt, 95%CI: 20.5 to 28.5); further, we observed evidence of mediation for number of condomless non-steady partners, which attenuated the estimated rise in 2017 vs 2010 by about 35%. STI testing uptake also accounted for a substantial increase in syphilis incidence signaling higher detection rates over time. While country-level PrEP use was linked to greater number of condomless partners, there was no substantial impact of population-wide factors, including GDP and PrEP use, on overall syphilis trends. Conclusions Risky sexual behavior changes, particularly condomless sex with non-steady partners, appears to be a major contributing factor to rising syphilis incidence. Further research is needed to understand what accounts for this substantial behavior change