Doxycycline post-exposure prophylaxis may dramatically reduce syphilis among gay, bisexual, and other men who have sex with men: a modeling study
Patrick A. Clay, David A. Jackson, Laura H. Bachmann, Ian H. SpicknallBackground:
Syphilis cases in the United States have surged over the past decade, and cases are highest among gay, bisexual, and other men who have sex with men (GBMSM). The CDC recommends doxycycline post-exposure prophylaxis (doxy PEP) for GBMSM recently diagnosed with bacterial STIs to mitigate this burden. Potential drawbacks include increased antimicrobial resistance and microbiome disruption. Here, we estimate doxy PEP coverage levels required to eliminate sustained community transmission among GBMSM while balancing the drawbacks of doxy PEP.
Methods:
We fit a modified susceptible-infectious-susceptible model of syphilis transmission among GBMSM to syphilis case counts by sex-of-sex-partner. We simulated a range of doxy PEP coverage and adherence levels among GBMSM linked to HIV treatment and prevention over a ten-year period. We tracked syphilis prevalence and incident cases among GBMSM with and without healthcare access.
Results:
With a lower bound efficacy estimate (41%) and 10% doxy PEP utilization (coverage × adherence) among GBMSM linked to HIV treatment and prevention, we estimated a 24% [Interquartile Range: 17%–30%] reduction in annual syphilis cases among GBMSM over the next 10 years. Under upper bound efficacy estimates (97%) and 50% utilization, we estimated a 99% [IQR: 88%–100%] reduction in annual syphilis cases among GBMSM over the next 10 years. Our model shows that if sub-populations of GBMSM have reduced access to healthcare, then the elimination of sustained community transmission is unlikely over the next decade.
Conclusions:
Doxy PEP may dramatically reduce syphilis incidence among GBMSM, but barriers to healthcare may impede the elimination of sustained community transmission of syphilis.