Countering the rise of syphilis: A role for doxycycline post-exposure prophylaxis?

Doxycycline post-exposure prophylaxis (PEP) holds the potential to mitigate increasing rates of syphilis among sexual minority men (SMM) in the US yet has received limited attention. Since evaluation of this intervention in actual populations is not currently feasible, we used agent-based models (ABM) to assess the population-level impact of this strategy. We adapted ABM of HIV and HPV transmission, representing a population of 10,230 SMM in Philadelphia, Pennsylvania, US. Parameter inputs were derived from the literature, and ABM outputs during the pre-intervention period were calibrated to local surveillance data. Intervention scenarios varied doxycycline uptake by 20, 40, 60, 80 and 100%, while assuming continued condom use and syphilis screening and treatment. Under each intervention scenario, we incorporated treatment adherence at the following levels: 0, 20, 40, 60, 80 and 100%. Long-term population impact of prophylactic doxycycline was measured using the cumulative incidence over the 10-year period and the percentage of infections prevented attributable to doxycycline at year 10. An uptake scenario of 20% with an adherence level of 80% would reduce the cumulative incidence of infections by 10% over the next decade, translating to 57 fewer cases per 1000 SMM. At year 10, under the same uptake and adherence level, 22% of infections would be prevented due to doxycycline PEP in the instances where condoms were not used or failed. Findings suggest that doxycycline PEP will have a modest impact on syphilis incidence when assuming a reasonable level of uptake and adherence. Doxycycline PEP may be most appropriate as a secondary prevention measure to condoms and enhanced syphilis screening for reducing infections among SMM.