title: Go big or go home: impact of screening coverage on syphilis infection dynamics.
abstract: OBJECTIVE
  Syphilis outbreaks in urban men who have sex with men (MSM) are an ongoing public health challenge in many high-income countries, despite intensification of efforts to screen and treat at-risk individuals. We sought to understand how population-level coverage of asymptomatic screening impacts the ability to control syphilis transmission.
  
  
  METHODS
  We developed a risk-structured deterministic compartmental mathematical model of syphilis transmission in a population of sexually active MSM. We assumed a baseline level of treatment of syphilis cases due to seeking medical care in all scenarios. We evaluated the impact of sustained annual population-wide screening coverage ranging from 0% to 90% on syphilis incidence over the short term (20 years) and at endemic equilibrium.
  
  
  RESULTS
  The relationship between screening coverage and equilibrium syphilis incidence displayed an inverted U-shape relationship, with peak equilibrium incidence occurring with 20-30% annual screening coverage. Annual screening of 62% of the population was required for local elimination (incidence <1 case per 100 000 population). Results were qualitatively similar in the face of differing programmatic, behavioural and natural history assumptions, although the screening thresholds for local elimination differed. With 6-monthly or 3-monthly screening, the population coverage required to achieve local elimination was reduced to 39% or 23%, respectively.
  
  
  CONCLUSIONS
  Although screening has the potential to control syphilis outbreaks, suboptimal coverage may paradoxically lead to a higher equilibrium infection incidence than that observed in the absence of intervention. Suboptimal screening programme design should be considered as a possible contributor to unsuccessful syphilis control programmes in the context of the current epidemic.
authors.affiliation: Institute of Medical Science and Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Ashleigh
authors.initials: A
authors.lastname: Tuite
authors.affiliation: Institute of Medical Science and Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada.
authors.forename: David
authors.initials: D
authors.lastname: Fisman
mesh_terms: D000328:Adult; D000553:Ambulatory Care / Q000706:statistics & numerical data*; D000900:Anti-Bacterial Agents / Q000008:administration & dosage*; D002170:Canada / Q000453:epidemiology; D016358:Contact Tracing / Q000706:statistics & numerical data; D003362:Cost-Benefit Analysis; D018451:Homosexuality, Male; D006801:Humans; D015994:Incidence; D008297:Male; D008403:Mass Screening / Q000458:organization & administration*; D008962:Models, Theoretical; D010342:Patient Acceptance of Health Care; D016730:Program Development; D012725:Sexual Behavior / Q000706:statistics & numerical data*; D012747:Sexual Partners; D013587:Syphilis / Q000635:transmission*
chemical_list: D000900:Anti-Bacterial Agents
keywords: CONTROL PROGRAMS; MATHEMATICAL MODEL; MODELING; SCREENING; SYPHILIS