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David Wilson: Modelling The Impact of Targeted Syphilis Interventions
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David Wilson: Modelling The Impact of Targeted Syphilis Interventions

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This presentation discusses the relationship between risk behaviours for syphilis and interventions targeting at-risk groups. This presentation was given at AFAO's syphilis forum in May 2009.

This presentation discusses the relationship between risk behaviours for syphilis and interventions targeting at-risk groups. This presentation was given at AFAO's syphilis forum in May 2009.

Published in Health & Medicine
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  • 1. Modelling the impact of targeted interventions on syphilis epidemics A/Prof David Wilson Richard Gray, Alex Hoare, Garrett Prestage, Basil Donovan, John Kaldor
  • 2. The Anderson-May equation
    • Equation for STI epidemics and control
    • Average number of secondary infections caused per infected person
  • 3. The Anderson-May equation
    • Consider a sexually active population
    Susceptible Infected
  • 4. The Anderson-May equation
    • Mathematically:
    Susceptible Infected Recover from infection after ‘D’ years Rate of becoming infected depends on the number of contacts (partnerships) ‘c’ and probability of transmission per partnership ‘ β ’
  • 5. Cartoon example >1 (epidemic growing) Susceptible Infected ‘ D’ years c β
  • 6. Cartoon example <1 (epidemic declining) Susceptible Infected ‘ D’ years c β
  • 7. Important factors for STI control
    • Contact rate
    • Partner acquisition
    • Sexual decision making
    • Abstinence
    • Monogamy
    • Duration of infectiousness
    • Screening
    • Timely diagnosis
    • Effective treatment
    • Transmission probability per partnership
    • Biology (host and parasite)
    • Minimise exposure
    • Frequency of sex and type of sex
    • Condoms, microbicides
    • Suppressive treatment
    • PEP
    • Non-specific (general STI), qualitative factors of importance
    • Not advocating, but listing possibilities
    • Changes in behaviour and clinical practice (screening) can change the course of epidemics
  • 8. Controlling specific epidemics
    • Requires greater understanding of the actual sexual behaviour, epidemiology, biology of the organism, and clinical practice in the population of interest
    Susceptible Infected Syphilis natural history Basic STI
  • 9. Syphilis Model
    • Individual-based model that simulates sexual partnerships and syphilis transmission in MSM populations
    Partnership network Transmission tracking Disease progression
  • 10. Model Calibration
    • Model calibrated to available behavioural and incidence data and accurately reflects epidemiological data
    Victorian Epidemic
  • 11. Interventions targeting all MSM Increasing testing coverage (at same frequency) has minimal impact currently 55-70%
  • 12. Interventions targeting all MSM Increasing testing frequency (at same coverage) can have substantial impact (goal: every 3 months)
  • 13. Interventions targeting all MSM Expected notifications Syphilis prevalence Synchronized (“blitz”) testing can only result in a noticeable reduction in incidence and prevalence if it occurs at least twice per year
  • 14. Contact tracing
  • 15. Interventions Targeting at Risk Groups
    • HIV + MSM already test relatively frequently. No substantial impact in targeting them further
    • Unsurprisingly, targeting MSM who engage in group sex and other men with high sexual activity (> 10 partners per year) could lead to significant reductions in syphilis if testing frequency increased
    • If testing of men of lower activity (< 10 partners per year) also occurs then the additional benefits are very modest; i.e. not effective (or cost-effective)
  • 16. Efficiency of interventions are highly variable
    • Contact tracing is highly efficient and should be done wherever possible
      • E.g. Tracing & testing 75% of regular and 5% of casual partners leads to a number needed to treat to prevent one infection (NNT) of ~36
    • Targeting MSM of high activity (>10 partners) is efficient
      • NNT ≈ 50-60 (for twice or four times per year)
    • Synchronised testing twice a year is moderately efficient (NNT ≈ 50) if twice per year
    • Testing all MSM (including low activity men) is not efficient (NNT ≈ 150)
  • 17. Model Predictions and Conclusions
    • Changes in behaviour and/or testing/treatment rates required to mitigate the epidemic.
    • Targeting ‘high-activity’ MSM (>10 partners per year) + MSM who engage in group sex can be highly effective.
    • Synchronising testing has additional modest benefits.
    • Increasing the average frequency of testing per MSM is predicted to be the only effective way to substantially control the current syphilis epidemic.
      • Every 3 months appears to be a theoretical target
    • Investigating other (feasible) scenarios