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.

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David Wilson: Modelling The Impact of Targeted Syphilis Interventions

  1. 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. 2. The Anderson-May equation <ul><li>Equation for STI epidemics and control </li></ul><ul><li>Average number of secondary infections caused per infected person </li></ul>
  3. 3. The Anderson-May equation <ul><li>Consider a sexually active population </li></ul>Susceptible Infected
  4. 4. The Anderson-May equation <ul><li>Mathematically: </li></ul>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. 5. Cartoon example >1 (epidemic growing) Susceptible Infected ‘ D’ years c β
  6. 6. Cartoon example <1 (epidemic declining) Susceptible Infected ‘ D’ years c β
  7. 7. Important factors for STI control <ul><li>Contact rate </li></ul><ul><li>Partner acquisition </li></ul><ul><li>Sexual decision making </li></ul><ul><li>Abstinence </li></ul><ul><li>Monogamy </li></ul><ul><li>Duration of infectiousness </li></ul><ul><li>Screening </li></ul><ul><li>Timely diagnosis </li></ul><ul><li>Effective treatment </li></ul><ul><li>Transmission probability per partnership </li></ul><ul><li>Biology (host and parasite) </li></ul><ul><li>Minimise exposure </li></ul><ul><li>Frequency of sex and type of sex </li></ul><ul><li>Condoms, microbicides </li></ul><ul><li>Suppressive treatment </li></ul><ul><li>PEP </li></ul><ul><li>Non-specific (general STI), qualitative factors of importance </li></ul><ul><li>Not advocating, but listing possibilities </li></ul><ul><li>Changes in behaviour and clinical practice (screening) can change the course of epidemics </li></ul>
  8. 8. Controlling specific epidemics <ul><li>Requires greater understanding of the actual sexual behaviour, epidemiology, biology of the organism, and clinical practice in the population of interest </li></ul>Susceptible Infected Syphilis natural history Basic STI
  9. 9. Syphilis Model <ul><li>Individual-based model that simulates sexual partnerships and syphilis transmission in MSM populations </li></ul>Partnership network Transmission tracking Disease progression
  10. 10. Model Calibration <ul><li>Model calibrated to available behavioural and incidence data and accurately reflects epidemiological data </li></ul>Victorian Epidemic
  11. 11. Interventions targeting all MSM Increasing testing coverage (at same frequency) has minimal impact currently 55-70%
  12. 12. Interventions targeting all MSM Increasing testing frequency (at same coverage) can have substantial impact (goal: every 3 months)
  13. 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. 14. Contact tracing
  15. 15. Interventions Targeting at Risk Groups <ul><li>HIV + MSM already test relatively frequently. No substantial impact in targeting them further </li></ul><ul><li>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 </li></ul><ul><li>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) </li></ul>
  16. 16. Efficiency of interventions are highly variable <ul><li>Contact tracing is highly efficient and should be done wherever possible </li></ul><ul><ul><li>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 </li></ul></ul><ul><li>Targeting MSM of high activity (>10 partners) is efficient </li></ul><ul><ul><li>NNT ≈ 50-60 (for twice or four times per year) </li></ul></ul><ul><li>Synchronised testing twice a year is moderately efficient (NNT ≈ 50) if twice per year </li></ul><ul><li>Testing all MSM (including low activity men) is not efficient (NNT ≈ 150) </li></ul>
  17. 17. Model Predictions and Conclusions <ul><li>Changes in behaviour and/or testing/treatment rates required to mitigate the epidemic. </li></ul><ul><li>Targeting ‘high-activity’ MSM (>10 partners per year) + MSM who engage in group sex can be highly effective. </li></ul><ul><li>Synchronising testing has additional modest benefits. </li></ul><ul><li>Increasing the average frequency of testing per MSM is predicted to be the only effective way to substantially control the current syphilis epidemic. </li></ul><ul><ul><li>Every 3 months appears to be a theoretical target </li></ul></ul><ul><li>Investigating other (feasible) scenarios </li></ul>