Syphilis Modelling


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Outcomes of Mathematical Modelling for the National Gay Men’s Syphilis Action Plan. Presentation given by David Wilson at the AFAO National Syphilis Forum, 23 October 2009.

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Syphilis Modelling

  1. 1. Outcomes of Mathematical Modelling for the National Gay Men’s Syphilis Action Plan Associate Professor David Wilson National Centre in HIV Epidemiology and Clinical Research Sydney, Australia [email_address]
  2. 2. Objective of this study <ul><li>To quantify the expected epidemiological impact of a large number of potential interventions </li></ul><ul><li>What will work and what will not work? </li></ul><ul><li>Interventions considered </li></ul><ul><ul><li>Behaviour change </li></ul></ul><ul><ul><ul><li>Increase condom use </li></ul></ul></ul><ul><ul><ul><li>Reduce partner acquisition </li></ul></ul></ul><ul><ul><li>Chemoprophylaxis </li></ul></ul><ul><ul><li>Targeted screening and treatment </li></ul></ul><ul><ul><li>Mass treatment </li></ul></ul><ul><ul><li>Contact tracing/partner notification </li></ul></ul>
  3. 3. Syphilis model <ul><li>Individual-based model that tracks sexual partnerships and syphilis transmission in MSM populations </li></ul>Partnership network Transmission tracking Disease progression
  4. 4. Included HIV infection and possible syphilis coinfection Uninfected Primary HIV Primary HIV Chronic HIV Chronic HIV AIDS / Late stage infection AIDS / Late stage infection 1 st Line Treatment Treatment Failure 2 nd Line Treatment Undiagnosed Infection Diagnosis
  5. 5. Sexual behaviour <ul><li>Number of casual sexual partners </li></ul><ul><li>34% of men with >5 partners per month have groups sex regularly </li></ul><ul><li>Men who engage in group sex have an average of 3.5 groups sex events per year </li></ul><ul><li>Average of 4 men per encounter </li></ul>
  6. 6. Disclosing HIV status <ul><li>80-90% among regular partners </li></ul><ul><li>Disclosing HIV status is common in casual partners </li></ul>
  7. 7. ‘ Strategic positioning’ among gay men <ul><li>Disclosure leads to strategic role positioning </li></ul>Data from HIM Study
  8. 8. Sexual behaviour within partnerships <ul><li>Model simulates anal and oral sex (average of 2 events per year in regular partnerships) </li></ul><ul><li>Casual encounters have an average of 1 oral sex act and 0.7 anal sex acts </li></ul><ul><li>Disclosure of HIV status to regular or casual partners </li></ul><ul><li>Condom use </li></ul><ul><ul><li>80->69% in discordant HIV disclosed partnerships </li></ul></ul><ul><ul><li>10% in concordant HIV disclosed partnerships </li></ul></ul><ul><ul><li>40% where HIV is not disclosed </li></ul></ul><ul><ul><li>None during oral sex </li></ul></ul><ul><li>10% of HIV-infected men disclose for serosorting </li></ul>
  9. 9. Syphilis testing <ul><li>Coverage and frequency </li></ul><ul><ul><li>High activity: 55% 1-2 times per year </li></ul></ul><ul><ul><li>Group sex: 65% once per year </li></ul></ul><ul><ul><li>On ART: 70% twice per year </li></ul></ul><ul><ul><li>Others: 55% once per year </li></ul></ul><ul><li>Test sensitivity 95% </li></ul><ul><li>15% of the population have never tested for syphilis or HIV </li></ul><ul><li>10% HIV prevalence </li></ul>
  10. 10. Simulating the sexual network
  11. 11. Reproducing the past epidemic Syphilis HIV Prevalence of syphilis in HIV+ and HIV- men in agreement with available data
  12. 12. Forecasting the future <ul><li>Current conditions </li></ul><ul><li>Impact of interventions: run simulations with new parameter values, incorporating the effect of intervention </li></ul>
  13. 13. Increased condom use Partnership status Currently Intervention HIV Concordant 10% 10% Unknown 40% 80% / 100% HIV Discordant ~69% 80% / 100%
  14. 14. Increased condom use Intervention for one month, three months, or indefinitely Long term
  15. 15. Reduction in partner numbers 25% decrease 50% decrease
  16. 16. Mass treatment Rollout 3 months > 10 partners Rollout 1 month > 25 partners Rollout 1 month >10 partners
  17. 17. Increasing testing coverage <ul><li>Current coverage </li></ul><ul><ul><li>Highly active men: 55% (1-2 times per year) </li></ul></ul><ul><ul><li>Engage in group sex: 65% </li></ul></ul><ul><ul><li>HIV+ on ART: 60% </li></ul></ul><ul><ul><li>Others 55% </li></ul></ul>Prevalence Notifications
  18. 18. Increasing testing frequency <ul><li>Increase testing frequency at baseline coverage </li></ul>Prevalence Notifications
  19. 19. Targeted screening at population groups
  20. 20. Targeting risk behaviour groups (increased frequency) 2 tests per year 4 tests per year
  21. 21. Blitz testing <ul><li>Testing over a short period </li></ul><ul><li>Similar to mass treatment </li></ul>
  22. 22. Partner notification
  23. 23. Follow-up testing
  24. 24. Efficiency of interventions Intervention Infections averted (10 years) Total tests (10 years) Tests per infection averted 85% coverage 6524 83004 13 Frequency 2 tests/yr 16466 239369 15 Frequency 4 tests/yr 20506 548767 27 Diagnosed HIV+ 6 tests/yr 6678 84535 13 >10 partners, 2 tests/yr 12865 83929 7 >20 partners, 2 tests/yr 12254 45329 4 >50 partners, 2 tests/yr 3003 8539 3 Follow up 75% every 3 months 2385 5036 2 Contact tracing 75% regular, 5% casual 8903 6199 0.7
  25. 25. Acknowledgements <ul><li>Modellers </li></ul><ul><ul><li>Richard Gray </li></ul></ul><ul><ul><li>Alex Hoare </li></ul></ul><ul><li>Advisory role and data provision </li></ul><ul><ul><li>Garrett Prestage </li></ul></ul><ul><ul><li>Basil Donovan </li></ul></ul><ul><ul><li>Andrew Grulich </li></ul></ul><ul><ul><li>Jeff Jin </li></ul></ul><ul><ul><li>Technical working group </li></ul></ul>The National Centre in HIV Epidemiology and Clinical Research is funded by the Australian Government Department of Health and Ageing and is affiliated with the Faculty of Medicine, The University of New South Wales.