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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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Modelling the impact of targeted interventions on syphilis epidemics
A/Prof David Wilson
Richard Gray, Alex Hoare, Garrett Prestage, Basil Donovan, John Kaldor
The Anderson-May equationEquation for STI epidemics and control
Average number of secondary infections caused per
infected person
0r cD
The Anderson-May equationConsider a sexually active population
Susceptible Infected
The Anderson-May equation
Mathematically:
Susceptible Infected
Recover from infection after ‘D’ years
dS 1
dtd 1
dt
ISc IS I D
I ISc IS I D
Rate of becoming infected depends on the number of contacts (partnerships) ‘c’ and probability of transmission per partnership ‘β’
Cartoon example
Susceptible Infected
‘D’ years
cβ
0 3r cD >1 (epidemic growing)
Susceptible Infected
‘D’ years
cβ
0 0.5r cD
Cartoon example
<1 (epidemic declining)
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
Important factors for STI control
0r cD
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
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 historyBasic STI
Syphilis ModelIndividual-based model that simulates sexual partnerships
and syphilis transmission in MSM populations
Partnership network
Transmission tracking
Disease progression
Model Calibration
Model calibrated to available behavioural and incidence
data and accurately reflects epidemiological data
Victorian Epidemic
Interventions targeting all MSM
Increasing testing coverage (at same frequency) has minimal impact
currently 55-70%
Interventions targeting all MSM
Increasing testing frequency (at same coverage) can have substantial impact (goal: every 3 months)
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
Contact tracing
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)
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)
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
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