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