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Pertussis modelling
Contributions of natural, vaccine immunity to epidemiology
A/Prof Jodie McVernon, Head, Modelling & Simulation Unit,Melbourne School of Population & Global Health, The University of Melbourneand Murdoch Childrens Research Institute
Pertussis: biology, epidemiology and prevention
Les Pensieres, Fondation Merieux Conference Centre, 11-13 November 2015
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Overview
Pertussis resurgence – Australia as a case study
Modelling Australian pertussis trends
Understanding the past, predicting the future
Conclusions, context and next steps
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Pertussis resurgence - background
Resurgence of pertussis (whooping cough) observed
recently in a number of developed countries
Initial uncertainty re: ascertainment bias
Associated infant deaths in US, UK indicate true
increase
How might vaccination have contributed to rising
disease?
Waning immunity, loss of boosting
Lower effectiveness of new vaccines
Changes in bacterial population
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Australia as a case study – in context
Jackson & Rohani, Epidemiol Infect 2014
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Use of pertussis vaccines in Australia
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Campbell et al PLoS ONE 2012
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Pertussis resurgence in Australia
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Campbell et al PLoS ONE
2012
aP
PSB
aP
infant
Swap toddler for
adolescent
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Australian pertussis seroepidemiology
8
Campbell et al PLoS ONE
2012
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Modelling Australian pertussis trends
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Vaccine/natural immunity interaction
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Campbell et al Vaccine 2015
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Pertussis model attributes
Compartmental - susceptible, infected, recovered
Age-structured – relates to disease risk, vaccine timing
Dynamic – prevalence drives incidence in susceptibles
Model of infection, as opposed to symptoms or disease
Model states are associated with pertussis AB levels: used as a marker of infection, vaccination and waning
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Model simulation and selection
Model parameters varied simultaneously Plausible parameter
distributions based on local data, literature review
200,000 parameter combinations tested
Qualitative match to 3 criteria and large epidemic post removing toddler dose
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2–5 year
epidemic
cycles
Highly
seropositive
adults
Large reduction in
infant incidence
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Selected simulations (n=2,321)
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a, d – infection decreases
with increasing coverage
b – infection increases
with decreasing coverage
e – infection increases
when toddler dose removed
*Naïve and low immune infections Campbell et al Vaccine 2015
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Understanding the past
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Natural immunity duration is far longer than either vaccine
Coverage fluctuations open up opportunity for outbreaks
Cycles of immunity and waning post outbreak have implications decades later
Schedule change has opened up susceptible ‘pockets’ to fuel the most recent epidemic natural whole cell acellular
010
20
30
40
50
60
70
80
90
10
0
immunity type
yea
rs
Campbell et al Vaccine 2015
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Looking backwards and forwards
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Campbell et al Vaccine 2015
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Impact of schedule changes
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2003-2013 2014-2020
Campbell et al Vaccine 2015*Best strategy: 43% reduction toddlers, 8% reduction infants
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Conclusions
Multiple factors have contributed to pertussis resurgence in
Australia and elsewhere
Ongoing transmission of pertussis seems inevitable
A six-dose vaccine schedule (reintroducing the toddler booster)
is expected to have some impact on transmission, and hence
disease
Additional mitigation strategies are needed to protect infants at
highest risk of severe outcomes
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Context and next steps
Other groups have used models to consider the drivers of
pertussis resurgence in their own context
Yoon Choi, Public Health England, UK
Manoj Gambhir, Centers for Disease Control, US
WHO-supported model comparison exercise underway to
assess model applicability to other data and settings
Development of models of maternal immunisation approaches
in household structured populations
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Acknowledgements
University of Melbourne
Trish Campbell
James McCaw
Terry Nolan
John Mathews
UNSW
James Wood
19
NCIRS
Peter McIntyre
Helen Quinn
Rob Menzies
Lyn Gilbert
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Thank
you!
Any questions?