National Institute
for Public Health
and the Environment
Targeting infection control to
maximize the impact of a limited
stockpile
Jacco Wallinga
Pandemic influenza
• previous influenza pandemics
- 1918 (H1N1 Spanish influenza), excess mortality 0.28 %
- 1957(H2N2 Asian influenza), excess mortality 0.02 %
- 1968 (H3N2 Hong Kong influenza), excess mortality 0.0 %
• governments are preparing for a new influenza pandemic
• projected number of available doses of vaccines will be
far smaller than the projected number of infections
National Institute
for Public Health
and the Environment
[email protected]
Purpose of stockpiling antiviral drugs
and components of pandemic vaccines
• prevent collapse of health care and of society
• reduce mortality and morbidity
National Institute
for Public Health
and the Environment
[email protected]
Costs and effects
• what are the costs of maintaining a stockpile?
• what are the expected effects?
- reduction of infections, cases, hospitalizations, deaths
National Institute
for Public Health
and the Environment
[email protected]
Problem of quantifying expected effects
• when dealing with a new emerging infection:
- limited knowledge of epidemiological key variables
• when available infection control measures are scarce
- control measures have to be allocated
• pro rata
• to key persons in society
• to reduce further transmission
• to minimize some measure of harm
• do the effects depend on which groups are targeted?
National Institute
for Public Health
and the Environment
[email protected]
Main purpose of our work
• develop a simple prediction rule to quantify expected
effects
- based on observable epidemiological characteristics
- based on allocation schedule
• develop a simple prediction rule to predict an optimal
allocation schedule that reduces further transmission
National Institute
for Public Health
and the Environment
[email protected]
Testing our predictions
• transmission model
• partition the population into 2 groups (children, adults)
• parameters are chosen to reflect an infection like pandemic influenza
• everyone is susceptible when infection is introduced
• find optimal allocation for this model
• using extensive simulations
• simulate data
• force of infection in each group, from day 7 to day 13 after introduction
• predict expected effects of intervention and optimal allocation
• using simulated data
• compare actual and predicted values
National Institute
for Public Health
and the Environment
[email protected]
Results
• change in reproductive number
R upon vaccinating one
individual among the children
(left) or adults (right)
•prediction, using simulated data for risk of
infection in initial phase of epidemic
•actual value, using complete information
on all model parameters
National Institute
for Public Health
and the Environment
[email protected]
• change in epidemic after vaccination, with
a stockpile that suffices to vaccinate 20%
of the population
What this means
• with high accuracy we can predict the expected change
in further spread of infection
- without knowing the precise epidemiological characteristics
of the infection
- if we have observed the group-specific force of infection
National Institute
for Public Health
and the Environment
[email protected]
Practicalities
• is it feasible to detect significant differences in risk of
infection during an emerging epidemic?
• yes: in a previous pandemic (1957 Asian influenza) there
were significant and large differences in force of infection
in the initial phase of the first wave
National Institute
for Public Health
and the Environment
[email protected]
What this means
• the observed large difference in force of infection implies
a large difference in the expected effect of targeting
specific groups
• on the positive side, if informed targeting is used, this can
be used to maximize the impact of a scarce resource
• on the negative side, if resources are shared pro-rata or
worse, the precious stockpile of scarce resources is
partly wasted
National Institute
for Public Health
and the Environment
[email protected]
Implications for epidemic surveillance
programs
• surveillance programs for emerging epidemics should
focus on monitoring incidence and force of infection over
regular short time intervals
• real-time estimates of incidence and force of infection
should be rapidly communicated and distributed
• existing plans for allocating scarce resources should be
adjusted if designated target groups experience a low
incidence and force of infection
National Institute
for Public Health
and the Environment
[email protected]
Implications for stockpiling
• the expected effects of a stockpile are highly dependent
on how the stockpiled resources are allocated when it is
needed
National Institute
for Public Health
and the Environment
[email protected]
National Institute
for Public Health
and the Environment
[email protected]