Quantifying the transmission potential of pandemic
influenza
This article reviews quantitative methods to estimate the basic reproduction
number of pandemic influenza, a key threshold quantity to help determine the
intensity of interventions required to control the disease. Although it is difficult
to assess the transmission potential of a probable future pandemic, historical epidemiologic
data is readily available from previous pandemics, and as a reference
quantity for future pandemic planning, mathematical and statistical analyses of
historical data are crucial. In particular, because many historical records tend
to document only the temporal distribution of cases or deaths (i.e. epidemic
curve), our review focuses on methods to maximize the utility of time-evolution
data and to clarify the detailed mechanisms of the spread of influenza.
First, we highlight structured epidemic models and their parameter estimation
method which can quantify the detailed disease dynamics including those
we cannot observe directly. Duration-structured epidemic systems are subsequently
presented, offering firm understanding of the definition of the basic and
effective reproduction numbers. When the initial growth phase of an epidemic is
investigated, the distribution of the generation time is key statistical information
to appropriately estimate the transmission potential using the intrinsic growth
rate. Applications of stochastic processes are also highlighted to estimate the
transmission potential using similar data. Critically important characteristics
of influenza data are subsequently summarized, followed by our conclusions to
suggest potential future methodological improvements.
The current vaccination strategy targeted at people at highest risk of severe disease outcome is suboptimal because current vaccines are poorly immunogenic in these population groups. Our results suggest that interrupting transmission of seasonal influenza would require a relatively high vaccination coverage (>70%) in healthy individuals who respond well to vaccine, in addition to periodic re-vaccination due to evolving viral antigens and waning population immunity.
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