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IJE Advance Access originally published online on December 3, 2007
International Journal of Epidemiology 2008 37(1):106-112; doi:10.1093/ije/dym240
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Published by Oxford University Press on behalf of the International Epidemiological Association © The Author 2007; all rights reserved.

An exploratory method for estimating the changing speed of epidemic waves from historical data

Andrew D Cliff1, Peter Haggett2,* and Matthew Smallman-Raynor3

1Department of Geography, University of Cambridge, Cambridge CB2 3EN, UK.
2Institute for Advanced Studies, University of Bristol, BS8 1UR, Bristol, UK.
3School of Geography, University of Nottingham, Nottingham NG7 2RD, UK.

*Corresponding author. Institute for Advanced Studies, University of Bristol, Royal Fort House, Bristol BS8 1UR, UK. E-mail: p.haggett{at}bristol.ac.uk


  Abstract

Background: Historical data are necessary to establish long-term trends in disease incidence but pose analytical problems since their accuracy and reliability may be poorly specified.

Methods: A robust measure of the spatial velocity, R0A, of epidemic waves from space-time series is proposed using binary data. The method was applied to the historical records of influenza morbidity for the island of Iceland over a 61-year period of influenza seasons from 1915–16 to 1975–76.

Results: The onset of influenza waves tended to speed up over the period studied and the three pandemic waves associated with viral shifts in influenza A [Spanish influenza H1N1 (1918–19), Asian influenza H2N2 (1957–58) and Hong Kong influenza H3N2 (1968–69)] spread more rapidly around the island and struck earlier in the influenza season than did inter-pandemic waves, even when the latter were equally intensive as measured by total number of cases and case incidence.

Discussion: The potential for using R0A in a real-time context is explored using French influenza data.

Conclusions: The new measure of wave velocity appears to be applicable to those historical time series where breakdown into regional or local areas is available. The study is being extended to (i) other countries where similar influenza time series are available and (ii) to other diseases within Iceland.

Keywords Time series, exploratory data analysis, epidemic velocity, influenza, pandemics, Iceland

Accepted 24 October 2007


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