Review

Extracting key information from historical data to quantify the transmission dynamics of smallpox

Hiroshi Nishiura¹ , Stefan O Brockmann^2,4 and Martin Eichner³

¹  Theoretical Epidemiology, University of Utrecht, Yalelaan 7, 3584CL, Utrecht, The Netherlands

²  Department of Epidemiology and Health Reporting, Baden-Württemberg State Health Office, Nordbahnhofstr. 135, D-70191 Stuttgart, Germany

³  Department of Medical Biometry, University of Tübingen, Westbahnhofstr. 55, D-72070 Tübingen, Germany

⁴  Department of Infectious Disease Epidemiology, Robert Koch Institute, Seestr. 10, D-13353 Berlin, Germany

author email corresponding author email

Theoretical Biology and Medical Modelling 2008, 5:20doi:10.1186/1742-4682-5-20

Published:	20 August 2008

Abstract

Background

Quantification of the transmission dynamics of smallpox is crucial for optimizing intervention strategies in the event of a bioterrorist attack. This article reviews basic methods and findings in mathematical and statistical studies of smallpox which estimate key transmission parameters from historical data.

Main findings

First, critically important aspects in extracting key information from historical data are briefly summarized. We mention different sources of heterogeneity and potential pitfalls in utilizing historical records. Second, we discuss how smallpox spreads in the absence of interventions and how the optimal timing of quarantine and isolation measures can be determined. Case studies demonstrate the following. (1) The upper confidence limit of the 99th percentile of the incubation period is 22.2 days, suggesting that quarantine should last 23 days. (2) The highest frequency (61.8%) of secondary transmissions occurs 3–5 days after onset of fever so that infected individuals should be isolated before the appearance of rash. (3) The U-shaped age-specific case fatality implies a vulnerability of infants and elderly among non-immune individuals. Estimates of the transmission potential are subsequently reviewed, followed by an assessment of vaccination effects and of the expected effectiveness of interventions.

Conclusion

Current debates on bio-terrorism preparedness indicate that public health decision making must account for the complex interplay and balance between vaccination strategies and other public health measures (e.g. case isolation and contact tracing) taking into account the frequency of adverse events to vaccination. In this review, we summarize what has already been clarified and point out needs to analyze previous smallpox outbreaks systematically.