Research article
Logistics of community smallpox control through contact tracing and ring vaccination: a stochastic network model
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* Corresponding author: Tomás J Aragón aragon@berkeley.edu
1 San Francisco Department of Public Health, Community Health and Epidemiology Section, Epidemiology and Effectiveness Research Unit, 101 Grove Street Suite 204, San Francisco, California 94102 USA
2 Center for Infectious Disease Preparedness, School of Public Health, University of California, Berkeley, USA
3 Surveillance and Epidemiology Section, Tuberculosis Control Branch, Division of Communicable Disease Control, California Department of Health Services, Berkeley, California, USA
BMC Public Health 2004, 4:34 doi:10.1186/1471-2458-4-34
Published: 6 August 2004Abstract
Background
Previous smallpox ring vaccination models based on contact tracing over a network suggest that ring vaccination would be effective, but have not explicitly included response logistics and limited numbers of vaccinators.
Methods
We developed a continuous-time stochastic simulation of smallpox transmission, including network structure, post-exposure vaccination, vaccination of contacts of contacts, limited response capacity, heterogeneity in symptoms and infectiousness, vaccination prior to the discontinuation of routine vaccination, more rapid diagnosis due to public awareness, surveillance of asymptomatic contacts, and isolation of cases.
Results
We found that even in cases of very rapidly spreading smallpox, ring vaccination (when coupled with surveillance) is sufficient in most cases to eliminate smallpox quickly, assuming that 95% of household contacts are traced, 80% of workplace or social contacts are traced, and no casual contacts are traced, and that in most cases the ability to trace 1–5 individuals per day per index case is sufficient. If smallpox is assumed to be transmitted very quickly to contacts, it may at times escape containment by ring vaccination, but could be controlled in these circumstances by mass vaccination.
Conclusions
Small introductions of smallpox are likely to be easily contained by ring vaccination, provided contact tracing is feasible. Uncertainties in the nature of bioterrorist smallpox (infectiousness, vaccine efficacy) support continued planning for ring vaccination as well as mass vaccination. If initiated, ring vaccination should be conducted without delays in vaccination, should include contacts of contacts (whenever there is sufficient capacity) and should be accompanied by increased public awareness and surveillance.