Despite eradication1, smallpox still presents a risk to public health whilst laboratory stocks of virus remain2,3. One factor crucial to any assessment of this risk is R0, the average number of secondary cases infected by each primary case. However, recently applied estimates have varied too widely (R0 from 1.5 to >20) to be of practical use, and often appear to disregard contingent factors such as socio-economic conditions and herd immunity4,5,6,7,8. Here we use epidemic modelling9 to show a more consistent derivation of R0. In isolated pre-twentieth century populations10,11,12 with negligible herd immunity, the numbers of cases initially rose exponentially, with an R0 between 3.5 and 6. Before outbreak controls were applied, smallpox also demonstrated similar levels of transmission in 30 sporadic outbreaks in twentieth century Europe1, taking into account pre-existing vaccination levels13,14 (about 50%) and the role of hospitals in doubling early transmission. Should smallpox recur, such estimates of transmission potential (R0 from 3.5 to 6) predict a reasonably rapid epidemic rise before the implementation of public health interventions, because little residual herd immunity exists now that vaccination has ceased.
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This work was funded by the Department of Health, UK. The views expressed in the publication are those of the authors and not necessarily those of the Department of Health. We thank C. Penn and G. Lloyd for their help with this work and the preparation of the manuscript, and D. Jones, S. Duncan, N. Gay, and members of the DH Steering Group for their comments and help with model parameterization.
The authors declare no competing financial interests.
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Gani, R., Leach, S. Transmission potential of smallpox in contemporary populations. Nature 414, 748–751 (2001). https://doi.org/10.1038/414748a
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