Transmission potential of smallpox in contemporary populations

Raymond Gani and Steve Leach ()
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Raymond Gani: Centre for Applied Microbiology and Research, Porton Down
Steve Leach: Centre for Applied Microbiology and Research, Porton Down

Nature, 2001, vol. 414, issue 6865, 748-751

Abstract: Abstract 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.

Date: 2001
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DOI: 10.1038/414748a

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