Transmission potential of smallpox in contemporary populations

Despite eradication(1), smallpox still presents a risk to public health whilst laboratory stocks of virus remain(2,3). One factor crucial to any assessment of this risk is R-0, the average number of secondary cases infected by each primary case. However, recently applied estimates have varied too widely (R-0 from 1.5 to >20) to be of practical use, and often appear to disregard contingent factors such as socio-economic conditions and herd immunity(4-8). Here we use epidemic modelling(9) to show a more consistent derivation of R0. In isolated pre-twentieth century populations(10-12) with negligible herd immunity, the numbers of cases initially rose exponentially, with an R-0 between 3.5 and 6. Before outbreak controls were applied, smallpox also demonstrated similar levels of transmission in 30 sporadic outbreaks in twentieth century Europe(1), taking into account pre-existing vaccination levels(13,14) (about 50%) and the role of hospitals in doubling early transmission. Should smallpox recur, such estimates of transmission potential (R-0 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.