Transmission potential of smallpox in contemporary populations

Abstract

Despite eradication, smallpox still presents a risk to public health whilst laboratory stocks of virus remain. 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 immunity. Here we use epidemic modelling to show a more consistent derivation of R0. In isolated pre-twentieth century populations 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 Europe, taking into account pre-existing vaccination levels (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.

The Effect of Social Mixing Controls on the Spread of Smallpox—A Two-Level Model

“…The basic reproductive rate R 0 is the number of secondary infections generated by an infectious individual if all the rest of the population is susceptible ( [22], p. 17). The range of R 0 values is consistent with the estimates in [23] (see also sensitivity analysis with respect to these parameters in Section 5). The infection rate α in a susceptible population of size X is given by α = R 0 X τ , where R 0 is the basic reproductive rate and τ is the average duration of the infectious period.…”

Differential efficacy of vaccinia virus envelope proteins administered by DNA immunisation in protection of BALB/c mice from a lethal intranasal poxvirus challenge

“…Routine vaccination against smallpox has not been practised for over 20 years, leaving the population of the world increasingly vulnerable. Today the majority of children and adults are not vaccinated against smallpox, and the consequences of a re-emergence of the disease, by whatever means, would be far reaching without effective public health interventions [2,3].…”

Daniel Bernoulli’s epidemiological model revisited

“…On the basis of his other numerical estimates, he obtained an endemic prevalence of susceptibles of around 15% which corresponds to a basic reproduction number of 6.67. Recently Gani and Leach [39] quote our paper in the context of their attempts to estimate R 0 for smallpox if it would recur.…”