What is the Case Fatality Rate of Smallpox?

This paper uses population smallpox mortality rates in eighteenth-century Sweden and the death toll from the 1707-9 smallpox epidemic in Iceland to estimate plausible ranges for the case fatality rate (CFR) of the deadly form of smallpox, Variola major, in both its endemic (Sweden) and epidemic (Iceland) form. We find that smallpox CFRs could be extremely high (40-53%) when smallpox was epidemic and attacked a population where both children and adults were susceptible as in Iceland. However, where smallpox was endemic and therefore a disease of childhood, as in Sweden, a better estimate of the CFR is 8-10%. This is far lower than the consensus CFR of 20% to 30%. Part of the differences between the CFRs studied here could be due to differences in the inherent virulence of smallpox in the two contexts. However, we argue that social factors are more likely to explain the differences. Where both adults and children were susceptible to smallpox, smallpox epidemics fundamentally disrupted household tasks such as fetching water and food preparation and prevented parents from nursing their sick children, dramatically increasing the CFR. Thus, when historians and epidemiologists give CFRs of smallpox, they should consider the population and context rather than relying on an implausible intrinsic CFR of 20% to 30%.