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 1707–09 smallpox epidemic in Iceland to estimate plausible ranges for the case fatality rate (CFR) of smallpox (Variola major). We find that smallpox CFRs could be extremely high (43–55 per cent) when smallpox 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, the estimated CFR is only 8–10 per cent: far lower than the consensus CFR of 20–30 per cent. We argue that social factors explain these differences. Where both adults and children were susceptible, smallpox epidemics fundamentally disrupted basic household tasks and nursing of the sick, dramatically increasing the CFR. Thus, when historians and epidemiologists give CFRs for smallpox, they should consider the population and context rather than relying on an implausible intrinsic CFR of 20–30 per cent.