What is the case fatality rate of smallpox?

Area of Science:

• Epidemiology
• Historical Demography
• Public Health

Background:

• Smallpox (Variola major) has historically posed a significant public health threat.
• The intrinsic case fatality rate (CFR) of smallpox is often cited as 20-30%, but this figure may not account for crucial contextual factors.

Purpose of the Study:

• To estimate plausible ranges for the case fatality rate (CFR) of smallpox.
• To investigate the influence of population susceptibility and social factors on smallpox CFRs.
• To challenge the consensus CFR for smallpox by incorporating historical demographic data.

Main Methods:

• Analysis of population smallpox mortality rates in 18th-century Sweden.
• Examination of the death toll from the 1707-09 smallpox epidemic in Iceland.
• Comparison of CFR estimates between populations with differing age susceptibility and endemicity patterns.

Main Results:

• Estimated smallpox CFRs were significantly higher (43-55%) in populations where both children and adults were susceptible (e.g., Iceland).
• Estimated smallpox CFRs were considerably lower (8-10%) in populations where the disease was endemic and primarily affected children (e.g., Sweden).
• The study identified social factors, such as disruption of household tasks and nursing care, as key determinants of increased CFR during epidemics.

Conclusions:

• The intrinsic CFR of smallpox is likely not a fixed value and is heavily influenced by social and demographic contexts.
• Historical and epidemiological analyses of smallpox should consider population susceptibility and societal disruption when reporting CFRs.
• Relying on a consensus CFR of 20-30% for smallpox may be an oversimplification and potentially inaccurate for specific historical populations.