Background

The threat of an avian influenza pandemic is causing widespread public concern and health policy response, especially in high-income countries. Our aim was to use high-quality vital registration data gathered during the 1918–20 pandemic to estimate global mortality should such a pandemic occur today.

Methods

We identified all countries with high-quality vital registration data for the 1918–20 pandemic and used these data to calculate excess mortality. We developed ordinary least squares regression models that related excess mortality to per-head income and absolute latitude and used these models to estimate mortality had there been an influenza pandemic in 2004.

Findings

Excess mortality data show that, even in 1918–20, population mortality varied over 30-fold across countries. Per-head income explained a large fraction of this variation in mortality. Extrapolation of 1918–20 mortality rates to the worldwide population of 2004 indicates that an estimated 62 million people (10th–90th percentile range 51 million–81 million) would be killed by a similar influenza pandemic; 96% (95% CI 95–98) of these deaths would occur in the developing world. If this mortality were concentrated in a single year, it would increase global mortality by 114%.

Interpretation

This analysis of the empirical record of the 1918–20 pandemic provides a plausible upper bound on pandemic mortality. Most deaths will occur in poor countries—ie, in societies whose scarce health resources are already stretched by existing health priorities.