Reassessing the global mortality burden of the 1918 influenza pandemic.

Mortality estimates of the 1918 influenza pandemic vary considerably, and recent estimates have suggested that there were 50-100 million deaths worldwide.

Method
We investigated the global mortality burden using an indirect estimation approach and two publically available datasets: the Human Mortality Database (13 countries) and data extracted from the records of the Statistical Abstract for British India. The all-cause Human Mortality Database allowed us to estimate mortality annually in 1916-1921 for detailed age groups.

Results
Three different calculation methods were applied to the data (low, medium and high scenarios) and we used a multilevel regression model to control for distorting factors (e.g. war and the underlying time trend in mortality). We found that total pandemic mortality was an estimated 15 million deaths worldwide in 1918 (2.5 million in 1919), after including the rates for British India and controlling for wars and the underlying mortality trend. Our validity analysis showed that simulations of total mortality above 25 million are not realistic based on the underlying mortality rates included in Human Mortality Database and in British India.

Conclusion
Our results suggest that the global mortality impact of the 1918 pandemic was important (17.4 million) but not as severe as most frequently cited estimates.