Differential All-Cause Excess Mortality of the 1918 Influenza Pandemic in the U.S. Registration Area

The 1918 influenza pandemic is one of the deadliest events to have occurred in recorded history. This pandemic remains significant to public health due to the potential of a 1918-like pandemic occurring today. The implications of a future influenza similar to that of 1918 necessitate investigation into the mortality of this pandemic. Previous studies examining mortality records in the U.S. during the pandemic found that mortality varied by city and state, and was associated with factors such as sex, age, urbanization, and population density and growth. The purpose of this study is to evaluate the age- and sex-specific all-cause excess mortality during the 1918 pandemic in the U.S. among twenty-four states with registered mortality data. A secondary objective of the study is to examine the correlations between all-cause excess mortality and suspected determinants of mortality during the pandemic. This study used mortality data from Vital Statistics reports of the U.S. Census Bureau for states that registered by 1915. Excess mortality was determined by age and sex as the deviation of mortality in 1918-20 from the average mortality rate in 1915-17 and 1921-23. Overall, the excess mortality rate was the highest in 1918, while in 1919 and 1920 the mortality rates were similar to the non-pandemic rates. With few state exceptions, excess mortality decreased substantially in 1919 and increased in 1920. Younger age groups had higher excess mortality in every year. In 1918, male excess mortality exceeded female excess mortality; however in 1919 and 1920 this was reversed. While some states demonstrated consistent age-specific and sex-specific patterns of excess mortality compared to the average rates, overall, the excess mortality varied greatly across all states. Population density, urbanization, and male to female ratios were moderately correlated with overall excess mortality, while military enlistment and influenza and pneumonia mortality were weakly correlated with excess mortality rates. These correlations varied by age and sex. The variation in excess mortality and the weak to moderate correlations with suspected determinants of mortality during the 1918 pandemic suggest further investigation of these determinants with regards to predicting excess mortality.