Background
Understanding age-specific mortality patterns across historic influenza pandemics is crucial for future pandemic preparedness. Prior research shows that, while the 1918 pandemic caused unprecedented mortality in younger adults, subsequent pandemics in 1957, 1968, and 2009 displayed varying mortality patterns, with elevated risks in some younger populations and elderly populations. However, cross-national comparative analyses of these patterns using harmonized all-cause mortality data remain lacking but are critical for informing public health strategies.

Methods
We analysed age-specific all-cause absolute and percentage excess mortality patterns across 48 populations during the 1918, 1957, 1968, and 2009 influenza pandemics by using data from the Human Mortality Database.

Results
While the 1918 pandemic consistently showed a peak in positive absolute excess mortality at younger ages (5–39?years), age-specific mortality patterns in 1918 also varied substantially across the populations, particularly at older and early-childhood ages; subsequent pandemics lacked this peak and revealed varied mortality patterns across the age groups, including inconsistent excess mortality rates among the elderly. The percentage of excess mortality also differed by country and pandemic, highlighting the complexity of age-based mortality risks.

Conclusion
This work demonstrates that reports of increased severity among young people as a universal feature of all historical influenza pandemics may have been exaggerated, influenced by the exceptional mortality among the young during the 1918 pandemic.