Another example is the 1920 pandemic, where the weirdly high mortality in young adults prompted theories that the dominant strains of influenza changed around 1

Another example is the 1920 pandemic, where the weirdly high mortality in young adults prompted theories that the dominant strains of influenza changed around 1890, and people born after that would be naive for H1N1. There've been some interesting calls for tailoring seasonal flu vaccines not just for currently or expected circulating strains, but for the age of the recipient.