In normal flu years, most of us have immunity, either from the vaccine or from having the flu in previous years, ... But in pandemics, we have no prior immunity, and it's just like being hit with a completely new disease that we've never built up any ability to fight. That's why the mortality tends to be high even in the age groups that don't usually get very sick from flu.  -   Marc Lipsitch     Quotes
In normal flu years, most of us have immunity, either from the vaccine or from having the flu in previous years, ... But in pandemics, we have no prior immunity, and it's just like being hit with a completely new disease that we've never built up any ability to fight. That's why the mortality tends to be high even in the age groups that don't usually get very sick from flu. Marc Lipsitch