It is difficult to predict when the next flu pandemic will occur or how severe it will be.  Historically, there has been a broad spectrum of severity in pandemic flu.  The pandemic in 1918 was a worst-case scenario during which more than 50 million people died worldwide.  The flu pandemic in 1968, caused by the H3N2 virus, was not significantly more severe than a "normal" flu season.

In a normal flu season, there are commonly 30,000-36,000 deaths in the United States among individuals with underlying medical problems or advanced age.  The Congressional Budget Office estimates that there is a 3% to 4% probability of a pandemic occurring in a given year.