I am not strongly for or against flu shots but I would love to know how they arrived at that figure. 

Assuming that everyone lost an average of $500 every time they got sick with the flu this would mean that 10 million got sick in the US alone. If you divide it evenly 5.1 Billion would be $15 for every man, woman, and child in the country (I think we are up to 330 million now) and from just doing a little googling it costs about $40 a pop for a does of flu vaccine.... :-)