Originally Posted by JGordon

The flu vaccine's effectiveness varies a lot year to year depending on how accurately researchers predict circulating flu strains -- the strains they put into this year's flu vaccine were decided on about a year ago based on historical data, flu strains that were in circulation then, and other factors. They have to decide this early so that they can ramp up production and perform appropriate testing on the vaccine.

Most numbers I've seen say that the flu vaccine is about 70% effective in any given year (i.e. it reduces your chance of getting the flu by roughly 70%), and that another 10-20% will have a milder version of the flu than if they hadn't been vaccinated.