The second most common excuse people use for not getting a flu shot, after "I didn't get one last year and I didn't get the flu", is that it's not even 100% effective.  It's true; the CDC says, "recent studies show that flu vaccination reduces the risk of flu illness by between 40% and 60%", which is indeed less than 100%.  But here's another way to look at it:  A flu shot is 50% effective, while not getting a flu shot is 0% effective. #science