It makes me sad that on a tech site, there are so many scientifically illiterate individuals, especially when it's already been stated in the thread that what they appear to believe is incorrect.
It has been known for a long time that the flu shot isn't 100% effective. It's usually about 60-70% effective.
Hmmmm... I'll attempt to explain using an analogy. The mathematical models are somewhat similar. Let's turn to nuclear bombs. I think just about everyone has heard of "critical mass" - below which, you're not getting a boom. Above which, bad shit starts to happen. It's similar to that in the population. If below a certain percentage of the people are vaccinated (and really, the vaccination is only affective about 3 to 4 out of 5 times), then the flu spreads very rapidly and infects a high percentage of individuals. This results in more people (young, elderly) dying from the flu. But, if the percentage of the population that is successfully vaccinated is high enough, then the flu spreads, but not as rapidly, and not infecting as high a percentage of the population.