I got my flu shot in Oct12. How do you tell the difference between the flu and a severe cold, fever?
I think, technically, they have to take a sample and have it analyzed. There are a lot of "flu like" illnesses this year, supposedly. However - this year the flu seems to be a 10 to 14 day affair.
Since a vaccination takes about 2 weeks before it is offering full protection, and since many people either don't know this or forget - they let their guard down after getting the shot by not washing their hands, staying away from people with the flu, etc... anyway... this accounts, I understand, for the the large number of people who claim the shot gave them the flu. When actually they got the flu the regular way before the shot took hold.
Also, the shot is only about 50% to 70% effective. So it's possible to get the shot. And still get the flu even after waiting the two weeks. That said, I understand that it is still a good idea to get the shot. Even if you don't need it, there are people around you who have compromised immune systems. If you get the flu, you may be putting their health at serious risk.