From what I understand from the info, it was probably because they use...some part of the carrying base for the vaccine (something to do with chicken eggs) that some people have a bad reaction to.

It's not the virus itself that makes you sick (because the cells are dead and can't do anything anymore), but the other components in it that people get an allergic reaction to.

Which sucks if you're one of those people. :/

Here's hoping you don't catch flu, then. Because it's miserable no matter the strain.