"Since October 1, 2012, CDC has antigenically characterized 521 influenza viruses, including 17 2009 influenza A (H1N1) viruses, 327 influenza A (H3N2) viruses and 177 influenza B viruses.
I'm not sure I understand how this relates to the flu shot, in terms of...521 different kinds of viruses. So one shot is protecting against 521 different kinds of flu? Or is it just that if you're safe from a type A and a type B, you're just about ready to deal with any of the others? How does that work?