Disclaimer: This is my field of expertise, but I am not a MD.
Influenza vaccine effectiveness is typically 50-65% and the benefits outweigh the risks. Two types of vaccines, the attenuated live vaccine and the "shot". This year, the experts picked 3 strains for the trivalent (H1N1, H3N2 and B) and 4 strains for the quadrivalent (H1N1, H3N2, and two B). Research shows that typically the intranasal (live) vaccine works better in kids.
Some of the problems encountered are when the specific strains are chosen (~6 months before influenza season) and also after the strains are chosen, they are grown in eggs or cell-culture. Sometimes the virus does not grow well or changes its antigenicity during growth.
Any specific questions?