flu vaccine, page-55

Hi Menta, re "When unvaccinated people are infected with the seasonal influenza virus, they have an opportunity to develop a robust "cell-mediated" immunity that not only protects against that strain of the virus, but is also cross-protective against other strains. People who’ve annually received the influenza vaccine, on the other hand, "may have lost multiple opportunities for infection-induced cross-immunity."

Well if that's true, why do people still get the flu, shouldn't their "robust cell mediated immunity" be kicking in to kill off the next strain. Doesn't seem to happen though.