It looks like this year's flu shot only offers limited immunity for this particular flu:
Dr Watson urged the over 65s to get vaccinated against influenza. He also suggested that children over 6 months who fall into "at risk" groups should also receive the jab. "The current influenza vaccine contains the [A/Panama/2007/99] strain of virus and is considered to offer some protection against [A/Fujian/411/2002 (H3N2)-like] strains and good protection against other strains that may also circulate," he said. "When influenza virus strains change over time, they are more likely to affect the young, who will have less immunity to them. For this reason, it is especially important for children over 6 months of age who fall into the at-risk groups to be vaccinated." Those in at-risk groups include those with severe asthma, people with chronic kidney disease, diabetes, or with lowered immunity due to any disease or treatment such as steroid medication or cancer treatment.