Normal flu vaccines are prepared each year and target a specific flu strain.  If however the flu strain mutates or is not the strain anticipated than often the vaccine provides limited protection.  It takes many months to develop the doses needed to protect the general population.

Ever wonder how a flu virus invades your body?

Flu Attack! How A Virus Invades Your Body

Over the past few years work has progressed toward a universal flu vaccine that would work against multiple strains.  As well as moving from egg based vaccines to cell cultured ones.  In a resent research report published August 22, UPen shared research on a candidate vaccine that looks at the HA Stalk rather than the HA Head (which often mutates).  The research report is available here.
Snip from the research report:

The team observed that after immunization, these strong antibody responses to the vaccine persisted through the thirty weeks of the experiment. At the end of this period the anti-stalk responses were even stronger than they had been four weeks after immunization. In addition to the mice, the researchers successfully repeated these experiments in ferrets and rabbits, other species commonly used as vaccine-development animal models.

Having established that the mRNA vaccine can elicit a strong antibody response, including an anti-stalk response, the scientists showed that this response actually protects mice from infection with influenza. A vaccine encoding the H1 subtype kept mice healthy when they were injected with otherwise lethal doses of three flu strains: the same H1 flu virus, a distantly related H1 flu virus, and an H5 strain.

Perhaps in the next year or two this research may completely change the way we produce flu vaccines and provide more robust protection – maybe, just maybe the flu virus is on its way out?