It's still very early in the flu season, but the Centers for Disease Control and Prevention says that most cases this year are caused by a mutated H3N2 strain, which the flu vaccine can't really protect against.
Flu vaccines are formulated several months before the season begins, and always target three or four strains. Strains "drift," or mutate, all the time, and although the vaccine can't do much to prevent infection with a drifted virus, there is a bit of cross protection.
"Though reduced, this cross protection might reduce the likelihood of severe outcomes such as hospitalization and death," the CDC said in a health advisory to doctors. "In addition, vaccination will offer protection against circulating influenza strains that have not undergone significant antigenic drift from the vaccine viruses, such as influenza A (H1N1) and B viruses."
In the United States, the flu kills anywhere from 4,000 to 50,000 people a year, and the CDC recommends that anyone older than 6 months get the vaccine.