Seasonal flu is a form of influenza ("the flu") that occurs yearly. It is a contagious respiratory disease caused by an influenza virus. Flu can show up as a mild illness or can result in complications that are serious and debilitating. Complications of seasonal flu can result in serious illness and thousands of deaths per year in the U.S.
You are encouraged to get your seasonal flu vaccine as soon as possible. According to the CDC the seasonal flu vaccine provides protection for at least one year following vaccination. Immunity from a flu vaccine received in September is expected to last the entire flu season.