Answer: forever, but….you should repeat the shot once per year
The flu shot is a mixture of particles from recent common flu viruses from around the world. This helps the body form an immune response to flu infections- the flu vaccine is known to prevent flu.
There are different formulas for the flu shot. The formula changes every year, in order to act against the most common viruses. The flu shot does not protect against all flu viruses- normally just two or three. Sometimes the vaccine does not cover an unexpected outbreak of a new type of flu virus. This is why clinical outbreaks occur despite widespread use of the flu shot.
Each year the flu viruses that affect people change, so it is important to have a new flu shot once per year. The effects of the immune response after one vaccine also decline during the year after treatment.