The flu shot becomes effective about 2 weeks after vaccination when the body develops antibodies. Its protection generally lasts for about 6 months, which is typically enough to cover the duration of the flu season. Immunity gradually decreases after this period, so annual vaccination is recommended to maintain protection against new flu strains each year. The flu vaccine is designed to protect against the most common strains for the season, but effectiveness can vary based on age, health, and how well the vaccine matches circulating viruses.