FactSentinel

The flu vaccine cannot give you the flu. Injectable flu vaccines are made using either inactivated (killed) virus or no virus at all, relying instead on a single protein to trigger immune protection. Neither of these can cause an infection. The nasal spray version uses a weakened live virus, but it is specifically engineered to replicate poorly in the warmer temperatures of the lungs, meaning it cannot establish the kind of infection that causes flu symptoms.

The persistence of this myth largely stems from timing and confusion with side effects. It takes about two weeks for the vaccine to produce full immunity, so someone who catches the flu shortly before or after vaccination may mistakenly attribute their illness to the shot itself. Additionally, flu season overlaps with cold and other respiratory virus season, so people who get vaccinated and then develop an unrelated illness often blame the vaccine. Mild side effects like soreness, low-grade fever, or fatigue are also sometimes mistaken for the flu itself, but these are simply signs that the immune system is responding and building protection, not symptoms of an actual influenza infection.

Extensive research and decades of clinical use, monitored by health organizations like the CDC and WHO, confirm that flu vaccines are safe and cannot cause influenza. While mild, temporary side effects are possible, they are the immune system's normal response to vaccination, not the disease itself.