How well the flu vaccine works to protect you from the flu can vary. The flu vaccine is generally more effective among people under 65 years old. Some older people and people with certain medical conditions may develop less immunity after receiving a flu shot.
Reviews of past studies have found that the flu vaccine is about 50% to 60% effective for healthy adults who are between 18 and 64 years old. The vaccine may sometimes be less effective.
Even when the vaccine doesn't completely prevent the flu, it may lessen the severity of your illness, and reduce the risk of serious complications and serious illness requiring hospitalization.