Because the flu virus changes from year to year, last year's immunity won't help you with the virus that causes problems this year. That's why you need a flu shot every year. Influenza is much worse than a cold. Even healthy young people can become quite ill. You might bring the influenza virus home to a baby, older relative, or someone with a medical condition who can get serious complications from influenza.
PVaccine effectiveness varies from one person to another.
Protection from the vaccine develops about two weeks after the shot, and may last up to one year. In elderly people, the vaccine can prevent pneumonia and hospitalization in about 6 out of 10 people. The viruses that cause influenza change often. Because of this, influenza vaccine is updated each year. People who receive the vaccine can still get influenza, but if they do, it is usually milder than it would have been without the shot.