Why do seasonal flu shots only last for a year?

Answer 1

There are thousands of different viruses and even the same virus will have mutated

over a year so it becomes a "cousin of a cousin's great great grandchild".

A vaccine is a small dose of the virus. Not enough to make you ill, but enough that

your body recognizes THAT PARTICULAR invader and learns to fight it.

Answer 2

Because the strains of viruses that cause the seasonal flu mutate very rapidly and by the time the virus has spread during the flu season and then moved on, it usually will have started to mutate and will no longer be prevented by the prior year's vaccines.

Answer 3

There are different kinds of influenza. So a vaccine against one, usually won't be effective against another. Also, the flu virus mutates easily and rapidly. When it mutates, the old vaccine is may no longer be effective, depending on how different the mutation is to the original strain. It is also under study to determine if there is a gradual loss of immunity from flu over long time periods as some believe occurs in some people with use of some vaccines and with some types of flu.

Answer 4

The flu virus changes over time. By next year, it isn't exactly the same as it is this year.