Why does the flu jab only last one year?

The flu is a nasty virus. It tends to make mistakes when it copies itself, so that when the virus made in one person is passed onto someone else, it is slightly different to the one you were infected with. As the flu goes around the world every year, by the time it's got back to where it started, it looks very different from the version that was here before. When you get the flu the next year, your immune system cannot recognise it because it looks completely different. So a vaccine is only good until the flu changes, which is why you have to have a new jab every year.