The flu vaccine provides the immune system with advance notice of the antigen, which can diminish the severity of symptoms if infection occurs, rather than preventing infection entirely.

“what does the flu vaccine do it gives the immune system a an advanced notice of what this antigen is and that has two effects what a lot of people believe is that it's going to protect you from ever getting infected it's not the case uh you can still get infected but what happens is that the symptomatology or the side effects of that infection will be greatly diminished”