Such a thing cannot be determined easily and effects can start showing after many years of the body not getting the flu.
You belive that it's easy to see how the body works: modern science can tell that only in isolation, but the body as a whole is not something we can predict the longterm effects for. Why do you think vaccines need 10-15 years of testing before getting approved (except the covid one, which tells you a lot about how much they know about the side effects....)