Let's just say that everyvyear, many people either get the vaccination, or the flu. These people are left immune to the strain of flu that they were vaccinated/infected with so the number of people who can get and pass on that form of flu (think of these people as that flu strain's "habitat") is reduced. If some of that flu mutates sufficiently that the immune systems of those previously infected vaccinated people no longer recognize it, then it's habitat is expanded back to the original size again.
Hope this helped :)
Have a great day
