They both fit the epdiemiology snugly.
Any free infection in an available population follows a sigmoid curve:
The summer infection hits one subset of the general population, and it's pretty much flared out: the summer infection curve is mostly played out. That's why the rate is slowing.
But flu likes it cool and damp. Flu spreads a lot better when it's cool and damp: the bug survives longer, and people clump together closer for longer out of sunlight. So all of a sudden, the available population increases massively because the environmental conditions change.
So the overall infection pattern is going to be a big sigmoid, in which the
summer curve is a ripple in the bottom corner.
Questions?