I don't see any evidence to do more than suggest it's seasonal. It may be, it may not be. It didn't seem to die back in summer, as flu and colds are wont to do.
When it hit, Australia and Brazil were in March, equivalent to our September. I don't see much evidence for seasonality based on their experience to date. When it hits, it hits, it seems, area under the curve.
I personally would consider not an upsurge in the winter as a second wave, it would just be a seasonal increase, if it occurs.
The 1918 flu had a distinct and obvious second, and third wave clearly evident. And yes, this could be seasonal, as noted.