My personal theory on why COVID is exploding in the south US but retreating in the north: people being inside.
Roughly April/May is when the weather is nice enough up north for people to spend lots of time outside - outdoor dining, recreating, back yard get togethers, golf, etc. After being stuck inside all winter, people look for any reason to be outside.
In the south, its reverse. Its too hot, and everyone retreats indoors. No one wants to be outside when it's 95 F and humid.
I think this thing spreads indoors almost exclusively. I think dispersion from naturally moving air helps, as does UV light. You get neither indoors.
I have nothing to back this up.