There is an odd ‘second wave’ pattern when you drive down to reduced aggregations. Namely, there doesn't appear to be second waves pretty much anywhere. Only first waves.

If you look at US states there are obvious two humps in new Covid cases in Louisiana, Illinois and Kansas (if you squint).

But drive down to the county level and, for counties that had a significant number of cases, it goes away. Here they are from the NYT. https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html

This is pretty striking. Explanation No.1: If you had a significant outbreak, you were more likely to socially distance properly and so a second one has been stopped.

Explanation No.2: This is evidence that perhaps the herd immunity threshold is lower than we thought.

At the moment, I am not sure how to distinguish these two hypotheses but I thought I'd put it out there as anything looking like a regularity is worth exploring.