Certainly for SF/LA, which is basically an extension of China because of the daily flights to SFO and LAX.
The SF tests show 3.6% of 78,000 exposed to corona, but nobody knows how accurate those tests are. The hospital dashboard has been flat.
I just checked, and the numbers have been flat (no growth) for over a month, with only 62 deaths to date and 38 in ICU now:
https://www.sccgov.org/sites/covid19/Pages/dashboard.aspx#ho...
All sources I've seen before say something different about those two things (i.e. supposedly effective R will maybe go a bit down in summer but in all likelihood not by much; and herd immunity is in most (or all) places still very far away), but I'm not following the science very close so I might very well be wrong. I'd be very grateful if you could dig up those references!
https://www.biorxiv.org/content/10.1101/2020.05.26.115832v1
https://www.biorxiv.org/content/10.1101/2020.05.18.100545v1
https://www.cell.com/cell/fulltext/S0092-8674(20)30610-3
https://www.medrxiv.org/content/10.1101/2020.04.17.20061440v...
https://hal-pasteur.archives-ouvertes.fr/pasteur-02569149
https://www.biorxiv.org/content/10.1101/2020.05.21.108308v1
https://www.medrxiv.org/content/10.1101/2020.06.02.20120477v...
https://www.medrxiv.org/content/10.1101/2020.04.27.20081893v...
There isn't much completed research about the seasonality of COVID-19 itself (understandably), the "cliff" is mostly an educated guess based on the strong seasonal pattern we see with every other respiratory virus. Here are a couple papers:
https://www.medrxiv.org/content/10.1101/2020.03.12.20034728v...
https://www.medrxiv.org/content/10.1101/2020.02.22.20025791v...
I don’t think this will happen in the northern hemisphere, thankfully. There is reason to believe both that effective R drops off a cliff in the summer months, and that we’re much closer to herd immunity (or whatever is possible with this kind of virus) than most people think. If you want references I can dig them up, but I guess we’ll see either way in a few weeks.