Let me back up a bit and relay my understanding of the concept of herd immunity and its application. Perhaps some will think I'm over-extending it. Here's a paper discussing it as pertains to COVID:
https://science.sciencemag.org/content/early/2020/06/22/science.abc6810
The basic (the article calls it "classic") herd immunity threshold is simply the fraction of people who need to be immune to get the reproduction rate down to/below 1:
hC = 1 – 1/
R0
I've seen 60% cited as a threshold based on a reproduction rate of 2.5: 1-1/2.5=60%
But this assumes a homogeneous society with zero mitigation effort of any kind. And society is of course not homogeneous and mitigation efforts aren't zero (government mandated or otherwise), nor are they consistent. The article addresses demographics and predicts 43% based on the level of social interaction for different groups.
Looking at the trend data*, Pennsylvania had a noticeable plateau at the end of May/early June before resuming its decreasing trend. Why? Probably because a week earlier, Memorial Day Weekend vastly increased the number of social interactions and vastly increased R.
PA's positivity rate bottomed-out on June 18 and started rising again. Why? Because between May 29 and June 26, all but one county entered the "green" phase of re-opening (in groups, roughly weekly). So this tells us that at some point we reached a threshold of social interaction where R went above 1 again. Zooming in; my county was one of the worst-hit and went green with that last batch on June 26. There's a fair amount of noise in the data at this level, but our positivity rate started increasing 10 days later, on June 5. We've since rolled-back on the reopening.
New York City moved to Phase 4 on July 20, 10 days ago, and was in the final batch in New York State. Nothing until now has moved the needle, so we'll see if this does.
Of course "green" or "phase 4" isn't a total return to normal. Restaurants are still only 50% capacity indoors and professional sports are still without fans. The "herd" is much thinner now than in normal times, and it follows that the associated green/4th phase herd immunity threshold is much lower. And even if we get off the "phases" altogether, I don't think life will go back to normal until a vaccine is approved. I don't think I'm going to a restaurant until then. So we may never get a true test of the "classic" herd immunity threshold.
If I'm over-extending the term "herd immunity", so be it. I'll re-phrase without using the term: I think that the primary reason for New York and some surrounding states' spectacular case curves vs most of the rest of the country is that the reproduction rate is being strongly suppressed by the fraction of people with immunity.
*I primarily use this data for state level:
https://coronavirus.jhu.edu/testing/individual-states/pennsylvania
I don't know of any good sources for county-level data and I have collected my own, from daily cumulative tallys.