The reason for case counts staying relatively low in those places has a lot less to do with "herd immunity" and a lot more to do with still being careful. Even though a huge number of people were infected, it was still a very small amount of the population overall, not anywhere near enough to affect the transmission rate with no restrictions.
In MA we're not even close to entirely reopened yet, but as we continue step by step the transmission rate is ticking back up unfortunately: https://rt.live/us/MA
https://www.cnbc.com/2020/07/07/herd-immunity-questioned-aft...
In MA we're not even close to entirely reopened yet, but as we continue step by step the transmission rate is ticking back up unfortunately: https://rt.live/us/MA