It's not technically immunity as usually defined, but the levels of immunity can start to reach the point where R naught drops. We COULD be where a third of the nation is resistant/immune. So, if an infected person might have passed it on to three people, now it's two people.
And R naught is thought to be slightly above 1.0, generally, so it doesn't take a lot to drop to 0.9.
I hope.