I agree , Ithink a better argument could be made that it's a combination of less social interaction, less travel, vaccines and those who have recovered.

If we assume that the number of people infected is 2-3 times higher than reported due to asymptomatic cases (big assumption), then we are around 1/3 of the country having some immunity. That would start to make a difference IMO.

But we are in a race against these mutations. Particularly the one out of South Africa. If we don't get ahead.of it, wf will be starting over.