“the 48 million or so previously infected & recovered cases”
Through the end of September 2021, CDC estimated that around 147 million Americans had been previously infected (https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/burden.html )
So even closer to that Herd Immunity Threshold.
It seemed that we were there for the original strain, after last Winter’s wave, but Delta needs a higher threshold.
After Delta and Omicron have had their runs, we will probably be over 90% of the population with some level of immunity.
Of course, out of the confirmed and/or suspected total case load in order to get the number of people walking around infected/recovered we'll have to reduce that number somewhat. Some passed away. Some may have left the US. Some unfortunate people are very susceptible and have caught it 2 or even 3 times. So the number of people walking around with some level of natural immunity will be somewhat less than the 147 million.
Then we have the 200 million or so vaccinated which should {snort} have some immunity. Obviously there will be some fairly significant overlap between the natural immunity group and the vaccinated group. Many got vaccinated after being sick (knowingly sick or not). Many got sick after being vaccinated.
But still, even if we knock out 20 million from the 147, and even if we go with as much as 50% overlap. That's 273 or so million out of 330 million or so. That's nearly 83%. You'd think for most nominally transmissible infections this should be about the herd immunity level. Unless of course the vaccines aren't really helping...