So, I think the proposed math is something like: 1/3rd of infected don't show antibodies so that 20% means 30% were infected. Another study showed pre-existing T cell reactivity (maybe immunity?) in 40% of people, so if you combine those you're at 70%.
I find it hard to believe, with the R0 numbers we were seeing initially, that 40% had a significant level of immunity. If the numbers were that high, then R0 among those without memory T-cell immunity would have been significantly higher, which suggests that the level of exposure/immunity necessary for herd immunity to set in would be significantly higher as well.
What you're saying makes sense. Looking at some tables of it, it seems like if we imagine it in a naive population having a 50% higher reproduction number, it seems like it only bumps the herd immunity number by about 10% because it is such a high proportion already, if I'm understanding the charts right. It isn't clear if the numbers come together, but it does make me wonder if it could be close.
