Whatever happened to large-scale antibody testing? You'd think that someone would be testing a few thousand random people each week to see how many people have been infected to date. That sort of thing was being done in the US back in April.[1][2] But it seems to have stopped.
If you're going to talk about "herd immunity", you need that info to get anywhere.
The antibody tests seemed to be consistently underestimating other measures of Covid-19 immunity, even conservative ones, so they're not considered as useful as we thought in April or May.
It's possible the tests aren't sensitive enough, or immunity is largely based on T-cells (more expensive to test for) rather than antibodies [0], or antibodies for other coronaviruses confer some level of immunity, or something else we still don't understand.
TLDR: "During the first wave of the COVID-19 pandemic, fewer than 10% of the US adult population formed antibodies against SARS-CoV-2, and fewer than 10% of those with antibodies were diagnosed."
The trends won't really tell you much as we get further into the pandemic. Just because someone lacks a detectible level of antibodies doesn't necessarily mean they are susceptible.
Unfortunately, even antibody testing is unreliable now because of waning antibodies.
(One might object and say that if someone has little to no levels of antibodies they must not be immune anymore, but immunity is complex and not solely determined by antibodies)
It's possible the tests aren't sensitive enough, or immunity is largely based on T-cells (more expensive to test for) rather than antibodies [0], or antibodies for other coronaviruses confer some level of immunity, or something else we still don't understand.
[0] https://www.eurekalert.org/pub_releases/2020-08/cp-mcc081720...