But antibodies aren't the only thing that matters; focusing only on antibody levels without acknowledging that you only care about antibody levels _as a proxy for protection_ is the error I talked about in my comment.

There are Memory T cells and Memory B cells that last for decades. So if these cells were able to fend off the virus by themselves, you could have zero antibodies and be fine.

Some possibly useful articles:

* https://en.wikipedia.org/wiki/Correlates_of_immunity

* https://en.wikipedia.org/wiki/Memory_B_cell