Lack of antibodies in the bloodstream does not equate to lack of immunity.

- There could be antibodies at the mucosal level (IgA)

- There could be cellular responses (T cells) that aren't detected

- There could be immunological memory at play after antibodies aren't detectable anymore

A critique of the work, a little heavy-handed:

https://statmodeling.stat.columbia.edu/2020/06/17/some-forec... (this specific comment)