> A central premise of immunity debt is that for many infectious diseases, repeat infections are milder than the first infection.

This is also the fundamental, well understood, mechanism of vaccines and herd immunity [1]: "Immunological memory" [2]. I think that sentence is very irresponsible.

[1] https://en.wikipedia.org/wiki/Herd_immunity

[2] Immunological memory: https://en.wikipedia.org/wiki/Immunological_memory