[pillusmany]

> It would be odd if our immune system, as had apparently been speculated, became less effective against pathogens that it saw a lot.

That actually can happen. It's called "original antigenic sin" or "immunological imprinting".

https://en.m.wikipedia.org/wiki/Original_antigenic_sin

It's especially prevalent in respiratory viruses. The flu is a well known case. The more flu shots you have (the yearly one), the less effective they become (while still more effective than nor having one).

[pazimzadeh]

That only happens in cases where the disease rapidly evolves. Which is the case for COVID, though not to the same degree as the flu, which has its genome split into 8 distinct RNA's which mutate and also get shuffled up when multiple viruses infect the same cell.