What’s your source for (a)? I think virtually every human being on the planet has been infected with some coronavirus common cold strain in the past.
Specifically this line: > Or, as a paper published Tuesday in Science suggested, it could trace back to previous encounters with other coronaviruses, such as those that cause the common cold.
This doesn't reflect reality, as its correlation isnt inverted.
To some degree. Immunity fades, which is the whole concept of booster shots.