You can think of it that way, but we never know, in practice, what level of actual immunity exists in a population. In any case, it's not relevant to my point:

You can think of it that way, but we never know, in practice, what level of actual immunity exists in a population. In any case, it's not relevant to my point: R0 is a contextual number, always defined by empirical data. It's not a fixed feature of the virus.