Viruses can’t multiply and replicate themselves, they need a host cell (human in this case). If someone is immune their cells effectively block the virus from using the human cell to reproduce and mean its dies without having multiplied. So as I understand it, an immune person can’t be a vector.
Herd immunity starts after about 60% of the population are immune. But that would kill a hell of a lot of people before we got to that stage. You’d have to have 80%+ of the population having it, and 60% survive for that to happen, and 20% is a shit load of people to die (edit: it would be about 1.4m people in the uk... staggering numbers)
I’m not an epidemiologist that’s just my understanding!