DY >> You don’t need 100% immunity to stop covig from spreading
It is not just an average number, it is about having a pool to propagate.
If there were no clusters, and we were to reduce everyone’s risk by a factor of 2, and R0 would decrease seriously below 1, then pandemic will be over.
BUT if a substantial CLUSTER of population continues being totally uncareful and in constant contact with each other, then covid will continue circulate among them and also touch everyone else. So, elderly will have 5x reduced risk due to their own vaccine, but they will still be exposed to that risk.
There is a lot of evidence that COVID moves in such non-uniform way. This explains, for example, nursing homes that are deadlier near universities. Medical students, even if being careful, eventually propagate covid from the student cluster to the nursing homes.