Herd immunity
If a substantial fraction of the population acquires immunity based on Vaccine or infection, the population may achieve “herd immunity”, where a new outbreak cannot be sustained. Therefore, it is often the target of vaccination campaigns.
However, it is not the whole picture. There are two aspects that make herd immunity not as important as it is often assumed to be. First, when there is an ongoing epidemic, the infection can “overshoot” the herd immunity threshold. In other words, reaching herd immunity does not mean that the disease will magically vanish at that point. Second, even if we don’t reach herd immunity, other interventions can be used to prevent epidemic outbreaks. Many emerging diseases, without any herd immunity, have been controlled by test-trace-isolate/quarantine strategy.
Without vaccines, some advocates a “herd immunity strategy”. In other words, let it spread through a younger population (in the case of COVID 19 pandemic) so that society can achieve herd immunity. However, this is probably not the best idea. First, it is almost impossible to isolate the disease within a particular population. Second, the diseases mutate. When let it spread freely, it is likely that the disease produces more dangerous variants. Third, especially with variants, immunity may not be sustained enough.