If you’re referring to the current Coronavirus pandemics, the reasons we want to mass vaccinate the population is to stop the spread by making it harder to find a host that it can use to replicate. The more times the RNA virus replicates, the more mutations are generated, the more likely a strain, that can escape the any immunity developed, can occur. Herd Immunity is what will help reduce the ability of a virus to spread. If the virus can’t find a host to replicate in (because the host is immune to the virus) the harder it is for the virus to spread.
