[MertsA]

It doesn't have to be perfect. If 90% of the at risk population where it's allowed to spread has immunity, then that's high enough that the virus would logarithmically decay. That's often referred to as "herd immunity". So long as on average cases result in less than one additional case you're fine. 1000 cases with an uninhibited net reproductive rate of 4 with a population that's 90% immune would result in the next "generation" having 400, then 160, then 64, then 26.... You get the point.