Eg: If a disease has an R0 of 2.0 (ie infects 2 people for each person that gets it), but 70% of the population is immune either because of vaccine or already has had the virus and has antibodies, the R0 is then reduced by 70% (2.0 * (1-.7) = 0.6). Since the R0 is then lower than 1, the virus dies off as the replacement rate is below the stock rate. Simple math.
I have fuzz brain but I do get what you mean. We want the virus to have as few people to infect. There are just two ways to accomplish that. 1. So many people die that there are too few left for it to spread. 2. Develop immunity, either through a vaccine or catching it and recovering.
One way to think about it is to consider how a lot parents dealt with outbreaks of chicken pox before a vaccine was available. It was thought better to let a child catch the virus and recover gaining immunity rather than risk them getting it at as and teen or adult when the disease has more serious complications.
In your example, are you saying that a person carrying the virus will infect only 2 others REGARDLESS of how many people are exposed to him? Or are you saying that given "shelter in place and social distancing" rules in place, the "average" infection rate is 2 infected for each carrier because the carrier will only come into contact with X number of vulnerable (i.e., non-immune) people before they recover and are no longer carriers?
And are you assuming that people who recover from the virus are now immune (this has not yet been established in fact IIRC. There have been situations where the virus has recurred a 2nd time in people who have recovered from a 1st bout)?
Thx again for helping me understand this.