It turns out that the R number that the government puts out is not the number who can potentially be infected by one person but the number who are actually being infected on average. I posted about the intrinsic "basic reproduction number" Ro and realised that this can never go below 1. Then I found this very clear explanation, not peer reviewed but written early in the outbreak. https://www.cebm.net/covid-19/when-will-it-be-over-an-introduction-to-viral-reproduction-numbers-r0-and-re/ It made a lot of sense to me. So there is an "effective reproduction number" Re and this is what the government are publishing. It gives an equation linking the two R numbers:
Re=Ro(1-Pi)
Where Pi is the proportion of the population who have developed immunity.
So if Ro=2.4, as has been assumed and Re is about 0.8 at the moment, you get Pi = 0.67. In other words, it would give two thirds of the population as being immune. This surely can't be the case. It must mean that the various distancing methods have worked to make the numbers be the same as if we were immune - a sort of virtual herd immunity. I think in Physics we'd call that an unstable equilibrium. It is nicely balanced but the removal of a key prop keeping it balanced would upset the current decline. Makes me keen to maintain the distancing measures.
