Coronavirus: Power of the herd can protect those most at risk of infection

We don’t think of ourselves as a herd. The idea of the needs of the collective subsuming those of the individual is antithetical to much of western culture. According to the government’s chief scientific adviser, we might have to reacquaint ourselves with the concept.

Because to control coronavirus, says Patrick Vallance, will require something called “herd immunity”. It will also require controlling who in the herd it is who gains that immunity.

Herd immunity does not require everyone in Britain’s “herd” to be infected. It happens before that, when just a proportion of them have. That proportion is the second most important number in epidemic modelling. To see how it is calculated — and perhaps even changed — requires, however, understanding the most important number