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.
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   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.

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   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