THE idea of herd immunity has had a bumpy ride as the coronavirus pandemic has played out. It was initially touted in some countries as a viable strategy for dealing with the spread of covid-19 , before being dismissed. Today, some headlines celebrate the fact that many places might have achieved herd immunity including Britain and pockets of London, New York and Mumbai . But others warn that millions will die before we get there.

The true picture is far messier, partly because scientists don’t even agree on what herd …