“Herd immunity”. In the absence of therapeutic treatments or a vaccine for the novel coronavirus, it suggests that there’s safety in numbers. Physicians say the reality is far more complex.

Patrick Vallance, the UK’s chief scientific adviser, said herd immunity is an option the government is exploring in its effort to grapple with the coronavirus-borne illness Covid-19. The aim would be to allow immunity to build up among members of the population who are least at risk of dying from Covid-19.