Basic models for COVID-19 suggest herd immunity is achieved when 60% of people are immune.

This is because in a population where everyone is susceptible to coronavirus, an infected person is estimated to infect on average an additional 2.5 people.

Yet if 60% of those theoretical 2.5 people are immune, then only one new infection can take place, and the outbreak cannot grow.

This is based on a very simple model, though. It assumes that everyone in the population mixes to the same degree and at random. It’s unrealistic.