In rapidly spreading epidemics such as the current coronavirus pandemic, it is usually expected that a majority of the population will be infected before herd immunity is achieved and the epidemic abates. The estimate of when the threshold for this is reached is usually based on models that assume all individuals in a population are identical. Researchers at the Max Planck Institute for the Physics of Complex Systems in Dresden have used a new model to demonstrate that herd immunity can be achieved at a lower threshold if some individuals are more easily infected than others.
Most epidemics abate before the majority of the population becomes infected. What exactly determines this turning point, and the extent of the epidemic, is not yet well understood. Many epidemic models are based on the assumption that individuals in a population are essentially identical. However, in an actual population, every person is different. For example, some people are less likely to become infected through contact with an infected individual, perhaps due to a more effective immune system, or better hygiene.
A team led by Frank Jülicher from the Max Planck Institute for the Physics of Complex Systems has been investigating the influence of this heterogeneity on the spread of an epidemic. When individuals differ in their susceptibility to an infection, it is primarily the most susceptible who become