Herd immunity is indirect protection against an infectious disease that occurs when enough people in a community are immune, usually through vaccination, so transmission is reduced or interrupted. This also helps protect people who cannot be vaccinated or who may not respond well to vaccines, such as some infants and immunocompromised individuals.

If an infected person has few susceptible contacts, the infection is less likely to spread through the population. Herd immunity applies mainly to diseases passed person-to-person.

In a simplified model, the immunity threshold is:

[ \text{Herd-immunity threshold} = 1 - \frac{1}{R_0} ]

where (R_0) is the average number of people one infectious person would infect in a fully susceptible population.

Vaccination is the safer way to achieve population immunity because acquiring immunity through natural infection can cause severe illness, disability, and death. The WHO explanation of vaccination describes vaccination as a safe way to build immune protection before exposure.