Herd immunity

Herd immunity, also called population immunity, is the protection a community gains when a large share of its members are immune to an infectious disease, either through vaccination or after recovering from infection. Because most people the pathogen reaches cannot catch it or pass it on, chains of transmission break and outbreaks fizzle out. The few people who are still susceptible are shielded indirectly, simply because they are unlikely to meet an infected person.

The share of immune people needed depends on how contagious the disease is, which epidemiologists capture with the basic reproduction number. A highly contagious virus such as measles needs roughly 95 percent of the population immune, while polio needs about 80 percent. Herd immunity has to be maintained, because births and migration keep adding susceptible people; when vaccination rates slip, diseases such as measles and whooping cough return.

This indirect protection matters most for people who cannot be vaccinated, such as young babies or those with certain medical conditions. Public health agencies aim to reach it through vaccination rather than by letting a disease spread, which would cause many avoidable illnesses and deaths.