Herd immunity (also known as community immunity) is defined by the
Centers for Disease Control and Prevention (CDC) as “a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely.”
In other words, where herd immunity exists—when lots of people in an area are vaccinated or have already been infected with a disease—fewer people get sick, and fewer germs are able to spread from person to person.
The CDC adds that even people who are not vaccinated, like newborn babies and individuals with chronic illnesses, have some level of protection because the disease can’t spread within the community.
The theory behind herd immunity is that when someone gets vaccinated, it’s not only that person who is protected from infection—they can’t transmit the disease to other people. Herd immunity protects people who cannot be vaccinated because their immune systems aren’t strong enough and are therefore the most vulnerable to serious illness.
An example of herd immunity via vaccination is the measles outbreak among preschool-age children in the US in the late 1980s. The attack rate decreased faster than coverage increased, and researchers who examined the association between incidence of measles and immunization coverage among preschool-age children concluded that immunization coverage of about 80% may be enough to stop sustained measles outbreaks in an urban community.
