What Is Herd Immunity?

To have herd immunity, a large number of people in a community must be protected from a contagious disease. Rafael Ramirez Lee/Shutterstock

When you get vaccinated, you're not just protecting yourself from getting sick. You're also protecting other people in your community.

Germs travel from person to person. But when enough people in an area are vaccinated against a contagious illness, fewer people get sick. That's because fewer germs are able to spread between people. This is called "herd immunity" or "community immunity."

"The whole principle is if you give a vaccine to somebody, you protect them from getting infected, but you also prevent them from transmitting the disease to other people," Michael Brady, M.D., associate medical director at Nationwide Children's Hospital in Columbus, Ohio, and a member of the hospital's Division of Infectious Diseases, tells WebMD.

Herd immunity can help protect people who can't be vaccinated or who might have issues with immunity. This includes those with poor immune systems, pregnant women, older people with weak immunity and children too young to be vaccinated, according to the American Academy of Pediatrics (AAP). It also protects people who refuse recommended vaccinations.

Herd immunity has made headlines because of recent measles outbreaks in the United States and around the world. There are places in the U.S. where vaccination refusal rates are exceptionally high. When there are significant groups of people who aren't immunized, often they are protected because of those in the community who do receive vaccinations.

The concept applies to a variety of contagious diseases, including influenza, measles, mumps, rotavirus and pneumococcal disease.

Determining a disease's threshold

herd immunity infographic
When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease. National Institutes of Health

But in order for herd immunity to work, a certain percentage of the people in a community must be vaccinated. The percentage of people that need to be vaccinated — known as a disease's threshold — differs for each disease because not all diseases have the same risk of transmission, says the AAP.

To set a disease's threshold, epidemiologists first determine the average number of people in an unprotected population who will catch a disease from one infected person, explains PBS. This is called a "basic reproduction number" or R0 (pronounced "R naught").

If a disease has an R0 of 10, for example, that means a person who has that disease will spread it to an average of 10 people in the community who haven't been vaccinated or who aren't immune to the illness. The higher a number is, the higher the immunity threshold needs to be in order to protect the community. The R0 for measles is between 12 and 18, while mumps is between 4 and 7.

In addition to using the R0, epidemiologists use several other factors to calculate a disease's threshold. They look at its severity and how easily it is transmitted. Because measles is very contagious and can be spread easily through the air, to get herd immunity, about 93 to 95% of people in a community must be vaccinated. For mumps, only 75 to 86% of the population must be vaccinated to protect the entire community.

Why herd immunity is important

If enough people are vaccinated against a disease, an infection will no longer circulate in a community. But even if a disease is wiped out in the U.S., people need to continue receiving vaccinations. When travelers go to other countries where those illnesses are still prevalent, they can return and spread those diseases back here.

In addition, protection can wane over time, so it's important to follow immunization recommendations. Sometimes people who skip vaccines rely on the safety of the herd to protect them. But if too many people rely on the rest of the vaccinated community to be safe, then protection drops, especially for those who might have the most compromised immune systems.