Immunization Issues

Community Immunity

Updated: June 23, 2006

Community immunity is an important concept in immunization science and policy. This article explains what community immunity is and its implications for the public health.

Unimmunized persons are protected—indirectly—against some infectious diseases by being surrounded by immunized persons. This is known as community (or “herd”) immunity. 1

For example, someone who has never had chickenpox or chickenpox vaccine is said to be susceptible to chickenpox. That means the person can become infected and develop the disease along with its potential complications. However, if everyone in proximity to the susceptible is immune to chickenpox (either by vaccination or prior infection), and so incapable of transmitting the infection to others, the chickenpox virus will be blocked from reaching the susceptible.

This is an especially important way to protect immunosuppressed children such as those being treated for leukemia. Children with leukemia usually cannot be immunized against chickenpox because both the vaccine virus and the wild type virus can cause severe—sometimes fatal—infection because of the child’s weakened immune system. But they can be protected if others around them are immunized.

Another example of indirect protection is the diphtheria vaccine. This vaccine protects against the worst forms of diphtheria illness; it also reduces the likelihood of becoming infected and spreading infection. Thus this is also a form of community immunity.

Community immunity only applies to diseases transmitted from person to person, such as measles, smallpox, rubella, and chickenpox, among others. Tetanus on the other hand is not spread from person to person but acquired from the environment, so there can be no community immunity—only the immunized person is protected.

Community immunity can only be achieved when the vast majority of the population is immune. In other words, a population needs high immunization rates to reap the benefits of community immunity.

Unvaccinated children are not only at greater risk of catching vaccine-preventable diseases but they can affect community immunity. 2 For instance, various studies have looked at the health consequences of exemptions from immunization laws. These studies have found that individuals claiming religious and/or philosophical exemptions from immunization (exemptors) are at a greater risk of contracting the diseases and thus put the rest of the population at risk by spreading infection.

For some diseases, such as measles, a population needs more than 94% immunity to achieve community immunity—or else the disease could spread in the susceptible population. This percentage is known as the community immunity threshold and it varies for each vaccine-preventable disease.

Reaching the thresholds for these diseases is important for the public health because no vaccine is 100% effective. For instance, measles vaccine protects 95% of those immunized; the other 5% are protected by the others being immune. One reason why a second dose of the measles vaccine is recommended is because it takes more than 94% of the population being immune to measles to get community immunity. In the end, only community immunity provides full protection for everyone.

It is important to understand that when a vaccine fails (which happens 5% of the time for measles), the chances of being infected are reduced if everyone nearby has also received that particular vaccine. In other words, people in whom the vaccine fails can still be protected by the immunity of those around them.

Two recent measles outbreaks can illustrate what happens when immunization rates reach the threshold of a particular disease.

In 2003 a measles outbreak occurred in the Republic of the Marshall Islands (pop. 56,000), where immunization rates were below 75%—less than the community immunity threshold for measles. An infected tourist from Japan started an epidemic that resulted in 703 cases of measles, 56 hospitalizations and three deaths. 3 Infection occurred in all age groups and in both immunized (vaccine failures) and unimmunized people. However, most cases occurred in infants below the age when vaccine is given and in older individuals who were not immunized.

In contrast, there were two separate introductions of measles into Mexico (pop. over 100 million) in 2003 but only 41 cases occurred, mostly among unimmunized infants. Measles immunization rates in Mexico are over 95%. 4

Not all vaccine-preventable diseases have thresholds as high as 95%. The threshold for smallpox, for example, is between 80% and 85%. The table below shows the thresholds for some other diseases. *

Community Immunity Thresholds Estimates
For Selected Vaccine-Preventable Diseases *


Vaccine-Preventable Disease

Level of Community Immunity




94% or more

Whooping Cough






The values in this table are estimates and can vary according to the method used to calculate them. The number of factors involved in community immunity makes the calculation of exact thresholds very complex.

The protection offered by community immunity is evident from results of many vaccination programs and from theoretical estimations. These estimations tell us that for an infection to persist (spread) in a community, each infected individual must be able to transmit the infectious agent to other individuals. Otherwise, the disease will disappear.

Several factors involved in the transmission of infectious diseases concern the measurement of community immunity:

  • The biologic properties of the viruses or bacteria: Some diseases are more easily transmitted than others, thus affecting community immunity thresholds. The greater the transmissibility, the higher the threshold needs to be to prevent spread of the disease.
  • The rate of contact among susceptibles and immunes in a population: Susceptibles are protected by community immunity only when they interact with immunes rather than with other susceptibles.
  • The number of susceptibles in the population: Immunes have to so far exceed the number of susceptibles in any community that contacts between susceptibles are rare events.

All these factors can be used to predict the impact of an immunization program in a population and to set up immunization coverage goals. Thus community immunity plays an important role in public health.

Additional Reference

UK Department of Health. Flash animation of herd immunity.