What Is Herd Immunity?

You may have become familiar with the term herd immunity in discussions of COVID-19. A simple herd immunity definition is that it's a type of immunity where most people in a population are immune to a disease or illness.

Herd immunity can occur naturally or with the help of vaccines, so it's potentially achievable in some cases. Read on to learn more about this kind of immunity, including the importance of herd immunity and other ways to reduce disease transmission.

Herd Immunity Definition

Herd immunity—or community immunity—occurs when enough people in a population are immune to a disease. This type of immunity helps make person-to-person transmission unlikely, and the infection, in effect, no longer has the upper hand.

What Is an Example of Herd Immunity?

The percentage of a community that needs to be immune to achieve herd immunity varies from disease to disease. Herd immunity against measles, for example, requires about 95% of the population to be inoculated; for protection from polio, it's about 80%.

Why Is Herd Immunity Important?

For many diseases, a certain percentage of the population must be capable of contracting the disease for it to continue to spread. However, once the proportion of the population that is immune to the disease exceeds the one capable of contracting it, the transmission of the disease will slow down.

How Is Herd Immunity Achieved?

Herd immunity can be achieved in one of two ways: through prior illness or vaccination.

Prior Illness

Your immune system naturally develops infection-fighting antibodies when it encounters a disease. After you recover, you're left with some level of antibody protection against the illness.

Vaccination

Vaccines, by contrast, work by training your immune system to create antibodies so that when you encounter the disease, your body will quickly mount a defense. That's why, in part, herd immunity has been considered for COVID, especially since COVID vaccines are available.

The more people who are vaccinated, the less opportunity the virus has to replicate and spread. At some point, the community—or "herd"—is protected, even people who aren't immune or cannot be vaccinated, such as newborn babies.

Is Herd Immunity Possible?

Herd immunity can be possible. The key to herd immunity is that there are too few susceptible hosts around to maintain transmission even if a person becomes infected. Those who have been vaccinated or have already had the infection cannot contract and transmit the virus. However, for some diseases and conditions, it can be hard or impossible to achieve herd immunity.

For example, the emergence of new variants of SAR-CoV-2—the virus that causes COVID—means that sterilizing immunity provided by infection or vaccination will be difficult to achieve. Sterilizing immunity means an individual can no longer be infected or infect others.

Also, while vaccines offer strong protection against severe illness, they are not 100% effective at preventing infection. Herd immunity is only relevant if there is a transmission-blocking vaccine. The only way to get herd immunity in the population without this type of vaccine is to give everyone a vaccine.

How Can You Slow the Transmission of Diseases?

There are different ways to help slow disease transmission, including:

• Avoiding contact with individuals who are sick
• Covering your nose and mouth—with a tissue or your elbow—when coughing and sneezing
• Getting vaccinated against diseases, when recommended, available, and appropriate
• Taking medications for treatment exactly as they are prescribed
• Using the proper protection like masks if you have to care for or have to have contact with someone who is sick
• Washing your hands with soap and water or using an alcohol-based hand sanitizer when soap and water are not available

A Quick Review

Herd immunity is the result of a population becoming immune to a particular disease. It may be achieved with natural immunity developing after becoming sick with an illness or through vaccinations.

How much of the population needs to be immune to reach herd immunity depends on how contagious the disease is. Though herd immunity may not always be possible, there are still ways to prevent or reduce the transmission of infectious diseases.