What Is Herd Immunity?

How herd immunity works. Credit: NIAID

Have you heard the term "herd immunity" but aren't quite sure what it means? It is usually used when discussing vaccines and how they are used to protect entire communities or populations from illnesses.

How It Works

Herd immunity - technically called community immunity - occurs when a majority of the population is immunized against a specific illness, so those who are unable to be vaccinated are still protected because the disease is not able to spread through the community.

Many people that are at highest risk from illnesses like Pertussis (Whooping Cough) and meningitis may not be able to get the vaccines. Young infants, pregnant women and people with weakened immune systems are often not eligible for vaccines, yet they are more likely to develop complications or die from vaccine preventable diseases. When the rest of us get vaccinated against these diseases, they aren't able to circulate in our communities, protecting not only us, but those who are at high risk and unable to be vaccinated as well.

Another way to think of it is like an actual herd of animals. Say you have a herd of elephants and the baby elephant is in the middle of the herd. He is most vulnerable to an attack because of his size but because he is surrounded by the larger and stronger adults, he is protected.

Getting vaccinated makes us all stronger and able to help protect those with weaker immune systems that are in our communities.

Key to Herd Immunity

An important part of herd immunity to remember is that it takes a majority of the population getting vaccinated for it to work. For example, polio has been eradicated in the United States because nearly everyone alive today has been vaccinated against it or has immunity to it due to exposure when they were young.

What was once a devastating illness that killed and disabled thousands of Americans is non-existent in this country today because of vaccines and herd immunity. Even those who can't be vaccinated against it are protected because there are no longer outbreaks.

On the other hand, we have a vaccine to protect us against influenza, but because only about 40% of the US population chooses to get it, there is little to no herd immunity for this disease. If more people were vaccinated against the flu each year, it would have more difficulty circulating and would make fewer people sick.


"Community Immunity (Herd Immunity)". Health & Research Topics 21 Oct 10. National Institute of Allergy and Infectious Diseases. National Institutes of Health. US Department of Health and Human Services. 10 Nov 14.

"Community Immunity: How Vaccines Protect Us All". NIH News in Health Oct 11. National Institutes of Health. US Department of Health and Human Services. 10 Nov 14.

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