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What is Herd Immunity?

Mary McMahon
Updated Mar 03, 2024
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In immunology, herd immunity refers to a situation in which a high percentage of a population is immune to a disease, essentially stopping the disease in its tracks because it cannot find new hosts. You may also hear this concept referred to as “community immunity.” The threshold for herd immunity varies, depending on the disease, with more virulent agents requiring vaccination of a higher percentage of the population to create the desired immunity. In addition to being used in disease prevention, community immunity is also utilized to fight ongoing outbreaks.

Most vaccination policies are focused on creating community immunity. Many countries require vaccinations in childhood, for example, protecting children from common diseases and ensuring that when these diseases enter the population, they cannot prey on children or adults, who have been previously vaccinated or exposed to the disease. The creation of herd immunity is especially important in crowded environments which facilitate the spread of disease, like schools.

Immunologists try to prevent the outbreak of diseases by creating widespread immunity, but they are not always successful. Sometimes a disease mutates or is entirely new, or a batch of vaccinations is faulty, or a large percentage of the population fails to get vaccinated, creating a situation in which an outbreak can occur, because much of the population is vulnerable. In the event of an outbreak of a major disease, agencies like the World Health Organization (WHO) can dispatch teams within days to figure out the cause of the outbreak and develop a vaccine, in the hopes of creating herd immunity to halt the outbreak in its tracks.

For some diseases, immunity thresholds are as low as 50%, especially when combined with good hygiene. In other instances, up to 90% of the population may need to be vaccinated to create the desired protection. It is also extremely important to receive regular boosters, as some vaccines lose their efficacy over time, leaving people vulnerable to an outbreak. Herd immunity led to the eradication of smallpox, and it explains why diseases such as polio and diphtheria are rare in developed nations with established vaccination policies.

The concept of herd immunity is often used to encourage reluctant parents to vaccinate their children. In addition to ensuring that their children are protected from fully preventable diseases, childhood vaccinations also benefit society at large by creating widespread protection from disease. Likewise, adults may be reminded to receive boosters to help protect their communities.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a The Health Board researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By anon280811 — On Jul 20, 2012

To those who don't want to take vaccines: Don't take the vaccines. Don't vaccinate your children. It's fine. You are not allowed to cry and moan when your children or grandchildren die of something which is easily preventable. Don't do it. Let your kids get polio, smallpox, measles and chicken pox. As for herd immunity, who cares? Let an evolved smallpox for which a vaccine will not work, wipe us all out.

By anon252246 — On Mar 05, 2012

Public cleanliness programs are what brought about the disappearance of these diseases, not vaccines. The numbers show the already declining numbers, well before vaccines came onto the scene.

People lie, numbers don't.

I should also add, in many other countries where they don't vaccinate, these diseases don't exist.

By anon160504 — On Mar 16, 2011

Vaccination is proven, galen. In 1967 alone, 2 million people died from smallpox. now thanks to huge vaccination campaigns across the world smallpox has been eradicated. Immunisation works.

By anon145773 — On Jan 24, 2011

you vaccinate for tetanus because a tetanus vaccine protects you from tetanus for 10 years.

By anon140442 — On Jan 07, 2011

Galen, I think you might have something there. Otherwise, why to crop scientists worry so much about crop refuge areas?

By StreamFinder — On Oct 30, 2010

So what about the logic behind immunizations for tetanus and the like. There's not a herd immunization theory attached to that, since tetanus isn't contagious.

Could you give me some more vaccine logic information? I'm really interested about the whole herd immunity thing, and I can see how it would work for something like a hepatitis immunization, but what about other conditions?

By galen84basc — On Oct 30, 2010

I had heard that herd immunity was a myth. There's a book by Dr. Russel Blaylock that claims that the whole herd immunity theory is responsible for mass over-vaccinations that could actually be dangerous for society.

It's kind of like the logic behind the over-use of antibiotics. If you immunize everyone, then the disease can just grow stronger and stronger until it finds a way to overcome the vaccine.

And if that's not scary enough, then think of all the immunization side effects -- some people even die from getting immunizations!

Although I'm not totally against vaccinations and immunization programs, I do think that it merits some thought. We really don't want to be trading out a short term immunity for a long-term fight with super-diseases.

Just some food for thought.

By pharmchick78 — On Oct 30, 2010

You really can't stress enough the importance of herd immunity in enclosed areas like schools and very tightly packed urban cities.

Disease spreads quickly through human contact, and if immunity is not acquired, then much of the "herd" or group can fall ill.

This is why you so often hear of entire schools coming down with a cold at the same time, or epidemics starting in big cities. When everybody is just so closely packed together, it's difficult to avoid the spread of disease.

However, if you have taken the precautions of creating or encouraging a herd immunity, the disease suddenly stops. It may not have died out per se, but with the herd no longer vulnerable to it, there's no longer any risk of the disease becoming epidemic within the group.

And herd immunization is especially important for children, who tend to have less capable immune systems than adults. That's why one of the biggest proponents of herd immunity is the CDC. When you take the precautions to have hepatitis vaccines and the like -- in this case, an ounce of prevention is worth much more than a pound of cure.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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