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Herd Immunity Explained

Written by Christine Jahn Santos

June 20, 2020

Herd Immunity has been talked about more often because of the rise of the Covid-19. So what exactly is it?

Simply explained, herd immunity is when the majority of the population, usually around 80%, has developed an immunity to an infectious disease. This results in the minority, who haven’t developed immunity, to gain indirect protection against the disease. This is because the more people who are immune, the harder it is for the virus to infect a whole population. Herd immunity, however, is only relevant when talking about infectious diseases spread through person to person contact. The best example of an exception would be tetanus. No matter how many people around you have been vaccinated against tetanus, they will not help in preventing you from tetanus.

There are two ways for people to get immunity: through vaccination, or through contracting the infection and fighting it off.

To better understand the idea of herd immunity, imagine 10 balls in a box just rolling around and hitting each other. Now imagine that there is one red ball (the infected) and the other 9 are yellow (the compromised). In a population that does not have herd immunity, almost everyone can easily get infected. In this first scenario, every ball that the red one hits will also be infected. It’s easy to imagine how all of the balls would turn red in a short amount of time. Now, imagine the same scenario but this time with one red ball, 7 green balls (the immune), and 2 yellow balls. This would represent a population with herd immunity. The red ball cannot turn the green balls red because the green is immune. When the red ball starts to hit the other balls, there is less chance of hitting one of the yellow balls because there is a larger amount of green balls than there are yellow and red. This means that there would be fewer people infected with a disease and the disease would spread slower in a population with herd immunity.

The immunocompromised benefit greatly from herd immunity because they can’t be vaccinated. If they did get vaccinated, they risk actually contracting the disease even if the vaccine is weakened because of their compromised immune systems. This group of people includes the elderly, very young kids, and people with underlying health issues. They practically have to rely on herd immunity to keep safe from the infection.

The best example of a herd immunity working is measles. Before the release of an effective measles vaccine, there were 400-500 deaths annually. But when most of the population in the U.S. was vaccinated, measles was announced to be eliminated in 2000.

So what does this mean for the COVID-19 situation?

Even though many scientists and researchers have been working on an effective vaccine for the public, the process may take an extended amount of time. The best way to cope with this pandemic is to be patient for the development of a vaccine and establish herd immunity, practice social distancing, and follow state “shelter-in-place laws” as described by Johns Hopkins Bloomberg School of Public Health.

Works Cited:

D'Souza, G., & Dowdy, D. (2020, April 10). What is herd immunity and how can we achieve it with COVID-19? Johns Hopkins Bloomberg School of Public Health.


Helft, L., & Willingham, E. (2014, September 5). What is herd immunity? PBS.

The Infographics Show. (2020, May 15). Coronavirus herd immunity - explained

[Video]. Youtube.

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