What Is Herd Immunity?

Immunity across a population is called ‘herd immunity’. It is the indirect immunity that people in a population have to an infection because so many are already immune to it.

It’s been a rough couple years on Earth, and everyone is waiting for this dreadful pandemic to be over. People are constantly reminded to wear masks, get their booster dose, follow social distancing, eat right, exercise and follow every other little guideline for health and safety. The whole world is together in this crisis, patiently waiting for COVID-19 to cease being a pandemic. One way we could achieve this is through “herd immunity”.

That is true to some extent, but it isn’t that simple. Let’s find out why.

What does herd immunity mean?

Immunity across a population is referred to as ‘herd immunity’. It is the indirect immunity that people in a population have to an infection because so many are already immune to it.

Let’s look at herd immunity with a simple example. Suppose you are susceptible to a mean little pathogen (for example, a bacterium) and are sitting in a small classroom with five other students. The bacterium is cruising around, looking for hosts to infect. Only after it infects someone can it grow, multiply and then spread to infect others.

Fortunately for you, four out of the five students are immune to the bacterium. This means that if the bacterium tries to infect them, their immune system will attack and kill it. It won’t be able to grow and will eventually die out, keeping you and the other susceptible person safe.

It’s not like those four other people’s immune systems protect you from infection. They are healthy hosts that stop the bacteria from growing inside them and spreading. This lowers the probability of the bacterium successfully infecting someone and multiplying, thus reducing its overall infectiousness. That’s herd immunity! It provides indirect protection by preventing disease outbreaks.

Herd immunity indirectly protects susceptible people. (Photo Credit : BrainCityArts/Shutterstock)

On the other hand, in that classroom, let’s say that instead of four, only one person was immune to the bacterium. The chances that the bacteria can infect a susceptible person are higher. If this happens, the bacterium can grow, multiply and infect other susceptible people much easier. Thus, there is a greater risk of infection. Herd immunity only works when the majority of a population are immune to infection.

A diagram illustrating herd immunity’s protective effect. (Photo Credit : Joebite/Shutterstock)

The term herd immunity was coined by Dr. Wilson in 1923. He published a paper at the time on bacterial infection spreading. It was the first published piece where ‘herd immunity’ was mentioned.

How does herd immunity form?

Immunity to a pathogen can come naturally or it can be artificially acquired. The difference lies in the way we are exposed to the pathogen.

If you’re exposed to a pathogen from the environment over the normal course of events, it builds natural immunity. To understand how herd immunity forms naturally, we need to look at infection dynamics. Let’s take a look at a slightly more fun example.

Suppose a small meteorite crashes into town. Luckily, the impact doesn’t harm anyone, but the meteorite does carry an unknown and highly infectious space germ. No one has any pre-existing immunity to it.

The new germ makes its way into the town’s environment and starts an epidemic. In the beginning, everyone is susceptible to it, and the germ easily infects any individual it comes in contact with. Luckily for us, this new germ doesn’t kill people, it just makes them feel sick for a few days. As newly infected people come across more susceptible people, the pathogen spreads and infections rise dramatically.

Eventually, people start to recover. As each person in the town gets sick and recovers, that’s one less susceptible person. Finally, there’s a point where the majority have contracted the new infection and have recovered. Those people have obtained natural immunity and won’t easily fall sick again when they reencounter the mystery space germ.

That’s the herd immunity threshold. It is the number of people in a population that need to become immune to achieve herd immunity.

Acquired immunity is vaccine-oriented. Artificially injecting pathogens via vaccines forces pathogen exposure, making our body launch an immune response.

Vaccines help in acquiring herd immunity. (Photo Credit : Syda Productions / Shutterstock)

Sometimes, letting people naturally acquire diseases isn’t a smart choice. Many germs aren’t as kind as our mystery space germ and can cause life-altering diseases, such as the polio or measles virus. This is when public health officials hold vaccination programs to immunize populations safely. Vaccines help in reaching the herd immunity threshold faster and safely.

However, that’s not the whole story, as herd immunity can fade with time.

How is herd immunity lost?

Unfortunately, herd immunity isn’t permanent, though there are a few reasons why populations can lose it.

Sometimes, a person’s immunity to an infection fades over time, such as what happens with whooping cough. According to the CDC, immunity to whooping cough can last anywhere between 4-20 years. After that, an individual can once again go back to being susceptible. In other words, a population can lose its herd immunity with time as their immunity levels fall below the threshold value.

Sometimes, immunity can be lifelong, but another reason herd immunity is lost is “the circle of life”. There is one thing every living being must face – death. As immune people die and susceptible new lives are born, the population’s immunity decreases.

With time, even the pathogen undergoes mutations, increasing its potential to evade immune systems and become more infectious.

Populations can suddenly or gradually lose their herd immunity.

Herd immunity is lost in all these different ways. However, vaccines help fight this. Having rolling vaccination programs, such as those for polio or rubella, help to maintain the herd immunity threshold.

A Final Word

Herd immunity is vital for controlling infectious diseases. We don’t have 100% effective preventive measures, so obtaining herd immunity provides an extra protective layer to susceptible people.

That said, it’s important to note that herd immunity isn’t foolproof. It prevents disease outbreaks and reduces the chances of susceptible people coming into contact with the pathogen. It does not, however, make the risk ZERO.

Furthermore, for herd immunity to be at its best, immunity should be distributed evenly amongst a population. For example, take a populated city like New York. If only a small part of the city has 100% immunity, but the remaining doesn’t, this will not be very helpful. The herd immunity threshold requires immunity to be randomly and almost evenly distributed.

As we’ve witnessed with the pandemic, simply waiting for herd immunity isn’t going to be painless or easy. It can take years, and we may not be out of the woods even after achieving herd immunity. That’s why we should do our part to maintain it any way we can!

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