Herd immunity, how we achieve it, and why it’s crucial

Herd immunity, how we achieve it, and why it’s crucial

The COVID-19 pandemic introduced us all to terminology previously familiar only to doctors and scientists. Among those was “herd immunity.”

But just because you’ve heard or read it doesn’t mean you fully understand it, so let’s take a complete look at the term herd immunity, what it means, and why it is important in situations such as a pandemic.


What is herd immunity?

Herd immunity occurs when a large part of the population of a specific area becomes immune to a specific disease. When this happens, the disease itself – whether it stems from a virus or bacteria – has nowhere to go and thus either disappears from that community or becomes extraordinarily rare.


Why is herd immunity important?

Herd immunity allows a community to realize relative safety from specific infections. And that is most important for those members of the population who are at risk the most from disease – including those whose immune systems are either not fully developed (children) or are weakened (elderly and those who are immunocompromised due to other diseases or treatments such as chemotherapy).


How does herd immunity happen?

Herd immunity can occur through two methods:

  • Natural immunity, which occurs over time.


  • Vaccination immunity, which can be achieved quickly, but only if enough members of a community receive the vaccine.


Natural herd immunity

As a population is exposed to a disease, the body’s immune system creates antibodies to combat the virus or bacteria. Your body will get sick from the initial infection, but the process gives your immune system a knowledge of how to defeat the source of the infection. When this has happened to enough members of a community, herd immunity is achieved – though it could cause plenty of harm in the interim, as it takes a long time to achieve natural herd immunity.


Vaccine herd immunity

Vaccinations – also referred to as immunizations – do for your body what infection does without all the risk of actual infection. Many vaccinations – like those that combat COVID-19 – inject dead viruses into your body. Your immune system will see these dead viruses and attack them like they would a live virus – which can produce mild symptoms just like being sick. However, the difference is that the dead virus cannot reproduce inside your body and thus threaten your actual health. And yet it still allows your immune system to learn about the virus, produce antibodies and learn how to deal with the disease in question.

For instance, this is what happened with polio eradication in the United States. It also worked extraordinarily well for measles until its recent reemergence with the majority of reported cases among people who were not vaccinated against measles.


How do you know when herd immunity is achieved?

Scientists and doctors estimate that 70-90 percent of a community population must be immune to a specific disease to achieve herd immunity. But the only true determination is by measuring a disease’s reproduction number – also referred to as R0.

The reproduction number indicates the average number of uninfected people that a single infected person can pass the disease to – assuming those people aren’t already immune.

The R0 of each disease lets you know just how communicable/transmissible/contagious it is. The higher the R0, the more important herd immunity is. For example, a virus with a high R0 will easily pass from person to person, so if you are not immune to it, you are much more likely to become infected and thus sick.

Some viruses have extraordinarily high R0. For instance, measles is between R12-18, while chickenpox has an R10-12, meaning that one person with measles may pass the disease on to 12-18 people – and someone with chickenpox may pass it to 10-12 people (those of us old enough will remember just how common chickenpox outbreaks once were in the United States).1

What about COVID-19? The original version of COVID-19 was considered between R2-4, but mutations have since proven much more contagious, with the Delta variant registering R5.082 and Omicron registering between R8.23.

The higher a disease’s R0, the more people in a community must be immune in order to achieve herd immunity. For the initial strain of COVID-19, between 50-67 percent of the population of a community would need to be immune in order to protect the entire community.

Unfortunately, when COVID-19 exploded onto the globe it was completely new to humans – which is why it was referred to as a novel coronavirus – so it was able to pass from person to person unchecked for many months. In the interim, it mutated, becoming more transmissible in the process – which only increased the need for vaccination to help speed up the process of herd immunity. The uniqueness of the novel coronavirus also means that we are still learning about how long our immune systems maintain antibodies to counter the virus.

That is why vaccination, as well as boosters, is crucial to eventually achieving herd immunity. The good news is that vaccines are safe, reliable, and now readily available at a multitude of sources. Doctors and scientists are also constantly working and studying to help us overcome this latest virus and attain a goal of herd immunity.

1 https://www.news-medical.net/health/What-is-R0.aspx

2 https://pubmed.ncbi.nlm.nih.gov/34369565/

3 https://pubmed.ncbi.nlm.nih.gov/35262737/