What is Herd Immunity? 

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Remember, in the early days of the coronavirus pandemic, epidemiologists and other experts were debating whether it was better to lockdown or let the virus spread to achieve herd immunity.

It is a question we’re always faced with when a new disease (for which we have no protection) suddenly develops in a population.

In this article, we’ll explore what herd immunity is, how it works, and why it’s such an important concept in epidemiology.

What is Herd Immunity?

Herd immunity, also known as community immunity or population immunity, is an indirect form of protection from infectious diseases.

It occurs when a specific proportion of a population becomes immune to an illness. Once a threshold is reached, the spread slows down or stops altogether.

This means individuals who aren’t personally immune (either they’ve not previously had the disease or have not received the vaccine) receive a degree of protection thanks to the prevalence of immunity around them.

After all, you can’t contract a disease from a person who can’t have it.

How It Works

Imagine a society where nobody is immune to a particular contagious disease. If a single person gets infected, they can easily pass the disease on to others they come in contact with.

However, if a large portion of that society becomes immune, the infected individual will likely encounter immune people, preventing the disease from being passed on.

That confers protection to the entire community, including vulnerable groups like infants, elderly individuals, and those who cannot be vaccinated for medical reasons.

Think it another way: envision a scenario in which many individuals are densely packed together, like dominoes. If one domino (representing an infected individual) falls, it can potentially knock over many others in its path.

However, if certain dominoes are removed from this setup (representing immune individuals), the chain of falling dominoes can be interrupted or stopped altogether.

The key is to have enough immune individuals (or removed dominoes) to disrupt the chain and prevent extensive spread. The percentage of the population that must be immune to achieve herd immunity varies for each disease.

For some diseases, such as measles, a very high percentage (around 95%) of the population needs to be immune to stop its spread. Other diseases might require a lower percentage.

How to Achieve Herd Immunity

There are two primary ways to achieve this immunity:

  1. Natural Immunity: Some people gain immunity after surviving and recovering from the actual disease. However, this method can be risky, as the disease might cause severe complications or death in some individuals.
  2. Vaccination: A safer and more controlled method is through vaccination. Vaccines mimic the disease in a way that triggers the immune system but doesn’t cause the disease itself. This way, the body “learns” how to fight the real disease without having to suffer through it.

Herd immunity doesn’t mean a disease will be eradicated entirely, but it can keep the disease under control and protect vulnerable groups more susceptible to complications.

Achieving herd immunity is especially critical in preventing outbreaks and is a primary goal in public health strategies worldwide.

How Effective Is Herd Immunity

Herd immunity relies on collective participation. Simply put, it only works when the majority of the population actively supports it.

The Association for Professions in Infection Control and Epidemiology (APIC) points out this significant limitation. Herd immunity cannot be achieved unless a community holds the same view on vaccination or natural immunity.

A classic example of herd immunity’s vulnerability is the resurgence of measles cases during the mid-2010s.

Despite measles being eliminated in the US in 2000, a spike in cases emerged a decade or so later due to reducing immunization following the measles-mumps-rubella (MMR) vaccine scandal in the 1990s.

Not only did these unvaccinated clusters contract the illness, but they also increased the spread among the population threatening individuals who could not receive the vaccine or chose not to.

Another aspect to consider is the performance of vaccines and the body’s corresponding immune response. Recent research has revealed a phenomenon called “waning immunity” associated with the measles-mumps-rubella (MMR) vaccine, especially concerning mumps.

Studies indicate that despite receiving a complete vaccination course, some individuals can see their immunity to mumps diminish over time. Such instances underscore the importance of booster vaccinations, especially during outbreaks.

Final Thoughts

Herd immunity is a cornerstone of epidemiology: a communal shield against infectious diseases. But for it to work well, most of us need to be on the same page, either through past infections or vaccines.

When too many skip vaccines or are misinformed, gaps appear in our shield, putting everyone at risk. So, we all must learn, understand, and act together for everyone’s health.


Written by: Emmanuel J. Osemota

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