What is Herd Immunity? 

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.