What's Herd Immunity or the Herd Effect?

Herd immunity obtained through a vaccine can be key to the disappearance of diseases. Today, we'll discuss what it is and how it works.
What's Herd Immunity or the Herd Effect?

Last update: 21 August, 2020

The terms “herd immunity” or “herd effect” are relatively unknown to most people. Recently, however, the concept has become widely discussed due to certain statements in the political arena and is at the center of many debates.

But what is herd immunity? Also, what are its collective and individual benefits?

Continue reading to find out!

A vision on an individual scale

First, let’s be clear about what immunity is:

  • A person becomes resistant to many illnesses once they’ve overcome them. Their immune system can remember certain elements of the threat, thus recognizing it faster in reinfections and sending the antibodies to destroy it before it reproduces.
  • This can happen with or without symptoms, depending on the virus. Occasionally, there are asymptomatic cases that develop immunity by going through the disease without even knowing it.

Having briefly defined this term, let’s take a look at herd immunity.

Herd immunity in society

This term refers to an indirect method of individual protection that happens when a large percentage of the population is immune to a disease. Therefore, those who haven’t undergone it are less likely to contract it.

An illustration about COVID-19.

The spread of a disease is sort of like a spider web:

Each infected person can transmit the pathogen to several healthy people. The transmissibility of a virus is represented by the R0 value or basic reproductive rate.

For example, if the R0 is two units, it means each infected person will transmit the disease, on average, to about two other healthy people. Thus, it creates a network in which each infected person will infect more people over time.

The basic principle of herd immunity is to cut this expansive dynamic. The fact that the virus reaches an immune person represents a stop in the transmission, as it cannot be spread further. This can directly break and stop the spread of a given disease.

The more immune people there are, the more dead ends the virus will encounter as it tries to spread.

Vaccines base their existence on this mechanism, as they provide individual protection against diseases to healthy people. Thus, immunosuppressed individuals who cannot process vaccines will have a certain degree of protection. This is because they’ll be among people who are immune with the help of a vaccine.

A mathematical question

Herd immunity, like all epidemiological terms, follows a mathematical model. When a critical proportion of the population is immune to disease – either by infection or vaccination – one can say there’s herd immunity.

From this point on, the pathogen should disappear over time. This point comes when the disease shows a continuous endemic state in which the number of infected people doesn’t increase or decrease exponentially.

In the calculation of this parameter, the R0 value mentioned above comes into play. S is the proportion of the population susceptible to contracting the disease:

R0 * S = 1

Without going into more detail about numbers, we’ll limit ourselves to saying the lower the value S (susceptible population), the lower the value of R0. Thus, the more immune people there are, the less the disease will spread.

A group of medical staff wearing masks.

The herd effect and pandemics

It may seem tempting, then, to let a high percentage of the population become infected. Logically, this would eventually stop the disease. (At least according to the theory we mentioned above).

However, this would only be feasible if the virus was relatively harmless.

When there’s even a tiny percentage that a given disease could be bad for groups at high risk of contracting it, health authorities will immediately dismiss this strategy. It might be feasible, numerically speaking, but the lives at stake are real – not just numbers.

In any case, we must develop vaccines with which to quickly stop diseases. The existence of immunization will greatly decrease the proportion of healthy susceptible people, thus, slowing the spread of the disease and eventually ending the pandemic.

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