What is herd immunity?

Herd immunity obtained through a vaccine may be the key to the disappearance of the current pandemic. Here we discuss what this term implies.

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The term herd immunity is unknown to most people . Recently, due to certain statements in the political arena, it has become fashionable and is at the center of many debates regarding the coronavirus.

But what is herd immunity? How does it work both individually and collectively? We will see it in detail below.

A vision on an individual scale

In the first place, before entering fully into the subject, it is not superfluous to clarify what individual immunity is:

  • A person can become resistant to an illness once they have overcome it. The immune system is able to 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, depending on the pathology, with or without symptoms. In the case of COVID-19 there are many asymptomatic cases that develop immunity through the disease without even realizing it.

Having briefly defined this term, let us enter fully into the world of herd immunity.

Herd immunity in society

Herd immunity is a term that refers to an indirect method of individual protection . This happens when a large percentage of the population is immune to a disease and, therefore, those who have not passed it are still less likely to contract it.

We must see the spread of a pathogen as a spider web:

  • Each infected person can transmit the pathogen to a number of healthy citizens. The transmissibility of a virus is represented by the R0 value or basic reproductive rate .
  • If the coronavirus R0is 2 units, for example, this means that each infected person will transmit the disease, on average, to 2 other healthy people.
  • Therefore, a network is created in which each infected translates into more patients 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 stalemate, since it cannot be transmitted further. This may allow for direct braking or stopping the spread of a disease.

The more immune people, the more dead spots the virus will find when it spreads.

Vaccines base their existence on this mechanism , as they provide individual protection against diseases to healthy people. Thus, immunosuppressed individuals, who cannot be vaccinated, will have a certain degree of protection because they will be surrounded by people who are.

You may also be interested in: Terminology to consider in a pandemic.

Mathematics question

Herd immunity, like all epidemiological terms, follows mathematical models . When a critical proportion of the population has become immune to disease – either by infection or vaccination – the herd immunity limit (HIL) is reached .

From this point on, the pathogen is destined to disappear in time . This point comes when the disease shows a continuous endemic state , in which the number of infected does not increase or decrease exponentially.

In the calculation of this parameter, the R0 value mentioned above comes into play, since its formula appears such that, S being the proportion of the population susceptible to contracting the disease:

R0 * S = 1

Without going into more numbers and headaches, we will limit ourselves to saying that the lower the value S (susceptible population), the lower the value of R0. Thus, it is confirmed that the more immune, the less the disease spreads .

To find out more: Patient zero, research during the pandemic

Herd immunity and coronavirus

It may seem tempting, then, to let a high percentage of the population become infected, since logically this would end up stopping the disease according to the theory we have presented. This could be feasible if it were a harmless virus.

When there is a percentage, albeit a minimal one, of the possibility of the pathology being complicated in risk groups, this strategy should be automatically dismissed. Numerically speaking it might be feasible, but the lives at stake are an ethical and moral rather than a utilitarian issue.

However , for this reason, the coronavirus vaccine is being sought so quickly . The existence of an immunization will greatly decrease the proportion of healthy susceptible people, thus slowing the spread of the disease and, finally, ending the pandemic.


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