Herd Immunity: What Is It And How Is It Achieved?

Herd immunity obtained through the use of a vaccine can be fundamental for the disappearance of some diseases. In this article we explain the meaning of this expression.
Herd immunity: what is it and how is it achieved?

Following some political statements, herd immunity has been at the center of numerous beats in recent times.

But what is meant by herd immunity? How does it work individually and collectively? We talk about it in the following lines.

A vision on an individual scale

Before getting to the heart of the subject, it may be useful to clarify the meaning of individual immunity:

  • A person can develop resistance to a disease after they are cured. The immune system remembers some elements that belong to the pathogen, thus recognizing it more quickly in the case of relapses; it therefore activates the antibodies necessary to destroy it before it can reproduce.
  • Depending on the pathology, this phenomenon may or may not have symptoms. Some asymptomatic cases develop immunity by contracting the disease without knowing it.

After briefly defining immunity at the individual level, we invite you to get into the full concept of herd immunity.

Herd immunity in society

Herd immunity refers to an indirect method of individual protection. It occurs when a large percentage of the population becomes immune to a disease, consequently others are less likely to contract it.

The immune system's response to viruses.

We need to consider the spread of a pathogen like a spider’s web :

  • Each infected person can pass the pathogen to a number of healthy individuals. The transmissibility of a virus is represented by the R0 value or base reproduction number.
  • If, for example, R0 corresponds to 2 units, it means that each infected person will transmit the disease, on average, to 2 healthy individuals.
  • Thus, a network is created in which each infected individual leads to an increase in the number of sick people over time.

The fundamental principle of herd immunity is to stop this dynamic of expansion. An immune person is a dead end for the virus, because at this point its spread will stop. It is thus possible to slow down or directly stop the spread of a disease.

Vaccines base their raison d’ĂȘtre on this very mechanism, because they provide healthy people with individual protection from disease. In this way, immunosuppressed individuals, who cannot be vaccinated, will enjoy a certain degree of protection, because they will be surrounded by people who have received the vaccine, therefore immune.

A question of mathematics

Herd immunity, like all epidemiological expressions, develops according to mathematical models. When a critical part of the population has obtained immunity from a certain disease (whether by infection or by vaccination), the limit that determines herd immunity (English acronym: HIL) is reached.

From this moment on, the pathogen is destined to disappear over time. This point comes when the disease manifests a continuous endemic state, in which the number of infected individuals neither increases nor decreases exponentially.

In the calculation of this parameter, the R0 value mentioned above comes into play. Calling S the part of the population at risk of contracting the disease, its formula looks like this:  R0 * S = 1.

Without going into numerical issues that could give us a headache, we will just say that the lower the value of S (susceptible population), the lower the value of R0. Thus, it is confirmed that the more immune people are, the less the disease spreads.

Medical staff with masks.

Herd immunity and pandemics

Therefore, the temptation could arise to let a high percentage of the population become infected, because, according to the theory we have set out, the logical consequence would be to stop the infection. This course of action may be applicable if it is a harmless virus.

When there is a possibility, however small, that the disease could lead to death, this strategy must be automatically discarded. In purely numerical terms it might be applicable, but the lives at stake raise an ethical and moral question,  rather than a utilitarian one.

This is why vaccines are being worked on to stop the rapid spread of diseases. Immunity will dramatically decrease the number of healthy people at risk of contagion thus ending the epidemic.

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