Herd immunity: this is why it is so important

The term “herd immunity” is encountered again and again at the moment. But what does this type of immunity actually mean and how does it work? We have summarized the most important information for you.

Herd immunity is not just about being immune to certain pathogens and viruses, but about protecting the community.

First, what is immunity?

Basically, immunity describes a condition in which the organism is insensitive to pathogens and viruses. This means that a person can no longer get sick from these pathogens at least for a certain period of time.

In order to understand what immunity means and how it works, an excursus into biological processes is necessary: ​​In order for immunity to a pathogen or virus to develop, the immune system must first come into contact with one. Your immune system then forms antibodies so that it can react more quickly if there is a renewed infection and you usually no longer get sick for a certain period of time. However, immunity does not automatically mean that the virus cannot be passed on, i.e. that other people are not infected. This is only the case with so-called sterilizing or sterile immunity.

Vaccinations make use of this protective mechanism. Vaccines train the immune system and prepare it for a potential infection with a certain pathogen.

Herd immunity is about breaking chains of infection and stopping the spread of a disease. This is particularly important in the case of highly contagious viruses, such as the SARS-CoV-2 or Covid-19 virus.

This is how herd immunity works

Herd immunity is when a certain percentage of the population is immune to a disease. How large this proportion must be is calculated using the reproduction number of the respective virus, also known as the R value. The Reproduction number indicates how many other people an infected person infects on average. This R value varies depending on the virus.

• The Robert Koch Institute (RKI) sets the basic reproduction number R₀ for SARS-CoV-2 between 3.3 and 3.8 fixed. This base value describes the average infection of others if no hygiene and protective measures are taken and no one is immune. This means that each person without measures infects an average of 3.3 to 3.8 other people and each infected person again infects other people (keyword infection chain).
• The so-called effective reproduction number applies when protective measures are taken to contain the spread and some people are already immune. It is correspondingly lower than the basic reproduction number.

The goal is to get the R-value to reduceso that it goes to zero even without protective measures. This means that a sufficiently large part of a population must be immune to a virus, i.e. herd immunity must be created. Specifically, the R-value should drop below 1. Then one infected person infects less than one other person on average, the number of infections drops and, in the best-case scenario, the virus is no longer spread at some point.

To come full circle, in terms of immunity, if a person is immune, they can no longer (for a period of time) be infected by an infected person. This breaks the chain of infection. As a result, fewer people are infected or become sick with a falling R-value. The virus is still passed on among non-immune people, but the number of people infected is no longer increasing.

Immunity can be generated by a disease (formation of antibodies) or by a vaccination (preparation of the immune system), according to the Association of Research Drug Manufacturers eV. Especially for the corona virus, which is currently rampant, immunity has not yet been clearly demonstrated during recovery or vaccination, and is still not examined – but it is likely. A second infection (reinfection) is possible, but according to current knowledge, the symptoms then decrease milder out. Besides, there are already Studies, which show that the risk of a second infection is lower after recovery.

In principle, the following also applies: A vaccination helps to create herd immunity and thus to spread the virus fight.

This is why herd immunity is important and it has to be taken into account

Herd immunity comes with significant benefits that make this development so important:

• Containment: The spread of a virus / disease is contained and a pandemic can be better combated.
• Herd protection: As soon as enough people are immune, the broken chains of infection also protect others. Even those who have not been vaccinated, such as babies or immunocompromised people, are more protected.

The importance of herd immunity is illustrated by a Simulation of the RKI clearly visualizing different scenarios.

Even if herd immunity has a positive effect on the community, there are a few other aspects to consider:

• Average value: The R-value is an average value. It is possible that some infected people infect a few people and some infected very many people. As a result, it is not possible to give an exact indication of herd immunity and the proportion of immune people required for this. Thus, the limit to herd immunity cannot be generalized.
• Virus mutations: With regard to herd immunity, new mutations that are more contagious than the virus of origin are problematic. Then the R-value increases and a higher proportion of immunity in the general population is necessary before one can speak of herd immunity. That said, more people need to be immune to achieve herd immunity.
• Connection with protective measures: The combination of protective measures and a high proportion of immune people can ensure (supposed) herd immunity. If protective measures are not taken, it is possible that the number of infections and thus the R-value will rise again. This means that the proportion of those who are immune must in turn be higher.

The video by the doctor of chemistry and science journalist shows more about herd immunity and critical backgrounds Mai Thi Nguyen-Kimwho wrote the topic for ARD and ZDF (YouTube channel “maiLab”) clearly presented:

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