Herd Immunity

  • 20 Mar 2020

  • The outbreak of disease caused by the new coronavirus (COVID-19) has raised questions about a phenomenon known as "herd immunity" and whether it might play a role in how the pandemic progresses.

What is Herd Immunity?

  • It refers to preventing an infectious disease from spreading by immunising a certain percentage of the population.
  • While the concept is most commonly used in the context of vaccination, herd community can also be achieved after enough people have become immune after being infected.
  • For herd immunity, it doesn't matter whether the immunity comes from vaccination, or from people having had the disease. The crucial thing is that they are immune.
  • With the new coronavirus infection, as more and more people become infected, there will be more people who recover and who are then immune to future infection.

How does Herd Immunity work?

  • The scientific principle is that the presence of a large number of immune persons in the community, who will interrupt the transmission, provides indirect protection to those who are not immune.
  • It means that if a certain percentage of the population is immune, members of that group can no longer infect another person.
  • This breaks the chain of infection through the community (“herd”), and prevents it from reaching those who are the most vulnerable.
  • To estimate the extent of spread and immunity, epidemiologists use a measure called the ‘basic reproductive number’ (R0). This indicates how many persons will be infected when exposed to an single case; an R0 of more than 1 indicates one person can spread the infection to multiple persons.
  • On the basis of the available evidence from China, and according to various experts, R0 COVID-19 ranges between 2 and 3.
  • This means that, if no other measures are taken, herd immunity would kick in when between 50% and 70% of a population is immune.

Recent Example of Herd Immunity

  • Zika virus, a mosquito-borne illness that caused a epidemic panic in 2015 because of a link to birth abnormalities.
  • Two years later, in 2017, there was no longer nearly so much to worry about.
  • A Brazilian study found by checking blood samples that 63% of the population in the northeastern beach city of Salvador had already had exposure to Zika; the researchers speculated that herd immunity had broken that outbreak.

Spreading of Infection in Community

There are three ways in which an infection can spread in a community.

  • The first scenario looks at a community that is not immunised. With two infectious cases, there is a possibility of the entire community being infected, with a few exceptions.
  • In the second scenario, there may be some persons who have been immunised; and only these immunised persons will not be infected when at least two infectious cases are introduced in the community.
  • The third scenario is when the majority of the community is immunised. So, when two infectious cases are introduced, the spread can take place only in exceptional cases, like in the elderly or other vulnerable persons.Even in such a situation, the immunised persons protect the non-immunised by acting as a barrier — which is herd immunity.

When does a population achieve Herd Immunity?

  • It depends on multiple factors like-
  • how effective the vaccine for a given disease is
  • how long-lasting immunity is from both vaccination and infection
  • which populations form critical links in transmission of the disease.
  • Mathematically, it is defined on the basis of a number called “herd immunity threshold”, which is the number of immune individuals above which a disease may no longer circulate.
  • The higher the R0, the higher the percentage of the population that has to be immunised to achieve herd immunity.

For eg.

  • Polio has a threshold of 80% to 85%, while measles has 95%.
  • With the current data for COVID-19, experts have estimated a threshold of over 60%. That means more than 60% of the population needs to develop immunity to reach the stage of herd immunity.

Why is herd immunity as a strategy against COVID-19 questionable?

  • It is very risky to seek herd immunity by allowing a large proportion of the population to get infected.
  • Allowing the virus to pass through the population means a surge of patients, putting pressure on existing medical infrastructure.
  • Further, it can take months, or even longer, to build group immunity to COVID-19. During that time, the need is to protect people who are at greater risk.
  • However, the coronavirus is a new strain of which we do not know enough. Much about the behaviour of the COVID-19 is still unclear. Moreover, in this scenario, when there is a significant mortality rate, talking about herd immunity would be controversial at best.

So, what are the ways to minimise further spread of COVID-19 infection?

Flattening the Curve

  • Flattening the curve means preventing the number of daily cases from peaking over the numbers we can treat.
  • This will minimise burden on medical staff. For this, we need to practise interventions, mainly social distancing.

Social Distancing

  • This is the act of keeping social contact to a minimum to pre-emptively minimise the spread of the disease, by staying home as much as possible, and avoiding crowds, public places, transport. This is not the same as being in quarantine or isolation.

Quarantine and Isolation

  • Quarantine is needed in a situation where one may be infected, because he/she  have been exposed to high-risk people, but have not been tested yet.
  • Once you are tested positive, however, that person is put into an isolation facility. It has a strict criterion for the type of air filters, flooring and walls.