Herd Immunity

First sign of ‘herd immunity’ in small population groups is found in Pune.

  • In a first-of-its-kind finding for any Indian city, a new study conducted in Pune has revealed that close to 85% of the people who had been found infected with coronavirus in an earlier serosurvey had developed protective antibodies. In effect, they had acquired immunity from the disease.
  • The latest finding is the result of a follow-up study done in five prabhags (comprising three or four municipal wards each) of Pune where a serosurvey conducted earlier this year, in July and August, had found that, on an average, about 51% of the people had been infected with the virus.
  • Serosurveys, like the ones carried out in several other cities as well, estimate the extent of the disease spread by detecting antibodies among surveyed population groups.
  • However, the presence of antibodies only indicates that the person had been infected with the disease at some point of time. It does not mean that the infected person has also acquired immunity against the disease. Immunity comes only if the infected person develops what is called ‘neutralising’ or ‘protective’ antibodies, a small sub-set of antibodies.
  • This is the first study that followed up on an earlier serosurvey to detect the presence of ‘neutralising’ antibodies in infected persons. And though the researchers who carried out the study are careful not to suggest that the city was approaching ‘herd immunity’, this is the first documented case in the country where the infection rate in a population group had gone up so high that the concept of herd immunity could already be playing out.
  • Pune has so far reported 3.44 lakh confirmed cases of coronavirus infections, the third highest for any city, after Delhi and Bengaluru. These are the people who were tested and found to be positive. The city has also recorded over 7,200 coronavirus-related deaths, the maximum after Mumbai and Delhi.

What is herd immunity?

  • Herd immunity 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.
  • The premise is 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

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.
  • 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 a single case; an R0 of more than 1 indicates one person can spread the infection to multiple persons.
  • There are three ways in which an infection can spread in a community. The first scenario looks at a community that is not immunised. When two infectious cases, both with an R0 value of 1, are introduced, 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.

What is Serosurvey?

  • Serosurveys estimate the wider prevalence rate, because not every infected person shows symptoms and gets tested.
  • Seroprevalence studies (or serosurveys) estimate the share of the population that test positive for antibodies using serology tests. The presence of a specific antibody in a sufficiently high concentration will suggest that the tested person was previously infected.
  • Typically, such studies test individuals who are selected at random using sampling techniques that will allow scaling the results to the general population. You do not need to test everyone, or even a majority of the population —what we need is a randomly drawn set of individuals, provided that those who agree to participate in the test are not somehow systematically different from those who refuse.