Context: A recent serological survey in national capital found the presence of coronavirus-specific antibodies in about 23% of the samples tested.

More on the news:

  • The results of that survey are being interpreted to suggest that about 46 lakh people in Delhi could so far have been infected with the COVID-19, and that “herd immunity” could be approaching.
  • However, the scientists caution against drawing such broad conclusions. The serological surveys were carried out to answer a very specific question. 

About the recent serological survey

  • It was meant to detect whether the person being tested had developed antibodies against the COVID-19.
  • The recent survey was carried out to assess how widespread Covid-19 could have become.
    • As it is not possible to test everyone, it is still not clear how many people in the population are infected, especially since most of the patients are asymptomatic
    • However, detecting antibodies in random sets of people is an indirect way of estimating the extent of disease spread in a community.
  • Since random people were tested, the survey indicated that the spread of the disease was much wider than what diagnostic tests suggest.
    • Information about the extent of the spread of the virus is very important for authorities to make decisions and plan containment measures.
  • However, according to the scientists, the suggestion that herd immunity could be achieved in the near future was problematic.

Antibodies and immune system

  • The antibodies are proteins produced by the immune system to fight external organisms like viruses that try to enter the body. 
  • These antibodies are produced only after the infection has happened, and are specific to the attacking virus or bacterium. 
  • The presence of these is an indication that an infection by that particular virus or bacterium has already occurred. Subsequent attempts to infect the body can be thwarted by these antibodies.

Similar work of vaccine 

  • Vaccines also work in a similar manner. They inject harmless doses of a virus or a bacterium inside the human body to trigger the production of antibodies by the immune system. 
  • Thereafter, these antibodies can then fight off an actual attack by those viruses or bacteria.

Do antibodies ensure Immunity?

  • The mere presence of the antibodies does not mean that the person is protected against the disease. 
  • These are two different things that holds importance
    • The amount of antibodies present, and 
    • Whether it also includes what are known as “neutralizing antibodies”. (Neutralizing antibodies are the ones that actually fight the disease) 
  • However, serological surveys are not designed to assess either the quantity of antibodies or detect the presence of neutralizing antibodies.

About the herd immunity

  • Herd immunity is a stage of an epidemic disease in which some members of a population group remain protected from infection because a majority of those around them have already developed immunity.
    • This may be achieved either through vaccination or because they have been infected earlier. 
  • Therefore, everyone in the population group does not need to get infected before the epidemic is over. 
  • Once a certain proportion of the population gets infected by a disease the epidemic begins to slow down and eventually stop.
    • However, it is unclear what percentage of the population needs to be infected before herd immunity kicks in.
    • In the case of measles, herd immunity reaches only when 85% to 90% of the population attains immunity.
    • In some other diseases, the threshold could be lower. For current coronavirus, it has been suggested that between 55-70% of the population would need to be infected before herd immunity would develop.


  • It is extremely difficult to determine the level of disease spread necessary for herd immunity when the epidemic is still raging. 
  • Currently, every parameter in this calculation is dynamic and evolving. Only after the epidemic is over, we can reliably estimate at what point did herd immunity take over.


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