A recent serological survey in Delhi found the presence of coronavirus-specific antibodies in about 23% of the samples tested.
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 novel coronavirus, and that “herd immunity” could be approaching.
The antibodies are proteins produced by the immune system to fight external organisms like viruses that try to enter the body.
These are produced only after the infection has happened, and are specific to the attacking virus or bacterium.
The presence of antibodies, therefore, 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.
Vaccines 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. These antibodies can then fight off an actual attack by those viruses or bacteria.
Since it is not possible to test everyone, it is not clear how many people in the population are infected, especially since most of the patients do not show any symptoms of the disease.
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, it indicated that the spread of the disease was much wider than what diagnostic tests suggest. In Delhi, about 14% of those who have been tested for the virus have turned out positive.
Information about the extent of spread is very important for authorities to make decisions and plan containment measures.
The mere presence of antibodies does not mean that the person is protected against the disease. What is also important is the amount of antibodies present, and whether it also includes what are known as “neutralising antibodies”.
These are the ones that actually fight the disease. Serological surveys are not designed to assess either the quantity of antibodies or detect the presence of neutralising antibodies.
It is very well possible that a person has antibodies against Covid-19 disease but no protection. These two are not the same thing, and therefore we need to be cautious when we interpret scientific findings in this manner.
Herd immunity is a stage of an epidemic in which some members of a population group remain protected from infection because a majority of those around them have already developed immunity, either through vaccination or because they have been infected earlier.
So, 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, and thus builds immunity against the disease, the epidemic begins to slow down and eventually stop.