Serological Survey: Everything you need to know

What exactly is it?

A serological survey is an antibody test conducted on a sample of population to determine how many have been affected by a particular disease. It does this by detecting the presence of specific antibodies against the pathogen. Sometimes, it can also be conducted to check whether a person has developed immunity to a certain disease. India uses ELISA, Enzyme liked immunosorbent assay, test to determine whether an antibody, for any particular infection, has been developed by the immune system. The test-kits for this test have been developed by the National Institute of Virology and Zydus Cadila. ELISA tests for IgG and IgM antibodies in the population and has 97.7% specificity and 92.1% sensitivity. 

Why is it of significance?

It is difficult to assess the infection rates in a sample of population with the RT-PCR and Rapid antigen tests, and this is where Serological Survey comes into the picture. A Serological test indicates past infections and not active ones. It gives the authorities a rough estimate of the percentage of population that is vulnerable to the disease.

Conducting antibodies test in a random set of people is an effective way to know whether the population is approaching the stage of Herd Immunity. Although it is still unknown, in the case of COVID-19, what percentage of people need to be infected before reaching that stage. Some studies indicate that 55-70% of the population should be exposed to the virus, meaning that a serological survey should indicate that in a large sample of population almost 70% are carrying antibodies against the pathogen.

The serological survey in Delhi

The survey was conducted on a sample of 21000 people in Delhi. It indicated that 23 percent of the sample has already developed antibodies specific to Sars-Cov2. The seroprevalence study, published after the survey, also suggested that a majority of people exposed to the pathogen remained asymptomatic. Looking at these results, the authorities became very optimistic and assumed that the population is reaching the stage of herd immunity. Later studies, however, indicated that it would be naive to make such a statement as the percentage of people exposed to the virus remains low.

The Serological Survey in Mumbai

Sero-survey conducted in the slums indicates that 57% have been exposed to the pathogen. The percentage remains low for residential areas. The combined percentage for both the slums and residential areas is 40. Some experts believe that the population in slum regions may have already reached the stage of herd immunity. Although the sample, covering 6936 people, is way smaller than that in the case of Delhi, the seroprevalence study in Mumbai is indicative of the fact that soon the situation is going to become better in the financial capital.