Bengaluru: Sero survey to test for Covid-19 local spread

Bengaluru has been the biggest contributor to the state’s Covid-19 case tally in the past few days.

On Wednesday alone, the city reported 735 cases. To establish whether there is a community prevalence of the infection and to assess the extent of the spread, the state government is planning to launch a sero survey next week, starting with Bengaluru and Udupi.

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In all, 39,400 tests will be done across the state with 3,456 tests on samples drawn in Bengaluru alone. Dr. Giridhar Babu, a member of the state Covid-19 technical advisory committee, is the principal investigator.

Dr. Babu told DH, “The survey will be done among low, moderate, and high-risk groups. This is to find out the changing trends of the infection and whether the community prevalence of the infection is high or low.

This is the first time in the country when we will be assessing the presence of the infection and immunity status of the patient, which means we will be doing both antigen-based tests and antibody-based tests.”

An antibody test will tell whether an individual has been infected in the past or not and whether Immunoglobulin G (IgG) is present. The first set of antibodies that the body produces is IgM in response to initial exposure to an antigen.

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“If a person has IgG, and another person has only antigen but no antibodies, we can tell that among 100 tested, 50 already had the infection and another 50 have the acute infection now. So in one go, we get to know the total extent of the infection burden, both past and current. In this way, this study is unique and is also the largest study done so far since the start of the pandemic,”

The city will be divided into eight zones and 432 tests will be done in each zone. So is a low prevalence rate good news? Not really, say experts. A low rate would mean the worst is yet to come.

“The ICMR sero survey done in May end showed that Bengaluru had 0.25% prevalence rate which is way lower than the national average of 0.7%. All over the world, the range of seroprevalence has been 0 to 30%.

It doesn’t go beyond 30%. In Spain, which is the worst affected country, it is 25%. As the epidemic progresses the seroprevalence will increase. If the seroprevalence is low, we should be worried, not celebrate. It means the infection has not hit us yet,” Babu explained.

“If the sero prevalence is low, we need to be prepared with more beds in the days to come. If it is as high as 30%, then the epidemic is already at its peak and we are towards the receding phase until the next wave hits us,” he said.