The Kolkata Municipal Corporation on June 16 started IgG antibody tests among people in some of the worst-hit areas to check for community spread of the coronavirus, a senior official said.

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Samples were taken from residents of ward numbers 11, 28, 61, 82 and 90, all of which have a large number of containment zones, he said. Samples of 100 people in each of these wards were collected and they will be sent to the ICMR's laboratory in Chennai for testing. "We had got guidelines from the ICMR (Indian Council of Medical Research) and according to that we have started conducting these tests from today," the chairman of the KMC board of administrators said.

The apex health research body, ICMR, has advised states to conduct sero-survey to assess the proportion of population, including asymptomatic individuals, exposed to coronavirus infection. Sero-survey involves testing of blood serum of a group of individuals for the presence of antibodies against that infection to know who has been infected in the past and has now recovered. In its advice to the states, the ICMR said periodic sero-surveys are useful to guide policymakers and stressed that IgG ELISA test should be used for carrying out such surveys.

Antibodies generally start appearing after two weeks of the onset of infection, once the individual has recovered, and last for several months. "Therefore, the IgG test is not useful for detecting acute infection but indicates episode of SARS-CoV-2 infection in the past," the ICMR had said in a statement.

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