“Around 80 per cent of the population is still susceptible to SARS-CoV-2 infection in Chennai,” revealed the preliminary finding of the report. The serosurvey was done by a team of experts from the National Institute of Research in Tuberculosis and Indian Council of Medical Research.

For the survey, the Corporation has collected random blood samples from 12,405 individuals from 51 wards covering all the 15 zones of the city from July 18 to 28. “Of all the tested individuals, 2,673 had evidence of past SARS-CoV-2, with an unweighted seroprevalence of 21.5 per cent,” it said. A huge variation was seen in seroprevalence across zones with Tondiarpet topping with the prevalence of 44.2 per cent followed by Royapuram, a major hotspot of COVID-19 cases in the city, with 34.4 per cent. Madhavaram have lowest of 7.1 per cent while the Perungudi has 7.3 per cent.

Corporation Commissioner G Prakash said that the North Chennai zones with a high prevalence of antibodies saw a steady reduction in the number of Covid-19 cases when compared to other zones. “It is a natural process where the transmission speed will reduce in high prevalence zones. Our survey, which was taken 35 days ago, found that almost one in four persons were immune to SARS-CoV-2 infection. If we take a survey today, naturally the prevalence would much higher. This data will help us to focus much more on the zones where the prevalence is less,” he told reporters here.

Population-based serosurveys are recommended to measure the spread of infection in the community, it said, noting that as a part of the national COVID serosurvey conducted in the first week of May 2020, the survey was conducted in four districts from Tamil Nadu, including Chennai. “The unweighted seroprevalence of SARS-CoV-2 infection in Chennai was 2 per cent. The prevalence of infection in the five randomly selected containment zones in Chennai was 5 per cent,” the report said.