A man draws circle to maintain social distance in Chennai on Monday. (ANI)

One-fifth of Chennai’s population has been exposed to Covid-19, a population-based sero survey carried out by the National Institute of Epidemiology to check transmission of the infection in the Greater Chennai Corporation (GCC) region showed.

The study, released by the city corporation, said the results were out of samples tested for antibodies against SARS-CoV-2 on 12,405 individuals from the corporation’s 51 wards.

Of those tested, only 173 were found to have had a history of contact with Covid-19 patient. The field data collection for the study took place between July 18 and 28, and the positivity rate found at the end of July was around 10 per cent.

The study said “around 80% of the population is still susceptible to SARS-CoV-2 infection in Chennai”, and there is a “need to understand the reasons for varying seroprevalence across zones to improve our control measures”.

Seroprevalence is the percentage of individuals of the population to have antibodies against SARS-Cov-2. “There is huge variation in seroprevalence across zones ranging between 7.1% in Madhavaram and 44.2% in Tondiarpet,” the study reported.

According to the study, out of 6,493 female samples tested, 1,538 tested positive, against 5,785 male samples of whom 1,115 were found positive for Covid-19. At 23.7 per cent, the female group thus had higher seroprevalence than the male group’s 19.3 per cent, the survey reported.

“The reading (inference) is (that) 21.5 per cent of Chennai’s population reached the immunity levels by July-end,” GCC commissioner G Prakash said. “Now, the (immunity) levels will be more and it would also mean (comparatively) lower transmission rate.”

In the age group category, samples in the 10-19 years showed higher seroprevalence of 23.8 per cent against those above the age of 60, which had the lowest — 15.9 per cent. Seroprevalence among the 20-29 and 30-39 age group was 22.7 per cent each, and the 40-49 age category showed seroprevalence of 23.4 per cent.

“There is huge variation in seroprevalence across zones. Infection to Case Ratio is lower compared to some of the published studies (Santa Clara, National survey), indicating high testing performance. Infection to Fatality Ratio is comparable/lower to published studies,” the study said.

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