New Delhi: One-fifth of the population in Chennai has been exposed to COVID, said a Sero survey for the transmission of the infection in Greater Chennai Corporation areas. “Overall, one-fifth of the population in Chennai was exposed to SARS-CoV-2 (the virus, severe acute respiratory syndrome coronavirus 2) infection,” the GCC said in its preliminary finding.

The sample size was around 12,000 to detect a seroprevalence of 2 percent, for a relative precision of 20 per cent design effect of 2.5 and for 95 percent confidence interval. Of the individuals tested, 2,673 individuals had evidence of past SARS-CoV-2.

“The reading (inference) is 21.5 percent of Chennai’s population reached the immunity levels by July-end,” GCC commissioner G Prakash told reporters. “Now, the (immunity) levels will be more and it would also mean a (comparatively) lower transmission rate,” he said adding the scientific study would be useful to alter strategies to tackle COVID in Chennai.

Around 80 percent of the population is still susceptible to SARS-CoV-2 here and “we need to understand the reasons for varying seroprevalence across zones to improve our control measures,” the civic body said. “There is huge variation in sero prevalence across zones ranging between 7.1 percent in Madhavaram and 44.2 percent in Tondiarpet.”

The infection to case ratio is lower compared to some of the published studies (Santa Clara, National survey) – indicating high testing performance.

The field data/sample collection activities happened between 18 and 28 July and the testing of the samples was completed by August 6, the GCC said in an official release adding the mean age of individuals surveyed was 41.1.

The sero survey was conducted in the GCC areas to estimate the proportion of the population exposed to SARS-CoV-2. The study design was a multi-stage cluster sample survey among individuals aged more than 10 years. 

Meanwhile, Tamil Nadu today reported 5,928 new COVID cases pushing the tally to 4,33,969, while the death toll stood at 7,418 with 96 more fatalities, according to a health department bulletin.

The active cases stood at 52,379 and in total 3,74,172 people have got cured, with 6,031 recoveries, said the bulletin.

Out of the over 5,900 new infections, the state capital shared 1,084 and the remainder was scattered across Tamil Nadu. The state tally of 4.33 lakh plus positive cases till date includes Chennai’s 1,36,697 infections.

As many as 75,165 samples were put to RT-PCR tests and cumulatively 48,88,312 specimens have been examined.