A fifth of Chennai people exposed to COVID: Sero survey

One-fifth of the population in Chennai has been exposed to COVID, a serosurvey for the transmission of the infection in Greater Chennai Corporation areas has revealed. "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 sero-prevalence of 2 per cent, for a relative precision of 20 per cent design effect of 2.5 and for 95 per cent confidence interval. Of the individuals tested, 2,673 individualshad evidence of past SARS-CoV-2.

"The reading (inference) is 21.5 per cent 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.

Around 80 per cent 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 per cent in Madhavaram and 44.2 per cent 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 were 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 population exposed to SARS-CoV-2. The study design was a multi-stage cluster sample survey among individuals aged more than 10 years.