Chennai: One-fifth of the population here has been exposed to COVID, a sero survey 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 percent confidence interval.Of the individuals tested, 2,673 individuals had 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.