A fifth of the population of Chennai has been exposed to the Covid-19 disease causing Sars-CoV-2 virus, a serological survey conducted in the capital city, has found.

The Greater Chennai Corporation (GCC) on Tuesday released its findings. Chennai at present accounts for a third of the total cases in Tamil Nadu. A serological survey provides an understanding of the extent of the virus transmission.

The GCC tested 12,405 individuals of which 2,673 people or 21.5% were found to have developed IgG antibodies to the virus. Around 80% of the population is still susceptible to Covid-19, the report stated. Chennai has a population of more than 8 million, and a floating population of 400,000, including workers from other districts and states. All 15 zones of the city were covered.

“Since the findings are based only till the end of July, the prevalence rate will be higher now,” said corporation commissioner G Prakash.

The results from the exercise jointly conducted by the corporation and Indian Council of Medical Research’s (ICMR) National Institute of Epidemiology in Chennai are a month old as the field activities of drawing blood samples and data collection happened between 18 and 28 July. The testing of samples was completed by August 6.

“There is huge variation in seroprevalence across zones,” the survey report stated. “In zones where there is a high prevalence, more people have developed immunity so the virus transmission has been slower,” said G Prakash. “The data helps us to identify zones where prevalence is low so we can intensify measures to control the virus.”

A seroprevalance of 20% is enough to indicate that a city has reached its peak from which it will be a downward trend, says Dr Jacob John, retired professor and former director of the ICMR’s Centre of Advanced Research in Virology. “But this is still not good news. It only means that the frequency of people getting infected will be less but the magnitude will be the same. It takes more time for a 100 people to get infected during the downward part of the curve than during the upward slope.”

He added that Chennai’s 21.5% of the serosurvey has to be compared with the proportion of people who turned positive in RT-PCR tests to understand the magnitude of the disease. “For example, if RT-PCR tests indicate that 2% of Chennai’s population is positive, it means for every detected case, there are 10 undetected cases.”

As a part of the national serosurvey conducted in the first week of May , four districts of Tamil Nadu, including Chennai were studied. The unweighted sero-prevalence of Sars-CoV-2 infection in Chennai was found to be 2%. The prevalence of infection in the five randomly selected containment zones in Chennai was 5%.

A host of other cities including Delhi, Mumbai, Ahmedabad, Pune, Amritsar among others have also conducted serological surveys on a range of sample sizes. Delhi recorded a prevelance of 27.7% in a sample size of 15,000.

Starting Tuesday, under Unlock 4, Tamil Nadu relaxed a majority of pandemic restrictions including Chennai. Public transport, malls, restaurants, businesses and industries were once again open with rules of wearing masks, social distancing and standard operating procedures. Tamil Nadu on Tuesday reported 5,928 cases taking the total to 4,33,969 with 96 deaths. Chennai accounted for 1,084 cases with an overall of 1,36,697 cases.