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 complete 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 outline 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 once completed by August 6.
“There is immense variation in seroprevalence across zones,” the survey outline stated. “In zones where there’s 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 will be able to escalate measures to keep an eye on the virus.”
A seroprevalance of 20% is sufficient to indicate that a city has reached its peak from which it’ll 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 would be the same. It takes more time for a 100 people to receive infected all through the downward a part of the curve than all through the upward slope.”
He added that Chennai’s 21.5% of the serosurvey needs to be compared with the proportion of people that turned positive in RT-PCR tests to understand the magnitude of the disease. “As an example, whether RT-PCR tests indicate that 2% of Chennai’s population is positive, it means for each and every detected case, there are 10 undetected cases.”
As 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 once found to be 2%. The prevalence of infection in the five randomly selected containment zones in Chennai was once 5%.
A host of other cities including Delhi, Mumbai, Ahmedabad, Pune, Amritsar among others have also conducted serological surveys on a range of pattern sizes. Delhi recorded a prevelance of 27.7% in a pattern 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 complete to 4,33,969 with 96 deaths. Chennai accounted for 1,084 cases with an overall of 1,36,697 cases.