A fifth of the total number of residents in Greater Chennai Corporation’s areas have been exposed to Covid-19, a serological survey has found, reported PTI.

Serology tests “look for antibodies in blood”, according to the US’ Centers for Disease Control and Prevention. “If antibodies are found, that means there has been a previous infection,” it says. “However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.”

Serological surveys show the number of people that may have been infected with the coronavirus in an area. However, they do not show how many are immune to the virus. It is still unclear how long antibodies last in infected persons and how many antibodies are required to protect a person from reinfection.

“Overall, one-fifth of the population in Chennai was exposed to SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus-2) infection,” the preliminary report said. Blood samples were collected from 12,405 individuals from 51 wards in all the 15 zones of the Corporation. The samples were collated between July 18 and July 28 and were tested by August 6 for IgG antibodies (start appearing in an individual’s system by the end of the first week since infection), according to The News Minute. Out of those tested, 2,673 people exhibited signs of being infected.

“The reading [inference] is 21.5% of Chennai’s population reached the immunity levels by July-end,” GCC commissioner G Prakash said. “Now, the [immunity] levels will be more and it would also mean a [comparatively] lower transmission rate,” he said. Prakash added that the study would be helpful in altering strategies to combat the disease.

Dr Jayaprakash Muliyil, an epidemiologist and former Principal of the Christian Medical College told The News Minute that if the results were extrapolated to the residents of Chennai of 71 lakh, it would indicate that more than 15.26 lakh were exposed to the infection. However, the overall cases in Chennai stood at 1,36,697 as of Tuesday.

The survey also displayed a significant variation in seroprevalence – or those who test positive based on blood serum samples – across the city’s zones, according to The Times of India. In Tondiarpet, 44.2% of people tested were exposed to the infection as opposed to only 7.1% of people in Madhavaram.

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