A new serological survey conducted by the Delhi government has shown that 29% of people in the national Capital may have been exposed to the coronavirus, Health Minister Satyendar Jain said on Thursday. These individuals were asymptomatic and may have quietly recovered from Covid-19, given the presence of antibodies in their bodies.

“29.1% prevalence has been found in the second sero survey,” Jain said, according to NDTV. “Scientists say that herd immunity is when up to 40% population have virus antibodies.”

According to the US’ Centers for Disease Control and Prevention, serology tests are “those that look for antibodies in blood”. “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 reveal how many people may have been infected with the novel coronavirus in an area but they do not show how many people are immune to the virus. It still isn’t clear how long antibodies last in infected persons and how many antibodies are needed to protect a person from reinfection.

The serological surveillance was conducted in the first week of August. The survey involved a group of people undergoing blood tests to detect the presence of Immunoglobulin-G antibodies. The body’s immune system produces IgG antibodies on being exposed to a virus.

The survey collected 15,000 samples. The highest prevalence was in the southeast district, Jain said. The other two rounds of the survey are scheduled to be held in the first weeks of September and October.

As of Thursday morning, Delhi has a total of 1,56,139 cases of Covid-19, including over 4,000 deaths. India’s tally rose to 28,36,925 on Thursday morning as 69,652 new cases were reported in 24 hours. The toll increased by 977 to 53,866.

Earlier, a similar serological survey conducted in Delhi had shown that 23.8% of the respondents carried antibodies. The results of the Delhi survey, which was revealed in July, implied that the total number of cases in the Capital should be around 47 lakh in relation to the city’s population of two crore. The survey was conducted between June 27 and July 10, and a total of 21,387 samples were collected from 11 districts to test for antibodies.

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