A serological survey done in Delhi has revealed 23.48% of the participants developed Immunoglobulin G antibodies that indicate they may have been infected by the coronavirus, the Union Ministry of Health and Family Welfare said. The study also noted that a large number of those infected were asymptomatic.

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.” However, the CDC adds that:

“A positive test result shows you may have antibodies from an infection with the virus that causes COVID-19. 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.”

— CDC.gov

The results of the Delhi survey imply that the total number of cases in the Capital should be around 47 lakh in relation to the city’s population of two crore, according to The Print. The Union health ministry’s Tuesday morning update puts Delhi’s coronavirus tally at 1,23,747, including 15,166 active cases, 1,04,918 recoveries and 3,663 deaths.

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.

“For all the 11 districts of Delhi, survey teams were formed,” the statement from the Union health ministry read. “Blood samples were collected from selected individuals after taking written informed consent and then their sera were tested for IgG antibodies and infection using COVID KAVACH ELISA approved by Indian Council for Medical Research [ICMR]. It is one of the largest sero-prevalence studies conducted in the country using the ELISA [enzyme-linked immunoassay] testing.”

A high-level expert committee, led by NITI Aayog member Dr VK Paul along with All India Institute of Medical Sciences Director Dr Randeep Guleria among other experts, had decided to conduct the survey in Delhi, according to The Indian Express.

Experts believe that antibody tests are not diagnostic but instead look for antibodies that may have developed after a person has been infected. Delhi is currently conducting RT-PCR or reverse transcription polymerase chain reaction and antigen testing to diagnose Covid-19.

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Here are today’s top 10 updates on Covid-19

On July 9, the Indian Council of Medical Research dismissed reports based on a survey that showed that about one-third of the people living in containment zones and hotspots may have already been infected with Covid-19 and had also recovered.

In April, scientists found that 2% of residents of Germany’s Gangelt town were actively infected by the coronavirus, and a total of 14% had antibodies, which point to a potential prior infection.

On June 11, a study titled “Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Geneva, Switzerland” by British medical journal Lancet estimated the “weekly seroprevalence of anti-SARS-CoV-2 antibodies in the population of Geneva, Switzerland”. “Between April 6 and May 9, 2020, we enrolled 2766 participants from 1339 households, with a demographic distribution similar to that of the canton of Geneva,” the study said. “After accounting for the time to seroconversion, we estimated that for every reported confirmed case, there were 11·6 infections in the community.”

The Lancet study said that most people in Geneva “remained uninfected during this wave of the pandemic, despite the high prevalence of COVID-19 in the region”. “Assuming that the presence of IgG antibodies is associated with immunity, these results highlight that the epidemic is far from coming to an end by means of fewer susceptible people in the population,” it further noted.