A new seroprevalence study has revealed that 44.1% of the population in rural areas of Karnataka and 53.8% in urban spaces have been exposed to the coronavirus, and have therefore developed antibodies for the infection.

Antibodies are immune molecules produced by the body to fight pathogens. The presence of antibodies in the blood typically suggests that people infected with a virus would gain immunity for some period of time. Although having antibodies is not the same as having immunity to the virus, research shows that antibody levels are closely linked with the ability to disarm the virus. The presence of antibodies is only expected to grow within a population with an outbreak.

The new study, released by IDFC Institute, Mumbai, suggests that 44.1% of the population in rural areas and 53.8% in urban areas of Karnataka had developed antibodies to fight the infection. The institute said that the study was based on data collected from a representative sample of households in 20 districts between mid-June and August.

“It found that the state has 31.5 million Covid cases, with 44% of rural citizens and 54% of the urban population affected – significantly higher than the reported number of cases,” a statement said. India has so far reported 85,53,657 Covid-19 cases, and the toll stood at 1,26,611. There are 5,09,673 active cases and 79,17,373 recoveries in the country.

The study collated information on antibodies for recent or past Covid-19 infection and used a test that looks for the RBD (or receptor-binding region) spike protein, therefore identifying exposure to the virus better. The study also tested the same participants for existing infections using the RT-PCR test. The statement added that this pairing allowed the study to report current levels of immunity and forecast future immunity as “most of today’s infected population will, in a few weeks, join tomorrow’s immune population”.

The study, funded by ACT Grants (India), was led by Professor Manoj Mohanan from Duke University, Professor Anup Malani from Chicago University, and Anu Acharya from Mapmygenome along with Kaushik Krishnan from the Center for Monitoring Indian Economy.

“What makes this study unusual is that we were able to partner with the largest panel data survey in India conducted by the Center for Monitoring Indian Economy, which has been following these households for over five years,” Mohanan said in a statement. “All seroprevalence studies globally have had difficulty in getting consent – but our partnership helps us understand the factors that drive selection in such studies and also allows us to have better estimates of population prevalence.”

Malani said that the study showed high levels of active infections and transmission, especially in urban areas of Mysore and coastal districts during the study period. He added that 9.7% to 10.5% in these parts had tested positive for current Covid-19 infection.

“Our study also demonstrates that pooled RT-PCR tests are feasible in India,” Acharya added. “But if active infection rates are very high, too many pools might test positive and the strategy becomes less cost effective.”

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What is a sero survey?

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.