The Indian Council of Medical Research on Tuesday revealed its findings of the second serological survey that showed one in 15 people, aged over 10 years, may have been exposed to the coronavirus by the end of August.

“One in 15 individuals aged over 10 years were estimated to be exposed to Covid-19 by August 2020,” the research body’s chief Balram Bhargava said. “Earlier we had thought that it does not affect those below 18, but have now found there’s no difference. The prevalence is not different by age group or gender.”

The survey, conducted between August 17 and September 22, showed that the prevalence of Covid-19 among individuals over 10 years was 6.6%, and only among adults (over 18 years of age) was 7.1%. The ICMR also found that in urban slums the prevalence was 15.6% and non-slums was 8.2%, but the prevalence of cases in rural areas was 4.4%.

The ICMR chief explained that blood samples collected from the general population and tested for Immunoglobulin G (or IgG) antibodies are taken into consideration for the serological surveys. When a person is IgG positive, it means they were exposed to SARS-CoV-2 “sometime in the past”.

However, the United States’ Centers for Disease Control and Prevention has said that there may be a chance that positive result meant that the person had antibodies from an infection with a virus from the same family (called coronaviruses). This includes the one that causes the common cold.

Serological surveys are supposed to 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 those infected and how many antibodies are required to protect a person from reinfection.

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For the second sero-survey, blood samples were collected from 29,082 individuals, Bhargava said. The data collected included socio-demographic details, history of respiratory symptoms, written informed consent, ethics clearance, along with a blood sample.

“The survey was conducted in the same 700 villages/wards (in urban) from 70 districts from 21 states covered during the first survey,” the ICMR said. Bhargava added that the second survey’s aim was to estimate the prevalence of SARS-CoV-2 infection among individuals aged 10 years and above.

Bhargava said that susceptibility of a “considerable section of people, yet unexposed to SARS-CoV-2 exists”. During the same briefing, Union Health Secretary Rajesh Bhushan also said that the ICMR’s second sero survey report revealed that “a considerable population is still vulnerable to Covid-19”.

India’s coronavirus tally rose to 61,45,291 on Tuesday morning after the Ministry of Health and Family Welfare reported 70,589 infections in 24 hours. The country’s toll went up by 776 to 96,318. As many as 51,01,397 people have recovered so far. India’s recovery rate is 83.01%, while the death rate stood at 1.57%.

The ICMR’s first serological survey, conducted between May 11 and June 4 in the 21 states, showed that 0.73% adults in India were exposed to the infection. The findings also showed that for every confirmed case in May, there were 82-130 infections that went undetected.

The first survey, in which blood samples of 28,000 individuals were tested, showed that sero positivity was highest in the age group of 18 to 45 years at 43.3%. This was followed by those between 46 and 60 years at 39.5% and was the lowest among those above 60 with 17.2% positivity. A total of 64,68,388 adult infections were estimated in India by early May, the survey report said.