88mn in country may have been exposed: Sero survey

New Delhi One in every 15 individuals, or 6.6%, of people above the age of 10 years in India, have been exposed to Sars-Cov2, the virus that causes coronavirus disease (Covid-19), till August, according to the findings of Indian Council of Medical Research’s (ICMR) second national sero survey released on Tuesday.

If extrapolated across the country’s population, this would mean that nearly 88 million people may have been silently exposed to the virus.

The second iteration of ICMR’s national sero survey covered 29,082 individuals and was conducted between August 17 and September 22, in the same 700 villages and wards from 70 districts in 21 states that were covered in the first survey. The first countrywide sero survey (conducted between May 11 and June 4) showed overall infection prevalence to be 0.73%.

All other parameters about the two surveys were similar except for the targeted age bracket — in the first phase population selected was 18 years and above, and in the second phase samples were included from 10 years and above.

“We changed the age group in the second survey from 18 years to 10 years and above because infection was also seen in younger population. Sero survey gives us virus exposure prevalence, but you may or may not have developed the disease,” said Dr Balram Bhargava, director general, ICMR.

About 3ml-5ml of blood sample was collected to study the presence of IgG antibodies against Sars-Cov2 virus. IgG antibodies are the longer lasting antibodies that help determine a past infection.

“How much time in the past is a work in progress but these develop about 2-3 weeks after having contracted the infection and approximately stay for about 2-3 months,” said Dr Bhargava.

Urban slum (15.6%) and non-slum (8.2%) areas had higher Sars-Cov2 infection prevalence than that of rural areas (4.4%), and prevalence in adults (above 18 years of age) was also higher at 7.1%, the findings show.

“It goes on to show that a large percentage of population is still susceptible to contracting the infection which is why it becomes really important to follow Covid-19 appropriate behaviour like wearing a mask, maintaining social distancing, and observing hand hygiene,” said Dr VK Paul, member (health), Niti Aayog.

Experts say these types of sero surveys do not present the current picture. “This doesn’t tell us about the current situation and that’s the reason why it may not be really beneficial at making policy-level decisions. However, for academic purposes, if anyone wants to study the pandemic trajectory in India sometime later, it will be of help,” said Dr Jugal Kishore, head, community medicine, Safdarjung Hospital.

Several state governments have carried out independent sero surveys which have shown a large variance in findings. Delhi had shown 23.5 % and 29.1% prevalence of infection during the first and second rounds of surveys. The initial findings of the third round of sero survey in Delhi showed that roughly 33% people have been exposed. Such studies have also been conducted in Mumbai (57.8% in slums, 17.4% in non-slum), Ahmedabad (17.6%), Chennai (21.5%), Puducherry (4.9%, 22.7%) and Indore (7.8%) as well.

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