ICMR second serosurvey: Prevalence of antibodies highest in urban slums, 7.4 crore coronavirus infections till August

Indian Council of Medical Research has published the findings of its second national serosurvey. The findings which appeared in the Lancet Global Health pre-print indicates that nearly 7 per cent of India's population (1 in 15 individuals) aged ten and above was exposed to the novel coronavirus, amounting to 74.3 million (7.4 crore) infections till August.

Sero survey means collection and testing of serum (or proxy such as oral fluid) specimens from a sample of a defined population, over a specified period of time, to monitor trends in prevalence of the novel coronavirus, or SARS-COV-2, infection.

ICMR's second national serosurvey also reveals that seroprevalence or the level of Covid-19 antibodies in a population is highest in urban slum areas followed by urban non-slum and rural areas. This further indicates asymptomatic seroconversion among the general population.

The overall seroprevalence of below 10 per cent in India indicates that a large proportion of the population remains susceptible to novel coronavirus infection.

'The transmission of infection is expected to continue in most Indian states till the herd immunity threshold is achieved, either by natural infection or vaccination. While this threshold is unknown, most estimates place it above 50 per cent,' the report said.

The survey further stated that one in nine individuals who did not report any COVID-19-related symptoms in the past had the presence of SARS-CoV-2 IgG antibodies, indicating asymptomatic seroconversion among the general population in India.

Seroconversion was also documented among individuals without a history of known contact with a COVID-19 case, and among those without any prior SARS-CoV-2 testing.

Also, 26-32 infections were reported per case diagnosed in August 2020 as against 82-131 in May 2020, when the first round of serosurvey was conducted, underlining the consequence of the growth of testing outpacing the growth of infections.

The seroprevalence was higher in slum areas of Mumbai (57.8 per cent) compared to non-slum areas (17.4 per cent).

Of the 29,082 individuals tested for the presence of IgG antibodies against SARS-CoV-2, 3,135 tested positive, with an unweighted seroprevalence of 10.8 per cent.

Of the 29,082 people (aged 10 and above) surveyed from August 17 to September 22, 6.6 per cent showed evidence of past exposure to COVID-19 while the seroprevalence among adults was 7.1 per cent.

The seroprevalence was similar across age groups, sex, and occupation, the report said

The first nationwide serosurvey in India was conducted in May, when the entire country was under stringent lockdown, with the exception of conditional relaxation in areas deemed to be minimally affected, and indicated a low prevalence of 0.73 per cent among the general adult population.

The second household serosurve was conducted among individuals aged ten years and above in the same 700 villages and wards from 70 districts selected during the first serosurvey.

(With Agency Inputs)