NEW DELHI: A sero-survey exercise undertaken by the Indian Council of Medical Research in containment areas across 10 COVID19 hotspot cities in May had shown the highest—40 percent—seropositivity in a cluster in Ahmedabad, TNIE has gathered.
This means that 40 percent of individuals whose blood samples were tested to check for antibodies against the virus in a containment zone in the city were found to have a past history of infection—suggesting they had contracted the disease and recovered quietly.
Nearly 36 per cent of the samples collected from Mumbai’s Dharavi, Asia’s biggest slum with a population of over 8.5 lakh, also had antibodies against SARS CoV 2—indicating that the actual number of detected COVID19 cases in the area were just the tip of the iceberg.
Seropositivity range in most other clusters in the worst-hit cities where the exercise was carried out however was found to be in the range of 15-30 per cent, as reported by TNIE earlier.
These findings as part of a two-part sero-survey research study will soon be published by the ICMR’s Indian Journal of Medical Research after a delay of several weeks and severe criticism of the Council.
The cities where the samples from containment areas were tested included Mumbai, Ahmedabad, Pune, Delhi, Kolkata, Indore, Thane, Jaiupr, Chennai and Surat—which at that time contributed a total of 70 per cent of COVID19 caseloads in the country.
A total of 500 samples were collected from 10 randomly picked containment zones in each of these cities to test for presence of the antibodies.
The study had been undertaken by the ICMR, with support from the World Health Organization, India and the National Centre for Disease Control, in order to assess the true spread of the disease in May.
As IgG antibodies against the virus appear typically three weeks after the infection, it has been assessed that the first pan-India sero-survey would have given the disease spread status till April end.
It was only earlier this week that the full report of the sero-survey, carried out in about 20,000samples from 69 districts ahmin 21 states categorised on the basis of zero to high caseloads and 5000 samples in 10 hotspot cities, was shared with an expert committee on COVID19 surveillance and epidemiology under the ICMR before it is formally published.
“The report has almost confirmed the long-standing view that despite strict lockdown measures in the early period of pandemic outbreak in India, the virus had been transmitted rampantly in many cities,” said a committee member.
“The findings also explain the swift decline COVID19 cases in areas like Dharavi starting June where the disease at one point looked uncontrollable and suggests that many localities in badly hit cities may have reached herd immunity already, as expected epidemiologically.”
Another member said that finding also confirmed that the quality of ELISA testing kit used for the exercise was good.
“Very low seropositivity was found in areas with zero or low cases while it was remarkably high in areas with high caseloads—consistent with the detected spread of the disease,” said.
The government so far has revealed only one figure from the massive exercise saying that the overall infection rate in non-hotspot districts was found to be 0.73 per cent.
A second nationwide survey is already being planned and could begin later this month or in early August.