NEW DELHI: The first population-based serosurvey in India, conducted to assess the extent of exposure to SARS-CoV-2 -- the pathogen behind COVID-19 -- has revealed that as high as one-third of the population in many containment areas in hotspot cities may have already been infected by the virus and quietly recovered.
The survey conducted by the Indian Council of Medical Research, whose preliminary findings have been shared with the Union cabinet secretary and the Prime Minister's Office, has found that in several containment areas in high caseload districts, 15 to 30 per cent of the population has been exposed to the infection, government sources told The New Indian Express.
In a serosurvey, blood samples of people are collected to test for the presence of antibodies specific to a pathogen. In this case, it was meant to test for IgG antibodies against SARS-CoV-2 that normally appear 14 days after the infection and continue to be present in the blood serum for months.
The ICMR, with help from the National Centre for Disease Control, World Health Organisation's India office and the state governments had collected nearly 24,000 samples from 70 districts in the country from randomly picked individuals.
These included ten hotspot cities Mumbai, Ahmadabad, Pune, Delhi, Kolkata, Indore, Thane, Jaipur, Chennai and Surat that contribute nearly 70 % of India's total caseload. 500 samples each were collected from 10 randomly chosen containment areas in each of these cities.
In addition, 400 samples each from other 60 districts across 21 states -- categorized on the basis of low, medium and high caseload -- were also collected.
"Barring two hotspot cities Surat and Kolkata and six other districts, we now have results from all other sites and it shows that infection size in many containment areas in the worst-hit districts is 100-fold to 200-fold higher than the cases reported at those sites," an official privy to the survey report said.
Data for eight districts are still being analysed and will be added in the final report.
Another official said that sites, where infection size is 100 to 200 times higher, are mainly in Mumbai, Pune, Delhi, Ahmedabad and Indore.
"What we have understood so far is that the infection is much more widespread than what is reflected by the reported cases in many cities and containment efforts may not have fully paid off," he said. "In tier II and tier III cities, on the other hand, the spread of the virus is minimal."
In the survey, antibody tests have been carried out using a testing kit, "COVID Kavach Elisa" developed by the National Institute of Virology under the ICMR which the agency claims has very high specificity and sensitivity. In other words, the kit detects the presence of even low levels of antibodies against novel coronavirus almost accurately.
What is serosurvey?
It means testing blood samples in a population for a specific purpose. In this case, the blood samples collected from individuals under a testing protocol were tested for the presence of antibodies against SARS CoV 2, the virus that causes COVID 19. The presence of the antibodies confirmed in a particular sample meant that the person had been exposed to the virus and has recovered.
How has this serosurvey been conducted?
Nearly 24,000 samples from 70 districts across 21 states--including 10 hotspot cities--have been collected and tested. Data has been analysed for 62 districts or cities while the work related to sample collection and data analysis is on for rest of the places.
What has emerged from the data analysis so far?
* Very high exposure levels in some cities
* Very low exposure levels in rural sites
* In some areas with high exposure, 15 to 30 per cent population has been exposed to the infection
* In some sites, the infection size is 100 to 200 timer higher than the cases reported at those sites
What could be the implications of the findings?
*It gives health policy makers and epidemiologists data that can be used in disease control and mitigation measures
*Districts with low prevelance of virus can continue to be on alert
*As the disease may have been limited to mainly in clusters in big cities, population in tier II and tier III cities and rural areas is still very vulnerable
*In some badly hit areas, population is closer to herd immunity than other areas. It has been estimated that herd immunity for Covid 19 can be achieved when 60-70 per cent people is exposed to the virus.