The Indian Council of Medical Research (ICMR) is unlikely to immediately undertake a fourth national serology survey to estimate the extent of exposure to the coronavirus since January. Officials told The Hindu that though discussions were still on, undertaking such a study presented newer logistical challenges and the ongoing vaccination drive could read to erroneous inferences.
The ICMR has conducted three national serology surveys since May 2020 and found that the exposure to the virus was much higher than that was reported by confirmed cases. The third serosurvey that measured the spread of infection between August and December found that 21% of India's adult population and 25% of those in the 10-17 age group may have been infected.
However, India's devastating second wave began around mid-March amidst a vaccination programme that prioritised healthcare, frontline workers and senior citizens.
Serology surveys take blood samples from participants and measure antibodies to check past exposure to the virus. However, because vaccinations also trigger an antibody response after two weeks, they could be misleading. “Serosurveys lose their scientific relevance once vaccination starts in the population,” Dr Balram Bhargava, Director General, ICMR told The Hindu .
HCWs overburdened
Another official said that with hospitals and healthcare infrastructure in several districts being entirely occupied in dealing with the surge in cases, to be able to conduct a serology survey at this point would be challenging.
Also read: Serosurveys underestimate building of herd immunity
“We've had discussions on this. A fourth survey would give useful insights but there are logistical challenges in the districts. However we may revisit this question next month,” said Dr. Samiran Panda, Head, Epidemiology and Communicable Diseases, ICMR.
Dr Manoj Murhekar, Director, ICMR-National Institute of Epidemiology, Chennai, who has been part of the ICMR surveys, said only in some infectious diseases was it possible to differentiate antibodies from a vaccination from one in a naturally acquired infection. “A fourth survey would also have to account for the waning of antibody levels and may require an entirely new survey design.”
Independent experts however, said that with vaccinations on, conducting serology surveys on a smaller, defined group was possible. The Council for Scientific and Industrial Research has surveillances done studies on a section of its employees across constituent laboratories in India. “This is a defined group that we can easily follow,” said Dr Anurag Agrawal, Director, CSIR-Institute of Genomics and Integrative Biology.
Other than overarching numbers, the serology surveys have also revealed patterns of infection in urban, rural areas, slum and non-slum regions. It also gives an indication of whether a significant fraction within a region had reached threshold immunity levels to keep future outbreaks in check.
