Indian Council of Medical Research’s (ICMR) fourth round of sero survey to detect prevalence of Sars-CoV-2 , the virus that causes coronavirus disease, will begin this month in 70 districts across the country.
What is a sero survey? What information does it reveal? Here’s what you need to know.
What is a sero survey?
A serological survey identifies how many people in the population have been exposed to COVID-19, in order to understand how prevalent it is in that area. Blood samples are collected and rapid antibody tests are carried out.
How is the test done?
Blood samples in a sero survey are tested for the presence of IgG (Immunoglobulin G) antibodies that determine a past exposure due to the virus.
“IgM antibody indicates the presence of active COVID-19 infection and start showing up in the body a week after contracting the virus. But since IgM disappears soon, it does not give a clear picture of total exposure. On the other hand, IgG antibody develops after two weeks of the onset of the infection and remains in the body for a sustained and longer period,” explained Dr SK Singh, Director of National Centre for Disease Control (NCDC), elaborating on the two kinds of antibodies.
Who will be a part of sero survey in June?
The Indian Council of Medical Research’s (ICMR) fourth national sero survey will be conducted in the same 70 districts where the first three were conducted. It will cover:
- The general population aged six years and above.
- Healthcare workers working in district hospitals.
These states will be a part of the sero survey: Andhra Pradesh (Krishna, SPSR Nellore, Vizianagaram), Assam (Udalguri, Kamrup Metropolitan, Karbianglong), Bihar (Muzaffarpur, Purnia, Begusarai, Madhubani, Arwal, Buxar), Chhattisgarh (Bijapur, Kabirdham, Surguja), Gujarat (Mahisagar, Narmada, Sabar kantha), Jharkhand (Latehar, Pakur, Simdega), Karnataka (Bengaluru urban, Chitradurga, Kalaburagi), Kerala (Palakkad, Ernakulam, Thrissur), Madhya Pradesh (Dewas, Ujjain, Gwalior), Maharashtra (Beed, Nanded, Parbhani, Jalgaon, Ahmednagar, Sangli), Odisha (Rayagada, Ganjam, Koraput), Punjab (Gurdaspur, Jalandhar), Haryana (Kurukshetra), Rajasthan (Dausa, Jalore, Rajsamand), Tamil Nadu (Tiruvannamalai, Coimbatore, Chennai), Telangana (Kamareddy, Jangoan, Nalgonda), Uttar Pradesh (Amroha).
Why are children being included in the sero survey?
This is not the first time that children are being included in the sero survey. However, this is the first time children under the age of 10 are being tested.
While ICMR has not given a reason, this comes amid concerns that the probable third wave of COVID-19 in India could affect children below the age of 18.
When was the last sero survey conducted? What did it show?
The ICMR’s third sero survey was conducted between 7 December and 8 January, 2021.
This is what it revealed:
Antibodies to SARS-CoV-2 were detected in 21.4 percent of adults and 25.3 percent of 10 to 17 year olds surveyed. At the time, the survey indicated that only 3.5 percent infections were detected.
Urban slums had a seropositivity of 31.7 percent, as compared to 26.2 percent for urban non-slum areas. The sero-prevalence for healthcare workers was 25.7 percent. Blood samples of 7,171 healthcare workers were used.
Two other surveys were conducted earlier. One in May-June 2020 and another in August-September 2020.
Why are sero surveys important? Why should these surveys be continued even if numbers fall?
Sero surveys give us a broad picture over a period of time about the prevalence of the virus in a community.
Speaking to NDTV, Dr Jyotismita Pathak, Assistant Professor at Army College Of Medical Sciences, Delhi, explained:
“Iceberg has a visible section which floats above the waterline, however, there is a larger, invisible section submerged in the water. Similarly, in the case of any infectious disease, we get to know about the cases that are reported; like, the COVID-19 numbers you see on state and national health portal. But there are cases that don’t come to notice for various reasons – they are asymptomatic, or people didn’t report. So, a sero survey focuses on that submerged portion of the iceberg and tells us what the infection to case ratio is.”Dr Jyotismita Pathak
Not just this, experts stress that if one region or section of population shows higher antibody levels, the resources can be shifted to another area that is still vulnerable.
Dr Om Srivastava, member of the Maharashtra Covid task force told The Times of India:
“The biggest importance of sero survey now was to match the the COVID numbers in the respective areas. If you see COVID numbers falling in one area and increasing in others, then this survey becomes quite important and gives an idea of what is happening in a particular area in terms of infection spread."
Does this mean different states will have different sero-prevelance?
Yes. For example, early studies from Mumbai found that a rapidly spreading infectious disease will almost certainly spread differently in different parts of the state based on – when it was seeded, the level of mobility, the density in these areas, and whether people follow masking and distancing precautions.
What do sero surveys not tell us?
According to scientist Vineeta Bal’s article in The Indian Express:
“The sero-survey test does not detect ‘protective’ antibodies, just all antibodies (the ‘protective’ ones are much harder to test for on a large scale),” she explains.
She also said that one actually has ‘no idea’ about what levels of ‘protective antibodies’ were needed for actual protection.
“What proportion of the community should be immune-protected to reach the ‘herd immunity’ point differs from situation to situation. We have no idea what that point is for COVID-19. While sero surveys are useful in examining herd immunity, they do not tell us whether that point has been reached or not,” she added further.
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