Sero survey

Subject: Science and tech

Context:

Around 7% of India’s adult population may have been exposed to the coronavirus till the last fortnight of August, according to the second national sero-survey by the Indian Council of Medical Research (ICMR).

Concept:

• Sero-surveys use tests that examine the liquid part of blood, or ‘serum’.
• And these tests detect an immune response to the virus material, not SARS-CoV-2 virus material itself.
• Upon virus infection, the body comes up with many immune responses.
• One of these is making proteins called antibodies that stick (or ‘bind’) to the virus – these show up within a few days after infection.
• The infection itself typically disappears after a couple of weeks. But the anti-virus antibodies, especially the IgG kind, stay around in the blood for a fairly long time, at least for months.
• So, if a person was infected, virus material would be detectable in their nose, throat and mouth fluid for a couple of weeks at most. If testing was not done in that time, we would never know if the person had been infected by the virus. But IgG antibodies stay in the blood of such a person for a long time. So, if we test the blood for these antibodies at any point and find them (making the person ‘sero-positive’), it can be said that this person had indeed been infected in the recent weeks/months.
• Sero-surveys test blood samples of healthy people for anti-SARS-CoV-2 IgG antibodies. Everybody cannot be tested, only a few people chosen at random are tested. The results are an estimate of the proportion of people who have been infected in the past. This information gives a wide-angle picture over time of how the virus has spread in the community.