All you wanted to know about sero studies

The latest sero surveillance survey conducted by the Indian Council of Medical Research (ICMR) in August-September, has revealed that over 6.6 per cent of the Indian population above the age of 10 has already had a close encounter with the novel coronavirus, which is responsible for the Covid-19 pandemic. The survey — the second in row — indicated that the prevalence of the virus ballooned nearly 10-fold since the first one was conducted three months back.

What is it?

Sero surveillance surveys are a key tool used by epidemiologists (hunters of infectious diseases) to ascertain the prevalence of an infectious disease — like Covid-19 — in a susceptible population.

Once geographical areas are identified, the experts randomly choose volunteers keeping in mind a fair representation of the target population. Once informed consent is obtained from volunteers, a tiny amount of blood is drawn which is analysed for the presence of certain antibodies .

The presence of these biomolecules — produced by the body in its fight against the infection — indicates that an individual is exposed to the virus already.

Why is it important?

Apart from helping public health experts design containment strategies, sero studies can show how long it will take a country to achieve herd immunity, a sure shot sign that the infection is blowing over.

At the national level, epidemiologists led by the ICMR carried out two rounds of sero survey. The first was in May-June and the second in August-September. Both these surveys were carried out in the same 700 habitats/villages in 21 States. The volunteers who participated in both the surveys belonged to different households. In the first survey, 28,000 individuals were tested, while the number of participants in the second one was little over 29,000. While all the participants in the first sero survey were above the age of 18, the coverage expanded and included samples up to a minimum age of 10 as it became clear that even children are equally affected by Covid-19.

The second sero survey showed that nearly 6.6 per cent of the sample population was exposed to the virus, up from 0.73 per cent in the first survey. It also found that people living in slums had four times more exposure than their counterparts in villages and twice more than non-slum urban dwellers.

There were similar sero surveys in Ahmedabad, Chennai, Delhi, Mumbai, Pune, Puducherry carried out by local authorities. The one conducted in Mumbai in June-July with a sample size of 7,000 showed that the prevalence of Covid-specific antibodies in slum population was about 57.8 per cent while that in non-slum population it was 17.4 per cent.

Why should I care?

If you are a person from a vulnerable group — elderly, with serious co-mobidities, pregnant and or a child — there is a need to protect yourself against the infection. Even young adults — who are seldom affected by the infection — cannot be careless as they may be carriers and can infect elderly or vulnerable members in their families.

Since there is no vaccine or drug against the virus yet, there is a need to adhere to non-pharmaceutical measures such as wearing a mask, washing hands regularly, social distancing, keeping away from crowds, etc. People need to be doubly careful during festivals that attract crowds. More care is needed in as winter sets in in many parts of the country, as respiratory infections tend to aggravate in this season.

What’s the bottomline?

Over 90 per cent of the country is still to be infected by the virus. So herd immunity is some way away!

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