Sero-Prevalence Surveys: A Lot To Cheer Yet Enough Evidence To Caution Indian Cities

Kiran Limaye

Jul 30, 2020, 02:33 PM | Updated 02:33 PM IST

A test being conducted as part of the survey. 
A test being conducted as part of the survey. 
  • Sero-prevalence surveys seem to hint that many of us are able to deal with the virus with not much fuss, though we can leave it to the experts whether to call this immunity or not.
  • ICMR (Indian Council of Medical Research) study of Delhi based on sero-prevalence survey has found nearly 23 per cent of those whose samples have been analysed to have antibodies indicating past presence of coronavirus.

    Similar percentage has been observed for Mumbai (Greater Mumbai Municipal Corporation region precisely.)

    The latest survey conducted by Brihanmumbai Municipal Corporation (BMC), using random sampling, has found that nearly 57 per cent of slum households and 16 per cent of non-slum households show antibodies. This survey involved nearly 7,000 samples from three wards of Mumbai.

    These results are in line with studies in New York or London, where survey estimates are around 17 per cent to 20 per cent after about four months of the Covid-19 pandemic.

    Sero-prevalence studies do not identify the virus but look for the response human body is highly likely to have generated for coronavirus. Due to limited availability of tests, only those who show symptoms and those who are in their close contact take RT-PCR test to confirm the presence of virus.

    If we go by the survey, 23 per cent of population in Delhi in July, when survey was conducted, had the likely presence of virus.

    Using 2019 estimates of population (2 crore) and possibility of many migrants not being in city during the time of survey, the number of people who have had virus is between 35 to 40 lakh for Delhi.

    Delhi had around 1.1 lakh cases when sero-survey was undertaken. The implication is obvious: exceptionally large fraction of virus carriers shows no symptoms to even prompt the testing. Same implication holds for Mumbai or New York or London if we compare RT-PCR tests and sero-prevalence surveys.

    This is good news!

    Though we do not want to race ahead in conclusion, it hints that many of us are able to deal with virus with not much fuss. We can leave it to experts about whether we should call such behaviour as immunity or not.

    It is a respite we all deserve, after months of pandemic, which has scarred lives, livelihoods, and imagination of the whole country. I must highlight here a bold article which has foreshadowed this optimism, it is worth reading and re-reading.

    The other aspect of sero-prevalence survey is it also maps the progression of the pandemic to its logical end, where everyone is affected. About four-and-half months of pandemic, March to mid-July, with lockdown and reverse migration, seems to have affected a quarter of the population in two of our largest cities, which have many extremely dense pockets of population.

    So even if migrants do not return to cities in large numbers, there is still three-fourth of population, which can be infected with the virus in the coming months. Let us remember how health systems in Delhi and Mumbai were overwhelmed before state interventions restored a new equilibrium of living with Covid-19.

    BMC’s sero-survey also hints at how the future progression of disease in our cities might be. If restrictions on inter-person contact, closure of mass transit and absence of migrants continue then we will see stable pattern of Covid-19 cases or even a fall.

    Institutional quarantine and a large presence of affected people in slums seem to have reduced the potential for rapid rise of cases in slums.

    Non-slum households, better equipped for inter-personal distancing, with much lower rate of spread, are likely to be the new source.

    Unless interpersonal interactions increase drastically, as states ease more and more curbs, this new source is likely to generate cases with much slower speed.

    If economic activity in these cities stay subdued, we are highly likely to see a slowly declining trend.

    Mumbai and Delhi are showing such pattern and Chennai might soon join them. Same template might be in store for other cities in India, which are currently experiencing a rapid rise, for example, Pune or Bengaluru. (Figure 1-5)

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    Figure 5

    To err on the side of caution, we are not even midway out of the woods yet. What we are witnessing in Delhi, Mumbai and Chennai is an eerie calm, after a turbulent first wave of pandemic which showed stark deficiencies in our responses.

    There is marked improvement in health capacities of the megacities which are through the first wave, and we are likely to be better prepared for next upticks.

    Sero-prevalence survey tells us that there is still large fraction of population of these cities which is yet to be impacted by virus, but these future recipients are likely to be better equipped than their precursors.

    Slow growth of cases might make the pandemic linger longer, but it will keep it to a manageable level, where we can operate locality-based economic activities while restricting citywide commutes and large arrival of migrants to cities. It might be the best trade-off that we have before vaccine arrives.

    Evidence of sero-prevalence survey also hints at other important questions from knowledge point of view.

    First, can someone having a virus but not the symptoms spread the virus? If yes, how?

    Second, why incidence fatality rates, ratio of deaths to infected, is low in India and in some other countries?

    Is it due to exposure to other variants of coronavirus, early adoption of masks and the type of strain of coronavirus?

    Third, can one get the virus again and if one has not shown the symptoms in first presence, will they be similarly asymptomatic in next? As India is conducting these sero-prevalence surveys successfully, one can hope that answers to these questions might also emerge from India. It is an opportunity (sandhi) that waits in the disaster (sankat).

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