Banking on serology: On seroprevalence studies

The virus isn’t as lethal as originally feared, but is more infectious than previously believed

July 31, 2020 12:15 am | Updated 10:47 am IST

A recent serology survey that scientists used to estimate the spread of COVID-19 in Mumbai has found that nearly three in five, or 57% of those tested in slums had been exposed to the virus and had developed antibodies against it as compared to only 16% of those tested in residential societies. Results from Delhi’s seroprevalence study , earlier this month, found that nearly a quarter of the 21,000-odd samples tested had been exposed to the virus and some of the densest districts had over 20% prevalence. Of the nearly 7,000 tested in Mumbai, nearly 61% were slum-dwellers and the higher prevalence of the virus there showed that — as expected — those living in the densest urban agglomerations were most likely to have been infected by it. A large proportion of those in whom antibodies were detected — the numbers aren’t known — were asymptomatic and this pointed to the fact that the fatality rate in Mumbai may be “as low as 0.05-0.10 per cent, instead of the existing 5.5 per cent,” as per an estimate accompanying the results of the survey. Such serological surveys are increasingly being used by States. Ahmedabad’s civic body conducted one to conclude that only 17% of the city had been likely exposed to the virus and Tamil Nadu too is in the midst of conducting such a survey. A dominant theme driving State bodies that commission such surveys is to check for levels of ‘herd immunity’, or if 60%-70% of the population have encountered the infection. The argument is that this degree of exposure will, akin to a vaccine, also protect the rest of those uninfected. But given the absence of knowledge about how long antibodies last and the extent to which they protect from fresh infections, herd immunity isn’t a precise science and not something that ought to be pursued by a state as a matter of policy.

It is four months since India got its first 100 cases and very early in May, the Indian Council of Medical Research’s survey had shown that the number of those infected by the virus were many multiples of those that were being reflected in official confirmed-case statistics. The unrelenting advance of the virus shows that while it is much less of a killer, in aggregate, than expected, it spares few from infection. Therefore, in the absence of a reliable vaccine, the vast majority of people everywhere — irrespective of peaks and ebbs in daily caseloads — continue to be vulnerable. Serology surveys can at best be crude pointers to chronicle the progress of the pandemic and not a psychological palliative.

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